DC / DC converters, distributed power systems

Abstract

To suppress rush current to a DC bus when a system is activated.SOLUTION: In an H-bridge converter (21), a control unit (22) can select a step-down mode, a step-up mode, and a step-up / down corresponding mode when transmitting power from a first DC power supply (SB1) to a second DC power supply (Bdc). The control unit (22), when activating the present DC / DC converter (21) in a step-up mode in such a direction that power is transmitted from the first DC power supply (SB1) to the second DC power supply (Cdc), makes a transition to the step-up mode via the step-up / down corresponding mode.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present disclosure relates to a step-up / step-down DC / DC converter and a distributed power supply system. [Background technology]

[0002] In recent years, hybrid energy storage systems (also called linked energy creation and storage systems) that link energy storage systems and solar power generation systems have become popular. It is conceivable to use an H-bridge converter (see, for example, Patent Document 1) as a DC / DC converter connected between a DC bus that connects a solar cell and an inverter, and a storage battery.

[0003] The inventors have developed an H-bridge converter with three operating modes: boost mode, buck mode, and buck-boost compatible mode. While detailed operation of each operating mode (see Figures 3 to 5) will be described later, the buck-boost compatible mode allows operation using a common control pattern regardless of the magnitude relationship between the battery voltage and the bus voltage. Therefore, the buck-boost compatible mode does not need to switch control patterns in response to changes in the magnitude relationship between the battery voltage and the bus voltage. This eliminates chattering, which occurs when the control pattern frequently switches between the battery voltage and the bus voltage in the region where the battery voltage and the bus voltage are close to each other, resulting in stable control. However, the buck-boost compatible mode requires constant on / off control of all four switching elements that make up the H-bridge converter, resulting in lower efficiency compared to the boost mode and buck mode, which do not require this. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2009-153245 Summary of the Invention [Problem to be solved by the invention]

[0005] When discharging from the storage battery when the battery voltage is lower than the target bus voltage, it is desirable to operate in boost mode if efficiency is emphasized. When starting up the hybrid energy storage system in boost mode, a large inrush current flows from the storage battery to the DC bus until the capacitor connected to the DC bus is charged. Note that a similar phenomenon occurs in energy storage systems that do not have solar cells connected.

[0006] The present disclosure has been made in consideration of these circumstances, and its purpose is to provide a DC / DC converter and a distributed power supply system that can suppress inrush current into a DC bus when the system is started up. [Means for solving the problem]

[0007] In order to solve the above-mentioned problems, a DC / DC converter according to one aspect of the present disclosure includes: a first arm including a first switching element and a second switching element connected in series and connected in parallel to a first DC power source and a first smoothing capacitor; a second arm including a third switching element and a fourth switching element connected in series and connected in parallel to a second DC power source and a second smoothing capacitor; a reactor connected to a midpoint of the first arm and a midpoint of the second arm; and a control unit that controls the first switching element and the fourth switching element. When transmitting power from the first DC power source to the second DC power source, the control unit selects one of the following modes: a buck mode in which the third switching element is fixed to an ON state and the fourth switching element is fixed to an OFF state and the first switching element and the second switching element are controlled by a buck chopper operation; a boost mode in which the first switching element is fixed to an ON state and the second switching element is fixed to an OFF state and the third switching element and the fourth switching element are controlled by a buck chopper operation; a buck-boost compatible mode that controls the ratio between a period during which energy is stored in the reactor from the first DC power supply when the third switching element is in an off state and a period during which energy is released from the reactor to the second DC power supply when the first switching element and the fourth switching element are in an off state and the second switching element and the third switching element are in an on state, and when the DC / DC converter is started in the boost mode in the direction of transmitting power from the first DC power supply to the second DC power supply, the control unit transitions to the boost mode via the buck-boost compatible mode. [Effects of the Invention]

[0008] According to the present disclosure, it is possible to suppress inrush current into the DC bus when starting up the system. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is a diagram showing the overall configuration of a distributed power supply system according to an embodiment; [Figure 2] FIG. 1 is a diagram illustrating a configuration example of a DC / DC converter. [Figure 3] 3(a) to 3(d) are diagrams for explaining the control of the boost mode of the H-bridge converter. [Figure 4] 4(a) to 4(d) are diagrams for explaining the control of the step-down mode of the H-bridge converter. [Figure 5] 5(a) to 5(d) are diagrams for explaining control of the H-bridge converter in the step-up / step-down compatible mode. [Figure 6] FIG. 10 is a diagram showing an example of waveforms for explaining a first startup method in which startup is performed in a boost mode in the discharge direction. [Figure 7] FIG. 10 is a diagram showing an example of waveforms for explaining a second startup method in which startup is performed in a boost mode in the discharge direction. DETAILED DESCRIPTION OF THE INVENTION

[0010] Fig. 1 is a diagram showing the overall configuration of a distributed power system 1 according to an embodiment. The distributed power system 1 according to the embodiment constitutes a hybrid power storage system. The distributed power system 1 shown in Fig. 1 includes a solar cell PV1, a power storage unit SB1, a first power conversion device 10, and a second power conversion device 20.

[0011] The first power conversion device 10 is a power conditioner for the solar cell PV1, and includes a step-up chopper 11, an inverter 12, and a control unit 13. The second power conversion device 20 is a charge / discharge converter for charging / discharging the power storage unit SB1, and includes a DC / DC converter 21 and a control unit 22.

[0012] FIG. 1 shows an example of the configuration of a separated-type distributed power system 1. In the separated-type, the power conditioner and the charge / discharge converter are separated, so that a storage battery system can be retrofitted to an existing solar power generation system. In recent years, the purchase price of solar power generation under the feed-in tariff scheme has been falling, and there is an increasing need to retrofit a storage battery system to an existing solar power generation system in order to increase self-consumption of solar power generation. Note that the system configuration of the distributed power generation system 1 is not limited to the separated type, and it may also be an integrated type in which the first power conversion device 10 and the second power conversion device 20 are integrated into a single housing.

[0013] The solar cell PV1 includes multiple solar cell modules (solar panels) connected in series. Each solar cell module includes multiple solar cells connected in series. The solar cell utilizes the photovoltaic effect to directly convert light energy into DC power. The solar cell can be a heterojunction solar cell, a polycrystalline silicon solar cell, a single-crystalline silicon solar cell, a thin-film silicon solar cell, or a compound solar cell.

[0014] The solar cell PV1 is connected to a boost chopper 11 and outputs the generated power to the boost chopper 11. The boost chopper 11 is a converter connected between the solar cell PV1 and the DC bus Bdc, and adjusts the voltage of the DC power output from the solar cell PV1 and outputs it to the DC bus Bdc.

[0015] The inverter 12 is a bidirectional inverter connected between the DC bus Bdc and the distribution board 3. The inverter 12 can convert DC power input from the DC bus Bdc into AC power and output the converted AC power to the distribution board 3. In addition to the first power conversion device 10, a commercial power system (hereinafter simply referred to as system 2) and a load 4 are connected to the distribution board 3. The load 4 is a general term for loads within the home. The inverter 12 can also convert AC power supplied from the system 2 via the distribution board 3 into DC power and output the converted DC power to the DC bus Bdc.

[0016] The control unit 13 performs overall control of the first power conversion device 10. The control unit 13 can be realized by a combination of hardware resources and software resources, or by hardware resources alone. Analog elements, microcontrollers, DSPs, ROMs, RAMs, ASICs, FPGAs, and other LSIs can be used as hardware resources. Programs such as firmware can be used as software resources.

[0017] The control unit 13 can perform MPPT (Maximum Power Point Tracking) control of the step-up chopper 11 so as to maximize the output power of the solar cell PV1. The control unit 13 generates a voltage command value for maximizing the output power of the solar cell PV1 based on the relationship between the output voltage and output power of the solar cell PV1.

[0018] For example, the control unit 13 searches for the maximum output operating point by changing the operating voltage in predetermined step widths according to the hill-climbing method. Specifically, the control unit 13 generates a voltage command value for shifting the current operating voltage toward the higher voltage side on the low-voltage side of the maximum output operating point of the PV curve of the solar cell module, and generates a voltage command value for shifting the current operating voltage toward the lower voltage side on the high-voltage side of the maximum output operating point. When the maximum output operating point is captured, the control unit 13 generates a voltage command value to maintain the maximum output operating point. The boost chopper 11 performs switching operation in response to a drive signal based on the generated voltage command value.

[0019] As a basic control of the inverter 12, the control unit 13 controls the inverter 12 so that the voltage of the DC bus Bdc maintains a target value. Specifically, the control unit 13 detects the voltage of the DC bus Bdc and generates a current command value for matching the detected bus voltage to the target value. If the voltage of the DC bus is higher than the target value, the control unit 13 generates a current command value for increasing the output power of the inverter 12, and if the voltage of the DC bus Bdc is lower than the target value, the control unit 13 generates a current command value for decreasing the output power of the inverter 12. The inverter 12 performs switching operation in response to a drive signal based on the generated current command value.

[0020] It is desirable that the target value of the voltage of the DC bus Bdc is set to a value higher than and close to the voltage of the system 2. The smaller the difference between the input voltage and output voltage of the inverter 12, the higher the conversion efficiency of the inverter 12. For example, if the voltage of the system 2 is AC 200V, the target value of the voltage of the DC bus Bdc may be set to about DC 330V.

[0021] The power storage unit SB1 is capable of charging and discharging DC power and includes a storage battery such as a lithium-ion storage battery, a nickel-metal hydride storage battery, or a lead-acid battery. Note that instead of a storage battery, a capacitor such as an electric double layer capacitor or a lithium-ion capacitor may be included. In this embodiment, the power storage unit SB1 is assumed to be a stationary power storage unit, but it may also be an on-board power storage unit mounted on an electric vehicle. In that case, the second power conversion device 20 becomes a V2H (Vehicle to Home) converter.

[0022] Power storage unit SB1 is connected to DC / DC converter 21, and charging and discharging are controlled by DC / DC converter 21. The other end of DC / DC converter 21 is connected to DC bus Bdc in first power conversion device 10 via a DC cable Cdc. DC / DC converter 21 is a bidirectional DC / DC converter for charging and discharging power storage unit SB1.

[0023] The control unit 22 performs overall control of the second power conversion device 20. The control unit 22 can be realized by a combination of hardware resources and software resources, or by hardware resources alone. Analog elements, microcontrollers, DSPs, ROMs, RAMs, ASICs, FPGAs, and other LSIs can be used as hardware resources. Programs such as firmware can be used as software resources.

[0024] The power storage unit SB1 and the control unit 22 are connected by a communication line, and the control unit 22 periodically acquires monitoring data including the voltage, current, and temperature of the storage battery included in the power storage unit SB1 via the communication line. The control unit 22 of the second power conversion device 20 and the control unit 13 of the first power conversion device 10 are also connected by a communication line, and control signals are exchanged between them.

[0025] Control unit 22 controls DC / DC converter 21 to charge and discharge power storage unit SB1. DC / DC converter 21 performs constant current (CC) discharge, constant voltage (CV) discharge, constant current charge, or constant voltage charge of power storage unit SB1, based on a current command value or a voltage command value set by control unit 22. When power storage unit SB1 is operated so as to follow at least one of the amount of power generated by solar cell PV1 and the amount of power consumed by load 4, control unit 22 generates a current command value for DC / DC converter 21 according to the voltage of DC bus Bd.

[0026] Fig. 2 is a diagram showing an example of the configuration of a DC / DC converter 21. The DC / DC converter 21 shown in Fig. 2 is configured as an H-bridge converter. The H-bridge converter includes a first capacitor C1, a reactor L1, a first switching element Q1 to a fourth switching element Q4, and a second capacitor C2. For example, electrolytic capacitors can be used for the first capacitor C1 and the second capacitor C2. For the first switching element Q1 to the fourth switching element Q4, MOSFETs (Metal-Oxide-Semiconductor Field-Effect Transistors) or IGBTs (Insulated Gate Bipolar Transistors) can be used.

[0027] A first voltage sensor V1 and a first smoothing capacitor C1 are connected between the positive and negative wiring of the power storage unit SB1. A first arm including a first switching element Q1 and a second switching element Q2 connected in series is connected in parallel to the power storage unit SB1 and the first capacitor C1. A second voltage sensor V2 and a second smoothing capacitor C2 are connected between the positive and negative wiring of the DC bus Bdc in the second power conversion device 20. A second arm including a third switching element Q3 and a fourth switching element Q4 connected in series is connected in parallel to the DC bus Bdc and the second capacitor C2.

[0028] A third smoothing capacitor C3 is connected between the positive wiring and the negative wiring of the DC bus Bdc in the first power conversion device 10. For the third capacitor C3, for example, an electrolytic capacitor having a larger capacity than the first capacitor C1 and the second capacitor C2 is used.

[0029] A reactor L1 is connected between the midpoint of the first arm (the connection point between the first switching element Q1 and the second switching element Q2) and the midpoint of the second arm (the connection point between the third switching element Q3 and the fourth switching element Q4).

[0030] A first diode D1 to a fourth diode D4 are formed or connected in anti-parallel to the first switching element Q1 to the fourth switching element Q4, respectively. When N-channel MOSFETs are used for the first switching element Q1 to the fourth switching element Q4, parasitic diodes formed in the source-to-drain direction can be used as the first diode D1 to the fourth diode D4. When IGBTs are used for the first switching element Q1 to the fourth switching element Q4, external diodes are connected as the first diode D1 to the fourth diode D4.

[0031] The first voltage sensor V1 detects the voltage of the power storage unit SB1 and outputs the voltage to the control unit 22. The second voltage sensor V2 detects the voltage of the DC bus Bdc and outputs the voltage to the control unit 22. The current sensor A1 detects the current flowing through the reactor L1 and outputs the current to the control unit 22. The first voltage sensor V1 and the second voltage sensor V2 each include, for example, a voltage dividing resistor and an error amplifier. The current sensor A1 includes, for example, a CT sensor or a Hall sensor.

[0032] Control unit 22 performs PWM (Pulse Width Modulation) control on first switching element Q1 to fourth switching element Q4 based on the voltage of power storage unit SB1, the voltage of DC bus Bdc, or the current flowing through reactor L1. Control of DC / DC converter 21 will be specifically described below.

[0033] The problem that arises when the distributed power system 1 is started up is the inrush current that flows from the power storage unit SB1 to the DC bus Bdc, and therefore, hereinafter, attention will be focused on the discharge operation of the power storage unit SB1.

[0034] 3(a) to 3(d) are diagrams for explaining the control of the H-bridge converter in the step-up mode, and 4(a) to 4(d) are diagrams for explaining the control of the H-bridge converter in the step-down mode.

[0035] The boost mode is a mode in which the power storage unit SB1 discharges to the DC bus Bdc when the voltage of the power storage unit SB1 is lower than the voltage of the DC bus Bdc. In the boost mode, the control unit 22 fixes the first switching element Q1 to a constantly on state and the second switching element Q2 to a constantly off state.

[0036] 3(a), control unit 22 controls third switching element Q3 to be in the off state and fourth switching element Q4 to be in the on state. In the first state, energy is stored in reactor L1 from power storage unit SB1.

[0037] 3(b), the control unit 22 controls both the third switching element Q3 and the fourth switching element Q4 to the off state. The second state is a dead time, and power is transmitted to the DC bus Bdc from both the power storage unit SB1 and the reactor L1 via the third diode D3 and the first switching element Q1.

[0038] 3(c), the control unit 22 controls the third switching element Q3 to the on state and the fourth switching element Q4 to the off state. The third state is a synchronous rectification period, during which power is transmitted to the DC bus Bdc from both the power storage unit SB1 and the reactor L1 via the third switching element Q3 and the first switching element Q1.

[0039] As shown in Figure 3(d), the fourth state is similar to the second state shown in Figure 3(b). The synchronous rectification period shown in Figure 3(c) results in less loss than the dead time shown in Figures 3(b) and 3(d). The dead time is provided to prevent a shoot-through current from occurring when the third switching element Q3 and the fourth switching element Q4 are simultaneously turned on.

[0040] In the second state to the fourth state, the voltage obtained by adding the voltage of the power storage unit SB1 and the voltage of the reactor L1 is output to the DC bus Bdc. The voltage of the reactor L1 depends on the energy stored in the reactor L1 in the first state. In this specification, the operation of repeating the first state in which energy is stored in the reactor L1 using the third switching element Q3 and the fourth switching element Q4, and the second state to the fourth state in which power is transmitted from the power storage unit SB1 and the reactor L1 to the DC bus Bdc, is referred to as boost chopper operation.

[0041] In the step-down mode, when the voltage of the power storage unit SB1 is higher than the voltage of the DC bus Bdc, the power storage unit SB1 is discharged to the DC bus Bdc. In the step-down mode, the control unit 22 fixes the third switching element Q3 to a constantly on state and the fourth switching element Q4 to a constantly off state.

[0042] 4(a), the control unit 22 controls the first switching element Q1 to the ON state and the second switching element Q2 to the OFF state. In the fifth state, a voltage difference between the power storage unit SB1 and the DC bus Bdc causes a current to flow from the power storage unit SB1 to the DC bus Bdc. At that time, energy is stored in the reactor L1.

[0043] 4(b), the control unit 22 controls both the first switching element Q1 and the second switching element Q2 to the off state. The sixth state is a dead time, during which the energy stored in the reactor L1 is smoothed by the second capacitor C2 and released to the DC bus Bdc via the third switching element Q3 and the second diode D2.

[0044] 4(c), the control unit 22 controls the first switching element Q1 to be in the OFF state and the second switching element Q2 to be in the ON state. The seventh state is a synchronous rectification period, during which the energy stored in the reactor L1 is smoothed by the second capacitor C2 and released to the DC bus Bdc via the third switching element Q3 and the second switching element Q2.

[0045] The eighth state shown in Figure 4(d) is similar to the sixth state shown in Figure 4(b). The synchronous rectification period shown in Figure 4(c) results in less loss than the dead time shown in Figures 4(b) and 4(d). The dead time is provided to prevent a shoot-through current caused by the first switching element Q1 and the second switching element Q2 being turned on simultaneously.

[0046] In the sixth state to the eighth state, the voltage of the reactor L1 is smoothed by the second capacitor C2 and output to the DC bus Bdc. The voltage of the reactor L1 depends on the energy stored in the reactor L1 in the fifth state. In this specification, the operation of repeating the fifth state, in which energy is stored in the reactor L1 while power is transferred from the power storage unit SB1 to the DC bus Bdc, and the sixth state, the eighth state, in which energy is released from the reactor L1 to the DC bus Bdc, using the first switching element Q1 and the second switching element Q2, is referred to as step-down chopper operation.

[0047] In the boost mode, the control unit 22 PWM-controls the fourth switching element Q4 based on the duty ratio D, and PWM-controls the third switching element Q3 based on the duty ratio (1-D). In the boost mode, the relationship between the power transmitting side voltage Vin, the power receiving side voltage Vout, and the duty ratio D is defined by the following (Equation 1).

[0048] Vout=(1 / (1-D))·Vin···(Formula 1)

[0049] In the step-down mode, the control unit 22 PWM-controls the first switching element Q1 based on the duty ratio D, and PWM-controls the second switching element Q2 based on the duty ratio (1-D). In the step-down mode, the relationship between the power transmitting side voltage Vin, the power receiving side voltage Vout, and the duty ratio D is defined by the following (Equation 2).

[0050] Vout=D·Vin (Formula 2)

[0051] Control unit 22 calculates a current deviation based on the difference between a measurement value of the current flowing through reactor L1 obtained from current sensor A1 and a current command value. Alternatively, control unit 22 calculates a voltage deviation based on the difference between a measurement value of the voltage of power storage unit SB1 obtained from power storage unit SB1 or first voltage sensor V1 and a voltage command value for discharging or a voltage command value for charging. Control unit 22 performs PI control on the current deviation or voltage deviation to generate duty ratio D. Note that PID control or P control may be used instead of PI control. Control unit 22 performs PWM control on the fourth switching element Q4 and the third switching element Q3, or the first switching element Q1 and the second switching element Q2, based on the generated duty ratio D and duty ratio (1-D).

[0052] 5(a)-(d) are diagrams illustrating control of the H-bridge converter in the step-up / step-down mode. In the ninth state shown in FIG. 5(a), the control unit 22 controls the first switching element Q1 and the fourth switching element Q4 to the on state and the second switching element Q2 and the third switching element Q3 to the off state. In the ninth state, energy is stored in the reactor L1 from the power storage unit SB1.

[0053] 5(b), the control unit 22 controls all of the first switching element Q1 to the fourth switching element Q4 to the off state. The tenth state is a dead time, and the energy stored in the reactor L1 is smoothed by the second capacitor C2 and released to the DC bus Bdc via the third diode D3 and the second diode D2.

[0054] 5(c), the control unit 22 controls the first switching element Q1 and the fourth switching element Q4 to the off state and the second switching element Q2 and the third switching element Q3 to the on state. The 11th state is a synchronous rectification period, during which the energy stored in the reactor L1 is smoothed by the second capacitor C2 and released to the DC bus Bdc via the third switching element Q3 and the second switching element Q2.

[0055] The twelfth state shown in Figure 5(d) is similar to the tenth state shown in Figure 5(b). The synchronous rectification period shown in Figure 5(c) results in less loss than the dead times shown in Figures 5(b) and 5(d). The dead times are provided to prevent shoot-through currents that occur when the first switching element Q1 and the second switching element Q2 are turned on simultaneously, or when the third switching element Q3 and the fourth switching element Q4 are turned on simultaneously.

[0056] In the buck-boost mode, the power storage unit SB1 and the DC bus Bdc are electrically disconnected in either state. Except during dead time, the first switching element Q1 to the fourth switching element Q4 are always controlled in complementary operation. In the buck-boost mode, the H-bridge converter is controlled by common duty control, regardless of the magnitude relationship between the voltage of the power storage unit SB1 and the voltage of the DC bus Bdc.

[0057] In the step-up / step-down compatible mode, the control unit 22 PWM controls the first switching element Q1 and the fourth switching element Q4 based on the duty ratio D, and PWM controls the second switching element Q2 and the third switching element Q3 based on the duty ratio (1-D). In the step-up / step-down compatible mode, the relationship between the power transmitting side voltage Vin, the power receiving side voltage Vout, and the duty ratio D is defined by the following (Equation 3).

[0058] Vout=(D / (1-D))·Vin···(Formula 3) D>50% boost D=50% and no voltage conversion Decrease blood pressure when D<50%

[0059] Duty ratio D indicates the ratio of the period during which energy is stored from the power storage unit SB1 to the reactor L1, and duty ratio (1-D) indicates the ratio of the period during which energy is released from the reactor L1 to the DC bus Bdc. In the region where duty ratio D is greater than 50%, the larger the duty ratio D, the larger the step-up rate. In the region where duty ratio D is smaller than 50%, the smaller the duty ratio D, the larger the step-down rate.

[0060] Control unit 22 calculates a current deviation based on the difference between the measured value of the current flowing through reactor L1 obtained from current sensor A1 and the current command value. The current command value is set to a positive value when discharging from power storage unit SB1 and to a negative value when charging power storage unit SB1. When performing CC discharging or CC charging of power storage unit SB1, control unit 22 fixes the current command value of the reactor current to a predetermined value. When performing CV discharging or CV charging of power storage unit SB1, control unit 22 changes the current command value of the reactor current so that the voltage of power storage unit SB1 detected by first voltage sensor V1 becomes a constant value.

[0061] The control unit 22 performs PI control on the current deviation to generate a duty ratio D. The control unit 22 performs PWM control on the first switching element Q1 to the fourth switching element Q4 based on the generated duty ratio D and duty ratio (1-D).

[0062] As described above, when discharging from the power storage unit SB1 in a state where the voltage of the power storage unit SB1 is lower than the target value of the voltage of the DC bus Bdc, it is desirable to operate in the boost mode if efficiency is emphasized. When the distributed power supply system 1 is started in the boost mode in the discharge direction, an inrush current flows from the power storage unit SB1 to charge the third capacitor C3 connected to the DC bus Bdc because the voltage of the DC bus Bdc is 0 V in the initial state.

[0063] As shown in Figures 3(b) to 3(d) above, in the boost mode, a state occurs in which the power storage unit SB1 and the DC bus Bdc are conductive, and therefore, when the voltage of the DC bus Bdc is low, a large current flows from the power storage unit SB1 to the DC bus Bdc. In the state shown in Figures 3(b) to 3(d) above, a voltage obtained by adding the voltage of the power storage unit SB1 and the voltage of the reactor L1 in which energy is stored is applied to the DC bus Bdc, and therefore a large current is likely to occur.

[0064] Therefore, a possible startup method is to operate in step-down mode until the third capacitor C3 is charged and the voltage of the DC bus Bdc rises to the voltage of the power storage unit SB1, and then switch to step-up mode when the voltage reaches that of the power storage unit SB1. In step-down mode, if the voltage of the power storage unit SB1 is significantly higher than the voltage of the DC bus Bdc, the period during which the power storage unit SB1 and the DC bus Bdc are conductive (see Figure 4(a) above) becomes shorter, making it difficult for a large current to flow.

[0065] In the above startup method, the voltage of the DC bus Bdc rises and it is necessary to switch from the step-down mode to the step-up mode at the timing when the voltage of the power storage unit SB1 and the voltage of the DC bus Bdc match. However, in a hybrid power storage system, the voltage of the DC bus Bdc fluctuates depending on fluctuations in solar radiation, making it difficult to maintain a state in which the voltage of the power storage unit SB1 and the voltage of the DC bus Bdc match.

[0066] When the voltage of the power storage unit SB1 and the voltage of the DC bus Bdc are the same, the first switching element Q1 and the third switching element Q3 are controlled to the on state, and the power storage unit SB1 and the DC bus Bdc are in a conductive state. In this state, if the amount of solar radiation increases and the voltage of the DC bus Bdc rises, an unintended charging current may flow from the DC bus Bdc to the power storage unit SB1. Furthermore, in a region where the voltage of the power storage unit SB1 and the voltage of the DC bus Bdc are similar, if the magnitude relationship between the voltage of the power storage unit SB1 and the voltage of the DC bus Bdc changes frequently in response to fluctuations in solar radiation, chattering may occur between the step-down mode and the step-up mode.

[0067] A method for starting up in a boost mode in the discharge direction while suppressing the inrush current at the time of starting up will be described below.

[0068] FIG. 6 is a diagram illustrating an example of waveforms for explaining a first startup method for starting up in a boost mode in the discharge direction. In the first startup method, the control unit 22 starts the H-bridge converter in a buck mode. When the voltage VBUS of the DC bus Bdc rises to the voltage VSB of the power storage unit SB1, the control unit 22 switches to the boost mode after a predetermined period of the buck-boost compatible mode. For example, the buck-boost compatible mode is inserted for about 10 ms as an intermediate mode (transition period) between the buck mode and the boost mode. This stabilizes the transition from the buck mode to the boost mode. In the buck-boost compatible mode, a state in which the power storage unit SB1 and the DC bus Bdc are directly connected to each other does not occur, and therefore the transition is not affected by changes in the magnitude relationship between the voltage VBUS of the power storage unit SB1 and the voltage VSB of the DC bus Bdc. Therefore, stable switching to the boost mode is possible.

[0069] In the step-down mode shown in Figure 6, in order to further limit the current flowing through the DC bus Bdc, an example is shown in which synchronous rectification is not used and diode rectification is always used during the period in which the energy stored in the reactor L1 is released. When the voltage VBUS of the DC bus Bdc reaches the target voltage Vtg, the voltage VBUS of the DC bus Bdc is maintained at the target voltage Vtg.

[0070] 7 is a diagram showing example waveforms for explaining a second startup method for starting up in a boost mode in the discharge direction. In the second startup method, the control unit 22 starts up the H-bridge converter in a buck-boost compatible mode, and when the voltage VBUS of the DC bus Bdc reaches the target voltage Vtg, the control unit 22 transitions to the boost mode. As described above, in the buck-boost compatible mode, a state in which the power storage unit SB1 and the DC bus Bdc are directly connected to each other is not generated, and therefore, inrush current is suppressed.

[0071] As described above, according to this embodiment, when the H-bridge converter is started in the boost mode in the discharge direction, the inrush current to the DC bus Bdc at startup can be suppressed by transitioning to the boost mode via the buck-boost mode. That is, by gradually increasing the voltage VBUS of the DC bus Bdc from 0 V (soft start), the inrush current to the DC bus Bdc can be suppressed.

[0072] In particular, with the second startup method, the voltage VBUS can be operated in the same mode from 0V to the target voltage Vtg, eliminating the need for intermediate modes compared to the first startup method and reducing the number of mode switches.

[0073] The present disclosure has been described above based on the embodiments. The embodiments are merely examples, and it will be understood by those skilled in the art that various modifications are possible in the combination of the respective components and processing steps, and that such modifications are also within the scope of the present disclosure.

[0074] In the above-described embodiment, an example has been described in which the H-bridge converter according to the present disclosure is used as the DC / DC converter 21 for the power storage unit SB1 of a hybrid power storage system. In this regard, the H-bridge converter can also be used in a pure power storage system to which the solar cell PV1 is not connected.

[0075] The solar cell PV1 is an example of a DC power supply unit that outputs DC power, and a fuel cell or another storage battery may also be used as the DC power supply unit. The control method of the H-bridge converter according to this embodiment is particularly effective in applications where the H-bridge converter is connected between a DC bus Bdc connected to a solar cell or fuel cell whose output voltage fluctuates irregularly and the power storage unit SB1.

[0076] The embodiment may be specified by the following items.

[0077] [Item 1] a first arm including a first switching element (Q1) and a second switching element (Q2) connected in series and connected in parallel to a first DC power source (SB1) and a first smoothing capacitor (C1); a second arm including a third switching element (Q3) and a fourth switching element (Q4) connected in series and connected in parallel to a second DC power source (Bdc) and a second smoothing capacitor (C2); a reactor (L1) connected to a midpoint of the first arm and a midpoint of the second arm; a control unit (22) that controls the first switching element (Q1) to the fourth switching element (Q4), When transmitting power from the first DC power supply (SB1) to the second DC power supply (Bdc), the control unit (22) a step-down mode in which the third switching element (Q3) is fixed to an ON state, the fourth switching element (Q4) is fixed to an OFF state, and the first switching element (Q1) and the second switching element (Q2) are controlled by a step-down chopper operation; a boost mode in which the first switching element (Q1) is fixed to an ON state, the second switching element (Q2) is fixed to an OFF state, and the third switching element (Q3) and the fourth switching element (Q4) are controlled by a boost chopper operation; a buck-boost compatible mode is selectable that controls the ratio of a period during which energy is stored in the reactor (L1) from the first DC power supply (SB1) when the first switching element (Q1) and the fourth switching element (Q4) are in an on state and the second switching element (Q2) and the third switching element (Q3) are in an off state to a period during which energy is released from the reactor (L1) to the second DC power supply (Bdc) when the first switching element (Q1) and the fourth switching element (Q4) are in an off state and the second switching element (Q2) and the third switching element (Q3) are in an on state; The control unit (22) transitions the DC / DC converter (21) to the boost mode via the step-up / step-down compatible mode when starting the DC / DC converter (21) in the boost mode in a direction to transmit power from the first DC power source (SB1) to the second DC power source (Cdc). This makes it possible to suppress the inrush current from the first DC power supply (SB1) to the second DC power supply (Bdc) at the time of startup. [Item 2] The control unit (22) The DC / DC converter (21) according to item 1, wherein the DC / DC converter (21) is started in the step-up / step-down mode, and when the voltage of the second DC power supply reaches a target voltage, the DC / DC converter (21) is shifted to the step-up mode. This reduces the number of times the operating mode needs to be switched at startup. [Item 3] The control unit (22) The DC / DC converter (21) according to item 1, wherein the DC / DC converter (21) is started in the step-down mode, and when the voltage of the second DC power supply (Bdc) rises to the voltage of the first DC power supply (SB1), the DC / DC converter (21) is shifted to the step-up / step-down compatible mode after a predetermined time. This allows seamless switching from the step-down mode to the step-up mode. [Item 4] the first DC power supply (SB1) is a power storage unit (SB1) capable of charging and discharging DC power, The second DC power supply (Bdc) is a DC bus (Bdc) connected to a third smoothing capacitor (C), 4. The DC / DC converter (21) according to any one of items 1 to 3, wherein a system (2) is connected to the DC bus (Bdc) via an inverter (12), and a solar cell (PV1) is connected to the DC bus (Bdc). This makes it possible to suppress the inrush current from the power storage unit (SB1) to the DC bus (Bdc) at the time of startup in an application where the voltage of the DC bus (Bdc) changes irregularly. [Item 5] a DC power supply unit (PV1) that outputs DC power; a first power conversion device (10) provided between the DC power supply unit (PV1) and a grid (2) and including an inverter (12); a power storage unit (SB1) capable of charging and discharging DC power; a second power conversion device (20) provided between the power storage unit (SB1) and a DC bus (Bdc) connected to a DC side of the inverter (12), The distributed power system (1) includes the second power conversion device (20) including the DC / DC converter (21) according to any one of items 1 to 3. This makes it possible to realize a distributed power system (1) in which an inrush current from the power storage unit (SB1) to the DC bus (Bdc) at the time of startup is suppressed. [Item 6] 6. The distributed power system (1) according to item 5, wherein the DC power supply unit (PV1) is a solar cell (PV1). This makes it possible to suppress the inrush current from the power storage unit (SB1) to the DC bus (Bdc) at the time of startup in an application where the voltage of the DC bus (Bdc) changes irregularly. [Explanation of symbols]

[0078] PV1 solar cell, SB1 storage unit, 1 distributed power supply system, 10 first power conversion device, 11 step-up chopper, 12 inverter, 13 control unit, 20 second power conversion device, 21 DC / DC converter, 22 control unit, Bdc DC bus, Cdc DC cable, 2 system, 3 distribution board, 4 load, L1 reactor, Q1-Q4 switching elements, D1-D4 diodes, C1-C3 capacitors, A1 current sensor, V1-V2 voltage sensors.

Claims

1. a first arm including a first switching element and a second switching element connected in series and connected in parallel to the first DC power source and the first smoothing capacitor; a second arm including a third switching element and a fourth switching element connected in series and connected in parallel to the second DC power source and the second smoothing capacitor; a reactor connected to a midpoint of the first arm and a midpoint of the second arm; a control unit that controls the first switching element to the fourth switching element, When transmitting power from the first DC power supply to the second DC power supply, the control unit a step-down mode in which the third switching element is fixed to an ON state, the fourth switching element is fixed to an OFF state, and the first switching element and the second switching element are controlled by a step-down chopper operation; a boost mode in which the first switching element is fixed to an ON state, the second switching element is fixed to an OFF state, and the third switching element and the fourth switching element are controlled by a boost chopper operation; a step-up / step-down compatible mode is selectable in which a ratio of a period during which energy is stored in the reactor from the first DC power supply when the first switching element and the fourth switching element are in an on state and the second switching element and the third switching element are in an off state to a period during which energy is released from the reactor to the second DC power supply when the first switching element and the fourth switching element are in an off state and the second switching element and the third switching element are in an on state is controlled; The control unit transitions the DC / DC converter to the boost mode via the step-up / step-down compatible mode when the DC / DC converter is started in the boost mode in the direction of transmitting power from the first DC power source to the second DC power source.

2. The control unit 2. The DC / DC converter according to claim 1, wherein the DC / DC converter is started in the step-up / step-down mode, and when the voltage of the second DC power supply reaches a target voltage, the DC / DC converter is shifted to the step-up mode.

3. The control unit 2. The DC / DC converter according to claim 1, wherein the DC / DC converter is started in the step-down mode, and when the voltage of the second DC power supply rises to the voltage of the first DC power supply, the DC / DC converter is transitioned to the step-up / step-down compatible mode for a predetermined time.

4. the first DC power source is a power storage unit capable of charging and discharging DC power, the second DC power source is a DC bus to which a third smoothing capacitor is connected; 4. The DC / DC converter according to claim 1, wherein a system is connected to the DC bus via an inverter, and a solar cell is also connected to the DC bus.

5. a DC power supply unit that outputs DC power; a first power conversion device provided between the DC power supply unit and a grid and including an inverter; a power storage unit capable of charging and discharging DC power; a second power conversion device provided between the power storage unit and a DC bus connected to a DC side of the inverter, The second power conversion device is a distributed power system including the DC / DC converter according to claim 1 .

6. 6. The distributed power system according to claim 5, wherein the DC power supply unit is a solar battery.

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