Control method for power supply system and control device for power supply system

Abstract

This control method for a power supply system is a method for controlling a power supply system (1) that uses power from a solar cell (13) to charge a first power storage device (3) connected to the solar cell (13) and charges a second power storage device (5) by using converted power obtained by converting the power of the first power storage device (3). In this control method for the power supply system (1), a generated power value (Pn) of the solar cell (13) and a terminal voltage value (Vn) of the first power storage device (3) are acquired. Limit values (VUL, VLL) of an allowable variation range of the terminal voltage value (Vn) of the first power storage device (3) are determined on the basis of the acquired generated power value (Pn) and terminal voltage value (Vn). The limit values (VUL, VLL) are values for causing the allowable variation range of the terminal voltage value (Vn) due to charging and discharging of the first power storage device (3) to follow a maximum power point voltage (VMPP_n) of the solar cell (13).

Description

Power supply system control method and power supply system control device

[0001] The present invention relates to a method for controlling a power supply system and a control device for a power supply system.

[0002] Patent Document 1 describes a power supply device that charges a secondary battery with power generated by a solar cell. In this power supply device, an electric double layer capacitor is charged with power from the solar cell. When the voltage of the electric double layer capacitor reaches or exceeds a high-level threshold voltage, charging of the electric double layer capacitor with power from the solar cell is stopped, and the secondary battery is charged with power from the electric double layer capacitor boosted by a DC-DC converter. When the voltage of the electric double layer capacitor drops to a low-level threshold voltage lower than the high-level threshold voltage, charging of the secondary battery with power from the electric double layer capacitor is stopped, and charging of the electric double layer capacitor with power from the solar cell is resumed.

[0003] Patent No. 4133924

[0004] The power generation efficiency of a solar cell is maximized when the voltage of the load connected to the solar cell is the maximum power point voltage. In the power supply device disclosed in Patent Document 1, an electric double layer capacitor that stores the power generated by the solar cell and is used to charge a secondary battery serves as the load for the solar cell. The voltage of the electric double layer capacitor is limited to a range from a high-level threshold voltage to a low-level threshold voltage, which are fixed values. The maximum power point voltage of the solar cell fluctuates depending on changes in the solar cell's operating environment, such as the ambient temperature at the solar cell's installation location, the amount of solar radiation, and the angle of sunlight irradiation. If the maximum power point voltage of the solar cell falls outside the range from the high-level threshold voltage to the low-level threshold voltage of the electric double layer capacitor, the power generation efficiency of the solar cell will decrease unless power conversion is performed between the solar cell and the electric double layer capacitor.

[0005] The present invention has been made in consideration of the above circumstances, and an object of the present invention is to suppress a decrease in the power generation efficiency of a solar cell without performing additional power conversion when storing power generated by a solar cell and converting the stored power to use for charging a secondary battery.

[0006] According to one aspect of the present invention, a method for controlling a power supply system that solves the above-described problems is a method for controlling a power supply system in which a first power storage device connected to a solar cell is charged with power from the solar cell and a second power storage device is charged with power obtained by converting the power of the first power storage device. The method for controlling a power supply system includes acquiring a power generation value of the solar cell and a terminal voltage value of the first power storage device. Based on the acquired power generation value and terminal voltage value, a limit value of an allowable fluctuation range of the terminal voltage value of the first power storage device due to charging and discharging of the first power storage device is determined so that the allowable fluctuation range of the terminal voltage value of the first power storage device follows the maximum power point voltage of the solar cell.

[0007] According to the present invention, in a power supply system including a solar cell, it is possible to suppress a decrease in power generation efficiency of the solar cell without performing additional power conversion between the solar cell and the first power storage device.

[0008] FIG. 1 is a diagram showing the configuration of a power supply system for an electric vehicle having a control device according to an embodiment of the present invention. FIG. 2 is a flowchart showing an example of a processing procedure performed by the controller of FIG. 1 in a low-voltage battery charging mode in the first embodiment. FIG. 3 is a graph showing the generated power-voltage characteristics (P-V characteristics) of the solar cell of FIG. 1. FIG. 4 is a flowchart showing an example of a processing procedure performed by the controller of FIG. 1 in a high-voltage battery charging mode in the first embodiment. FIG. 5 is a diagram showing an allowable fluctuation range of the terminal voltage of the low-voltage battery determined by the controller of FIG. 1 in accordance with the maximum power point voltage value of the solar cell. FIG. 6 is a flowchart showing an example of a processing procedure performed by the controller of FIG. 1 in a low-voltage battery charging mode in the second embodiment. FIG. 7 is a flowchart showing an example of a processing procedure performed by the controller of FIG. 1 in a high-voltage battery charging mode in the second embodiment.

[0009] Hereinafter, an embodiment of the present invention and its modifications will be described with reference to the drawings. In the description of the drawings, the same parts are given the same reference numerals and the description thereof will be omitted.

[0010] [Configuration Example of Power Supply System According to Embodiment] In the following embodiment, a power supply system mounted on an electric vehicle (EV) (not shown) will be described. As shown in Fig. 1 , the power supply system 1 according to the embodiment includes a low-voltage battery 3, a high-voltage battery 5, electronic devices 7, a controller 9, a motor / inverter 11, a solar cell 13, and a bidirectional DC-DC converter 15. The power supply system 1 further includes a voltage sensor 17, a current sensor 19, and a reverse current prevention diode 21. In the power supply system 1, the low-voltage battery 3 directly connected to the solar cell 13 is charged with power from the solar cell 13, and the high-voltage battery 5 is charged with power obtained by converting the power of the low-voltage battery 3.

[0011] The low-voltage battery 3 is a secondary battery constituting a first power storage device that stores DC power of a first voltage value. The first voltage value is, for example, 14 V. The low-voltage battery 3 supplies power to electronic devices 7, a controller 9, and other components that are low-voltage system loads of the electric vehicle. The electronic devices 7 include auxiliary machinery mounted on the electric vehicle. The controller 9 functions as a control device for the power supply system 1. Details of the controller 9 will be described later. The high-voltage battery 5 is a secondary battery constituting a second power storage device that stores DC power of a second voltage value that is higher than the first voltage value. The second voltage value is, for example, 200 V. The high-voltage battery 5 supplies power to a motor / inverter 11 and other components that are high-voltage system loads of the electric vehicle. The motor / inverter 11 includes a propulsion motor driven by AC power from the electric vehicle and an inverter circuit that converts the DC power of the high-voltage battery 5 into AC power and supplies it to the propulsion motor.

[0012] The solar cell 13 includes a solar cell module. The solar cell module includes multiple solar cells that generate electromotive force corresponding to the amount of sunlight irradiated. The power generated by the solar cell 13 can be supplied to the low-voltage battery 3 connected to the solar cell 13 and a low-voltage load. The bidirectional DC-DC converter 15 performs bidirectional power conversion between the low-voltage battery 3 and the high-voltage battery 5. In the following description, the bidirectional DC-DC converter 15 is referred to as the bidirectional converter 15. The bidirectional power conversion by the bidirectional converter 15 includes a boost operation that converts DC power of a first voltage value to a second voltage value and a buck operation that converts DC power of the second voltage value to the first voltage value. During the boost operation of the bidirectional converter 15, the high-voltage battery 5 is charged with the power converted from the low-voltage battery 3, which has been boosted by the bidirectional converter 15. During the boost operation, the bidirectional converter 15 functions as a power converter that converts the power of the low-voltage battery 3 and outputs it to the high-voltage battery 5. The converted power from the low-voltage battery 3 may be supplied to the motor / inverter 11. During the step-down operation of the bidirectional converter 15, the low-voltage battery 3 is charged with the converted power of the high-voltage battery 5, which has been stepped down by the bidirectional converter 15. The bidirectional converter 15 has a built-in sensor (not shown) that measures the voltage and current on the low-voltage side connected to the low-voltage battery 3. During the step-down operation, the bidirectional converter 15 is controlled by the controller 9 so that the power on the low-voltage side is at a set value. While the bidirectional converter 15 is not operating, the low-voltage battery 3 is charged with the power generated by the solar cell 13. The voltage sensor 17 detects the terminal voltage of the low-voltage battery 3 as the voltage of the load connected to the solar cell 13. The current sensor 19 detects the current of the power generated by the solar cell 13 that is supplied to the low-voltage battery 3 and the low-voltage load. The backflow prevention diode 21 prevents current from flowing back from the low-voltage battery 3 to the solar cell 13 when the amount of power generated by the solar cell 13 decreases, for example, due to a decrease in the amount of sunlight irradiation.

[0013] The control method of the power supply system 1 can be executed by a controller 9 that functions as a control device for the power supply system 1. The controller 9 has a general-purpose microcontroller equipped with an arithmetic unit and an input / output unit. The arithmetic unit includes a central processing unit (CPU) and a memory. The memory includes a read-only ROM (read-only memory) and a readable / writable RAM (random access memory). The arithmetic unit of the controller 9 can virtually configure multiple information processing circuits by having the CPU execute a program stored in the ROM. The information processing circuits configured in the arithmetic unit of the controller 9 can configure, for example, a first acquisition unit 91, a second acquisition unit 93, and a control unit 95 of the controller 9. The information processing circuit of the controller 9 may be configured with dedicated hardware. The dedicated hardware may include devices such as application-specific integrated circuits (ASICs) and conventional circuit components arranged to perform the required information processing functions.

[0014] The first acquisition unit 91 acquires the power generation value of the solar cell 13. The power generation value of the solar cell 13 can be acquired by calculation, for example, by multiplying the terminal voltage value of the low-voltage battery 3 detected by the voltage sensor 17 by the current value of the power generated by the solar cell 13 detected by the current sensor 19. The second acquisition unit 93 acquires the terminal voltage value of the low-voltage battery 3 from the value detected by the voltage sensor 17. The control unit 95 controls the boost operation of the bidirectional converter 15 based on the terminal voltage value of the low-voltage battery 3 acquired by the second acquisition unit 93. To perform this control, the control unit 95 determines a limit value of the allowable fluctuation range of the terminal voltage value of the low-voltage battery 3 based on the power generation value of the solar cell 13 acquired by the first acquisition unit 91 and the terminal voltage value of the low-voltage battery 3 acquired by the second acquisition unit 93. The limit value determined by the control unit 95 is a value for controlling the fluctuation range of the terminal voltage value due to charging and discharging of the low-voltage battery 3 to follow the maximum power point voltage of the solar cell 13, which will be described later. This limit value defines at least one of the lower limit value and the upper limit value of the allowable fluctuation range. In the following embodiment, the control unit 95 determines the lower limit voltage VLL, which is the lower limit value of the allowable fluctuation range, and the upper limit voltage VUL, which is the upper limit value of the allowable fluctuation range.

[0015] The control unit 95 controls the charging and discharging of the low-voltage battery 3 so that the terminal voltage value acquired by the second acquisition unit 93 becomes a predetermined voltage value within the allowable fluctuation range during charging and discharging of the low-voltage battery 3. When the terminal voltage value acquired by the second acquisition unit 93 reaches the limit value of the allowable fluctuation range, the control unit 95 switches the charging and discharging state of the low-voltage battery 3 from one of the charging state and the discharging state to the other. When the low-voltage battery 3 is in the charging state, the operation of the bidirectional converter 15 is stopped, and the charging mode of the low-voltage battery 3 and the high-voltage battery 5 becomes the first charging mode. The first charging mode is a low-voltage battery charging mode in which the low-voltage battery 3 is charged with power generated by the solar cell 13. In the low-voltage battery charging mode, the terminal voltage value of the low-voltage battery 3 increases due to charging. When the low-voltage battery 3 is in the discharging state, the bidirectional converter 15 performs a boost operation, and the charging mode of the low-voltage battery 3 and the high-voltage battery 5 becomes the second charging mode. The second charging mode is a high-voltage battery charging mode in which the high-voltage battery 5 is charged with power obtained by converting the power of the low-voltage battery 3 using the bidirectional converter 15. In the high-voltage battery charging mode, the terminal voltage of the low-voltage battery 3 drops due to discharging to supply power to the bidirectional converter 15. The limit value determined by the control unit 95 is used as a threshold value for the control unit 95 to switch between the first charging mode and the second charging mode based on the terminal voltage value acquired by the second acquisition unit 93. When the terminal voltage value acquired by the second acquisition unit 93 reaches the limit value, the control unit 95 switches the on / off state of the power conversion operation of the bidirectional converter 15, which serves as a power conversion unit. The lower limit voltage VLL as a limit value can be used, for example, as a threshold value for switching from the second charging mode to the first charging mode, and the upper limit voltage VUL as a limit value can be used as a threshold value for switching from the first charging mode to the second charging mode. The lower limit voltage VLL may be used as a threshold for switching the low voltage battery 3 from a discharged state to a charged state, and the upper limit voltage VUL may be used as a threshold for switching the low voltage battery 3 from a charged state to a discharged state.

[0016] [First Embodiment] In the power supply system 1 of the first embodiment, the control unit 95 executes processing according to the procedure shown in Fig. 2 in the low-voltage battery charging mode. The control unit 95 acquires the power generation power Pn of the solar cell 13 acquired by the first acquisition unit 91 and the terminal voltage Vn of the low-voltage battery 3 acquired by the second acquisition unit 93 (step S101). The control unit 95 determines whether the sign of a ratio (ΔPn / ΔVn) obtained by dividing the change in power generation power Pn per unit time ΔPn by the change in terminal voltage Vn per unit time ΔVn has been inverted (step S102). This determination can be made, for example, by determining whether the value obtained by multiplying the current ratio (ΔPn / ΔVn) by the previous ratio (ΔPn / ΔVn-1) obtained by dividing the previous power generation power Pn-1 acquired by the control unit 95 in step S101 by the previous terminal voltage Vn-1 is less than zero. If the multiplication value (ΔPn / ΔVn)×(ΔPn-1 / ΔVn-1) is less than 0 (<0) (Yes in step S102), the control unit 95 updates the maximum power point voltage VMPP_n of the solar battery 13 to the current terminal voltage Vn of the low voltage battery 3 (step S103). If the multiplication value (ΔPn / ΔVn)×(ΔPn-1 / ΔVn-1) is 0 or greater (No in step S102), the maximum power point voltage VMPP_n is not updated, and the current value of the maximum power point voltage VMPP_n-1 remains (step S104).

[0017] The maximum power point voltage VMPP_n indicates the optimal voltage value at which a solar cell can maximize its output when generating power. Solar cells have output voltage-current characteristics (VI characteristics) that correspond to the characteristics of the solar cell's equivalent diode. The maximum power point voltage VMPP_n is the voltage at the operating point where the maximum amount of power generated is maximized on these characteristics. The maximum power point voltage VMPP_n can be shown in the solar cell's power generation power-voltage characteristics (PV characteristics) as shown in Figure 3. The PV characteristics of a solar cell change due to, for example, changes in the amount of sunlight irradiated onto the solar cell and changes in the environmental temperature around the solar cell. Even if the PV characteristics of a solar cell change, maximum power point tracking (MPPT), which tracks the solar cell's output voltage to the maximum power point voltage VMPP_n, can maintain high solar cell power generation efficiency. In the "hill climbing method," the voltage is changed in an increasing or decreasing direction in the PV characteristic of Figure 3, and when the output power of the solar cell changes from increasing to decreasing, the direction of the voltage change is reversed, thereby controlling the output voltage of the solar cell to follow the maximum power point.

[0018] In this embodiment, the output voltage of the solar cell 13 is equal to the terminal voltage Vn of the low-voltage battery 3 directly connected to the solar cell 13. The control unit 95 performs the processes of steps S101 to S102 in Fig. 2 to detect, by applying the hill-climbing method, whether the terminal voltage Vn of the low-voltage battery 3 has increased or decreased compared to the maximum power point voltage VMPP_n of the solar cell 13.

[0019] The control unit 95 sets an upper limit voltage VUL_MPP for performing maximum power point tracking control of the solar cell 13 based on the maximum power point voltage VMPP_n set in step S103 or step S104 (step S105). The upper limit voltage VUL_MPP for maximum power point tracking control can be, for example, the maximum power point voltage VMPP_n plus 1 / 3 × ΔV0. ΔV0 is a target value for the difference between the upper limit voltage VUL and the lower limit voltage VLL. The control unit 95 reads an upper limit voltage VUL_BAT for the terminal voltage Vn of the low-voltage battery 3 and an upper limit voltage VUL_ACC for the auxiliary devices in the electronic device 7 (steps S106 and S107). The upper limit voltage VUL_BAT of the low-voltage battery 3 specifies the maximum terminal voltage value of the low-voltage battery 3. The upper limit voltage VUL_BAT is a target value for preventing the low-voltage battery 3 from failing, and can be, for example, the fully charged voltage of the low-voltage battery 3. The upper limit voltage VUL_ACC of the auxiliary equipment defines the maximum operating voltage value of the load. The upper limit voltage VUL_ACC of the auxiliary equipment is a target value for preventing the auxiliary equipment from breaking down, and may be, for example, the rated voltage of the auxiliary equipment. The target value ΔV0, the upper limit voltage VUL_BAT of the low-voltage battery 3, and the upper limit voltage VUL_ACC of the auxiliary equipment are determined in advance and stored in the memory of the controller 9.

[0020] The control unit 95 sets the upper limit voltage VUL from the three upper limit voltages VUL_MPP, VUL_BAT, and VUL_ACC (step S108). For example, the control unit 95 can set the upper limit voltage VUL to the lowest of the three upper limit voltages VUL_MPP, VUL_BAT, and VUL_ACC. The control unit 95 determines whether the terminal voltage Vn acquired by the second acquisition unit 93 is lower than the upper limit voltage VUL (step S109). If the terminal voltage Vn is lower than the upper limit voltage VUL (No in step S109), the process returns to step S101. If the terminal voltage Vn has increased to the upper limit voltage VUL (Yes in step S109), the control unit 95 switches the charging modes of the low-voltage battery 3 and the high-voltage battery 5 from the first charging mode to the second charging mode. By this switching, the control unit 95 transitions the operating mode of the power supply system 1 from the low-voltage battery charging mode to the high-voltage battery charging mode (step S110).

[0021] In the power supply system 1 of the first embodiment, the control unit 95 executes the process shown in FIG. 4 in the high-voltage battery charging mode. In steps S201 to S204, the control unit 95 performs the same process as steps S101 to S104 in FIG. 2 . The control unit 95 sets a lower limit voltage VLL_MPP for maximum power point tracking control of the solar cell 13 based on the maximum power point voltage VMPP_n set in step S203 or step S204 (step S205). The lower limit voltage VLL_MPP for maximum power point tracking control can be, for example, the maximum power point voltage VMPP_n minus two-thirds of the target value ΔV0. The control unit 95 reads a lower limit voltage VLL_BAT of the terminal voltage Vn of the low-voltage battery 3 and a lower limit voltage VLL_ACC of the auxiliary equipment (steps S206 and S207). The lower limit voltage VLL_BAT of the low-voltage battery 3 specifies the lowest terminal voltage value of the low-voltage battery 3. The lower limit voltage VLL_BAT of the low-voltage battery 3 is a target value for preventing breakdown of the low-voltage battery 3, and may be, for example, the discharge end voltage of the low-voltage battery 3. The lower limit voltage VLL_ACC of the auxiliary equipment specifies the minimum operating voltage value of the load. The lower limit voltage VLL_ACC of the auxiliary equipment is a voltage value required for normal operation of the auxiliary equipment, and may be, for example, the minimum operating voltage of the auxiliary equipment. The lower limit voltage VLL_BAT of the low-voltage battery 3 and the lower limit voltage VLL_ACC of the auxiliary equipment are determined in advance and stored in the memory of the controller 9.

[0022] The control unit 95 sets the lower limit voltage VLL from the three lower limit voltages VLL_MPP, VLL_BAT, and VLL_ACC (step S208). The control unit 95 can set the lower limit voltage VLL to, for example, the highest of the three lower limit voltages VLL_MPP, VLL_BAT, and VLL_ACC. The control unit 95 determines whether the terminal voltage Vn acquired by the second acquisition unit 93 is higher than the lower limit voltage VLL (step S209). If the terminal voltage Vn is higher than the lower limit voltage VLL (No in step S209), the process returns to step S201. If the terminal voltage Vn has decreased to the lower limit voltage VLL (Yes in step S209), the control unit 95 switches the charging mode of the low-voltage battery 3 and the high-voltage battery 5 from the second charging mode to the first charging mode. This switching causes the control unit 95 to transition the operating mode of the power supply system 1 from the high-voltage battery charging mode to the low-voltage battery charging mode (step S210).

[0023] In the power supply system 1 of the first embodiment, the maximum power point voltage VMPP_n of the solar cell 13 is updated in response to changes in the terminal voltage Vn of the low-voltage battery 3 in both the low-voltage battery charging mode and the high-voltage battery charging mode. In the low-voltage battery charging mode in which the low-voltage battery 3 is charged, the upper limit voltage VUL of the terminal voltage Vn of the low-voltage battery 3 is updated in response to the update of the maximum power point voltage VMPP_n. As shown in FIG. 5 , when the maximum power point voltage VMPP_n increases, the upper limit voltage VUL is updated to a value higher than that before the increase in the maximum power point voltage VMPP_n. When the maximum power point voltage VMPP_n decreases, the upper limit voltage VUL is updated to a value lower than that before the decrease in the maximum power point voltage VMPP_n. In the high-voltage battery charging mode in which the low-voltage battery 3 is discharged, the lower limit voltage VLL of the terminal voltage Vn of the low-voltage battery 3 is updated in response to the update of the maximum power point voltage VMPP_n. When the maximum power point voltage VMPP_n increases, the lower limit voltage VLL is updated to a value higher than that before the increase in the maximum power point voltage VMPP_n. When the maximum power point voltage VMPP_n decreases, the lower limit voltage VLL is updated to a value lower than that before the decrease in the maximum power point voltage VMPP_n.

[0024] In the power supply system 1 of the first embodiment, when the voltage of the load connected to the solar cell 13 increases above a predetermined voltage value corresponding to the characteristics of the equivalent diode of the solar cell of the solar cell 13, the current flowing from the solar cell 13 to the load drops sharply. By acquiring the power generation power Pn of the solar cell 13 and the terminal voltage Vn of the low-voltage battery 3 directly connected to the solar cell 13, information on the current flowing from the solar cell 13 to the load, including the low-voltage battery 3, can be obtained based on these two values. By monitoring changes in the information on the current flowing from the solar cell 13 to the load, information on the relative relationship between the maximum power point voltage VMPP_n of the solar cell 13 and the voltage of the load can be obtained. By determining the upper limit voltage VUL and the lower limit voltage VLL of the allowable fluctuation range of the terminal voltage Vn based on the power generation power Pn and the terminal voltage Vn, it becomes easier to manage the voltage value of the load of the solar cell 13 via the terminal voltage Vn so that it fluctuates within a range close to the maximum power point voltage VMPP_n. In the power supply system 1 of the first embodiment, even if the maximum power point voltage VMPP_n of the solar cell 13 fluctuates, it is possible to prevent the difference between the voltage value of the load of the solar cell 13 and the maximum power point voltage VMPP_n from widening, which would otherwise cause a decrease in the power generation efficiency of the solar cell 13. By determining the upper limit voltage VUL of the terminal voltage Vn, it is possible to prevent the terminal voltage Vn of the low-voltage battery 3 from increasing to a voltage value that would cause a decrease in the power generation efficiency of the solar cell 13. By determining the lower limit voltage VLL of the terminal voltage Vn, it is possible to prevent the terminal voltage Vn of the low-voltage battery 3 from decreasing to a voltage value that would cause a decrease in the power generation efficiency of the solar cell 13. Because it is not necessary to match the terminal voltage Vn of the low-voltage battery 3 to the maximum power point voltage VMPP_n of the solar cell 13 by power conversion, it is not necessary to add a power conversion device between the solar cell 13 and the low-voltage battery 3.

[0025] In the first embodiment, the lowest value among the minimum voltage VLL_MPP of maximum power point tracking control, the minimum voltage VLL_BAT of the terminal voltage Vn, and the minimum voltage VLL_ACC of the auxiliaries is set as the minimum voltage VLL, thereby preventing unnecessary decreases in the minimum voltage VLL. In the first embodiment, the highest value among the minimum voltage VLL_MPP of maximum power point tracking control, the minimum voltage VLL_BAT of the terminal voltage Vn, and the minimum voltage VLL_ACC of the auxiliaries is set as the maximum voltage VUL, thereby preventing unnecessary increases in the maximum voltage VUL.

[0026] In the first embodiment, when the relative relationship between the maximum power point voltage VMPP_n of the solar cell 13 and the voltage of the load of the solar cell 13 changes, the sign of the ratio (ΔPn / ΔVn), which is the ratio of the change per unit time of the terminal voltage Vn to the power generation power Pn, is reversed. When the sign of the ratio (ΔPn / ΔVn) is reversed, the terminal voltage Vn becomes a voltage value that simulates the maximum power point voltage VMPP_n. By determining the upper limit voltage VUL and the lower limit voltage VLL of the allowable fluctuation range of the terminal voltage Vn based on the power generation power Pn and the terminal voltage Vn when the sign of the ratio (ΔPn / ΔVn) is reversed, the upper limit voltage VUL and the lower limit voltage VLL corresponding to the maximum power point voltage VMPP_n can be determined.

[0027] In the first embodiment, the charging and discharging of the low-voltage battery 3 is controlled so that the terminal voltage Vn of the low-voltage battery 3 remains at a predetermined voltage value within the allowable fluctuation range during charging and discharging. The predetermined voltage value is a voltage value equal to or lower than the upper limit voltage VUL and equal to or higher than the lower limit voltage VLL. By controlling the charging and discharging of the low-voltage battery 3 in this manner, the voltage value of the power generated by the solar cell 13, which is equal to the terminal voltage Vn of the low-voltage battery 3, can be constantly controlled to a voltage value that does not reduce the power generation efficiency of the solar cell 13. In the first embodiment, when the terminal voltage Vn reaches the upper limit voltage VUL or the lower limit voltage VLL, the low-voltage battery 3 is switched from one of the charging state and the discharging state to the other. This allows the voltage value of the power generated by the solar cell 13 to be controlled to a voltage value that does not reduce the power generation efficiency of the solar cell 13. In the first embodiment, in the first charging mode of the low-voltage battery 3 and the high-voltage battery 5, the terminal voltage Vn of the low-voltage battery 3 increases due to charging using power from the solar cell 13. In the second charging mode, the terminal voltage Vn of the low-voltage battery 3 decreases due to the discharge of the power supplied to charge the high-voltage battery 5. By using the upper limit voltage VUL and the lower limit voltage VLL as thresholds for switching between the first charging mode and the second charging mode, the voltage value of the power generation power Pn of the solar cell 13 can be controlled to fall within a voltage value that does not result in a decrease in the power generation efficiency of the solar cell when switching between charging modes.

[0028] Second Embodiment In a second embodiment, the control unit 95 executes processing according to the procedure shown in FIG. 6 in the low-voltage battery charging mode. In step S301, the control unit 95 performs the same processing as step S101 in FIG. 2. In step S311 following step S301, the control unit 95 determines whether the ratio (ΔPn / ΔVn) between the generated power Pn and the terminal voltage Vn obtained in step S301 has changed suddenly. Here, a sudden change includes both a sudden change in quantity and a sudden change in speed. This determination can be made, for example, using the difference dP / dV between the previous ratio (ΔPn-1 / ΔVn-1) and the current ratio (ΔPn / ΔVn), which indicates the amount of change in the ratio (ΔPn / ΔVn), and a first predetermined value used as an index for determining whether the ratio (ΔPn / ΔVn) has changed suddenly. In this case, if the absolute value of the difference dP / dV is equal to or less than a first predetermined value, it is determined that the ratio (ΔPn / ΔVn) has not changed suddenly. If the absolute value of the difference dP / dV exceeds the first predetermined value, it is determined that the ratio (ΔPn / ΔVn) has changed suddenly. The determination in step S311 may be performed using a derivative (dP / dV)dt of the absolute value of the difference dP / dV, which indicates the rate of change of the ratio (ΔPn / ΔVn), and a second predetermined value, separate from the first predetermined value, that serves as an index for determining whether the rate of change of the ratio (ΔPn / ΔVn) is sudden. In this case, if the derivative (dP / dV)dt is equal to or less than the second predetermined value, it is determined that the ratio (ΔPn / ΔVn) has not changed suddenly. If the derivative (dP / dV)dt exceeds the second predetermined value, it is determined that the ratio (ΔPn / ΔVn) has changed suddenly. The first and second predetermined values ​​are determined in advance and stored in the memory of the controller 9.

[0029] The determination in step S311 is performed to determine whether the power generation efficiency of the solar cell 13 has suddenly changed. The power generation efficiency of the solar cell 13 fluctuates due to changes in the environment in which the solar cell 13 is used. Examples of the environment in which the solar cell 13 is used include the amount of sunlight irradiated onto the solar cell 13 and the ambient temperature of the solar cell 13. Changes in the environment that cause a sudden change in the power generation efficiency of the solar cell 13 may be temporary. When the power generation efficiency of the solar cell 13 suddenly changes, the terminal voltage Vn of the low-voltage battery 3 may not represent the correct value of the maximum power point voltage VMPP_n of the solar cell 13. If the maximum power point voltage VMPP_n is updated to an incorrect value based on the terminal voltage Vn when the power generation efficiency of the solar cell 13 suddenly changes, the upper limit voltage VUL and the lower limit voltage VLL are determined to be incorrect values ​​based on the maximum power point voltage VMPP_n. If the ratio value (ΔPn / ΔVn) has not changed suddenly (Yes in step S311), the control unit 95 proceeds to step S302. If the ratio value (ΔPn / ΔVn) has changed suddenly (No in step S311), the control unit 95 proceeds to step S304. In step S302, the control unit 95 performs the same processing as step S102 in FIG. 2. In step S302, if the multiplication value (ΔPn / ΔVn) × (ΔPn-1 / ΔVn-1) is less than 0 (<0) (Yes), the control unit 95 proceeds to step S303. If the multiplication value is equal to or greater than 0 (No), the control unit 95 proceeds to step S304. In steps S303 and S304, the control unit 95 performs the same processing as steps S103 and S104 in FIG. 2. After steps S303 and S304, the control unit 95 performs processing not shown in FIG. 6. The processes not shown in the figure are the same as steps S105 to S110 in Fig. 2. In step S105, the upper limit voltage VUL_MPP is set based on the maximum power point voltage VMPP_n set in step S303 or step S304. In step S109, if the terminal voltage Vn is lower than the upper limit voltage VUL (No), the process returns to step S301.

[0030] In the second embodiment, the control unit 95 executes processing according to the procedure shown in Fig. 7 in the high-voltage battery charging mode. In step S401, the control unit 95 performs the same processing as step S401 in Fig. 4. In step S411 following step S401, the control unit 95 performs the same processing as step S311 in Fig. 6. If the ratio value (ΔPn / ΔVn) has not changed suddenly (Yes in step S411), the control unit 95 proceeds to processing of step S402, and if the ratio value (ΔPn / ΔVn) has changed suddenly (No in step S411), the control unit 95 proceeds to processing of step S404. In step S402, the control unit 95 performs the same processing as step S202 in Fig. 4. In step S402, if the multiplication value (ΔPn / ΔVn)×(ΔPn-1 / ΔVn-1) is less than 0 (<0) (Yes), the process proceeds to step S403. If the multiplication value is equal to or greater than 0 (No), the process proceeds to step S404. In steps S403 and S404, the control unit 95 performs the same processes as steps S203 and S204 in FIG. 4. After steps S403 and S404, the control unit 95 performs processes not shown in FIG. 7. The processes not shown are the same as steps S205 to S210 in FIG. 4. In step S205, the lower limit voltage VLL_MPP is set based on the maximum power point voltage VMPP_n set in step S403 or step S404. In step S209, if the terminal voltage Vn is higher than the lower limit voltage VLL_MPP (No), the process returns to step S401.

[0031] The second embodiment provides the same functions and effects as the first embodiment. In the second embodiment, if the absolute value of the difference dP / dV exceeds a first predetermined value or if the differential value (dP / dV)dt of the absolute value of the difference dP / dV exceeds a second predetermined value, it is determined that the ratio (ΔPn / ΔVn) has suddenly changed. If it is determined that the ratio (ΔPn / ΔVn) has suddenly changed, the maximum power point voltage VMPP_n is not updated to the terminal voltage Vn at the time when the sign of the ratio (ΔPn / ΔVn) was reversed, even if the sign of the ratio (ΔPn / ΔVn) was reversed. This prevents the upper limit voltage VUL and lower limit voltage VLL corresponding to the maximum power point voltage VMPP_n of the solar cell 13 from being determined to be erroneous values ​​based on the terminal voltage Vn of the low-voltage battery 3 at the time when the sign of the ratio (ΔPn / ΔVn) was reversed.

[0032] In the power supply system 1 of each embodiment, the controller 9 controls the upper limit voltage VUL_MPP and the lower limit voltage VLL_MPP of the terminal voltage Vn of the low-voltage battery 3 to follow the maximum power point voltage VMPP_n of the solar cell 13. This control causes the allowable fluctuation range of the terminal voltage value Vn due to charging and discharging of the low-voltage battery 3 to follow changes in the maximum power point voltage VMPP_n of the solar cell 13. By causing the allowable fluctuation range of the terminal voltage value Vn to follow changes in the maximum power point voltage VMPP_n, maximum power point tracking control is essentially performed, in which the output voltage of the solar cell 13 to which the low-voltage battery 3 is directly connected follows the maximum power point voltage VMPP_n of the solar cell 13. This maximum power point tracking control can be performed by the controller 9 controlling the on / off of the boost operation of the existing bidirectional converter 15 that converts power between the low-voltage battery 3 and the high-voltage battery 5. To perform this maximum power point tracking control, for example, it is not necessary to newly provide a dedicated power conversion device between the solar cell 13 and the low-voltage battery 3 and convert the output voltage of the solar cell 13, which has been made to track the maximum power point voltage VMPP_n, into the terminal voltage Vn of the low-voltage battery 3. The control method of each embodiment can be realized by utilizing an existing configuration without adding a new power conversion device dedicated to the solar cell 13, thereby suppressing the cost burden.

[0033] In the power supply system 1 of each embodiment, in the low-voltage battery charging mode, the low-voltage battery 3 is charged with power generated by the solar cell 13. This prevents the state of charge of the low-voltage battery 3 from excessively decreasing, even when the high-voltage battery 5 is running low on power, for example. In the low-voltage battery charging mode, the power generated by the solar cell 13 is supplied not only to the low-voltage battery 3 but also to low-voltage loads such as electronic devices 7. This reduces power consumption from the low-voltage battery 3 by the loads, improving the operational efficiency of the power supply system 1. In the low-voltage battery charging mode, the step-up operation of the bidirectional converter 15 is turned off and the step-down operation of the bidirectional converter 15 is not performed, thereby stopping power consumption by the high-voltage loads supplied with power from the high-voltage battery 5 of the electric vehicle. In the power supply system 1 of each embodiment, in the low-voltage battery charging mode, the power generated by the solar cell 13 is temporarily stored in the low-voltage battery 3. The power converted by the bidirectional converter 15 and supplied to the high-voltage battery 5 is supplied from the low-voltage battery 3 to the bidirectional converter 15 in the high-voltage battery charging mode. The low-voltage battery 3 can supply a large current, for example, a large power of 500 W level, to the bidirectional converter 15 compared to the solar cell 13, so the bidirectional converter 15 can perform power conversion with high efficiency.

[0034] The control method for the power supply system 1 according to the above embodiment may be performed, for example, while the electric vehicle is stopped or parked. The power supply system to be controlled by the control method or control device according to the present invention is not limited to a power supply system mounted on an electric vehicle. The present invention is widely applicable to power supply systems that charge a first power storage device connected to a solar cell with power from the solar cell, and charge a second power storage device with power obtained by converting the power of the first power storage device.

[0035] The above-described embodiment is merely an example of the present invention, and therefore the present invention is not limited to the above-described embodiment, and various modifications can be made to the design and other aspects of the present invention without departing from the technical concept of the present invention.

[0036] REFERENCE SIGNS LIST 1 Power supply system 3 Low-voltage battery (first power storage device) 5 High-voltage battery (second power storage device) 7 Electronic device (load) 9 Controller (control device) 13 Solar cell 15 Bidirectional DC-DC converter (power conversion unit) Pn Generated power (generated power value) VLL Lower limit voltage (limit value, lower limit value) VMPP_n Maximum power point voltage Vn Terminal voltage (terminal voltage value) VUL Upper limit voltage (limit value, upper limit value) VUL_BAT Upper limit voltage of low-voltage battery (maximum terminal voltage value of first power storage device) VUL_ACC Upper limit voltage of auxiliary equipment (maximum operating voltage value of load) ΔPn Change (amount of change in generated power value per unit time) ΔPn / ΔVn Ratio value

Claims

1. A control method for a power supply system that charges a first storage device connected to a solar cell with power from the solar cell and charges a second storage device with power obtained by converting the power of the first storage device, the control method comprising the steps of: acquiring a power generation value of the solar cell and a terminal voltage value of the first storage device; and determining, based on the acquired power generation value and terminal voltage value, a limit value of the allowable fluctuation range of the terminal voltage value due to charging and discharging of the first storage device, so that the allowable fluctuation range of the terminal voltage value due to charging and discharging of the first storage device follows the maximum power point voltage of the solar cell.

2. The method for controlling a power supply system according to claim 1, wherein the limit value defines the lower limit value of the allowable fluctuation range.

3. A control method for a power supply system as described in claim 2, wherein a load to which power is supplied from the first storage device is connected, and when at least one of the minimum operating voltage value of the load and the preset minimum terminal voltage value of the first storage device is higher than the lower limit value, the lower limit value is changed to the higher voltage value of the minimum operating voltage value and the minimum terminal voltage value.

4. The method for controlling a power supply system according to claim 1, wherein the limit value defines an upper limit of the allowable fluctuation range.

5. A control method for a power supply system as described in claim 4, wherein a load to which power is supplied from the first storage device is connected, and when at least one of the maximum operating voltage value of the load and the preset maximum terminal voltage value of the first storage device is lower than the upper limit value, the upper limit value is changed to the lower voltage value of the maximum operating voltage value and the maximum terminal voltage value.

6. A control method for a power supply system according to any one of claims 1 to 5, wherein the acquired terminal voltage value when the sign of the ratio obtained by dividing the amount of change per unit time of the acquired generated power value by the amount of change per unit time of the acquired terminal voltage value is inverted is set as the maximum power point voltage value of the solar cell, and the limit value corresponding to the maximum power point voltage value is determined based on the acquired generated power value and terminal voltage value.

7. A control method for a power supply system as described in claim 6, wherein when the ratio value is greater than a first predetermined value that is used as an indicator for determining a sudden change in the ratio value, the acquired terminal voltage value when the sign of the ratio value is reversed is not used as the maximum power point voltage value.

8. A control method for a power supply system as described in claim 6 or 7, wherein when the differential value of the ratio value is greater than a second predetermined value used as an indicator for determining a sudden change in the ratio value, the acquired terminal voltage value when the sign of the ratio value is reversed is not used as the maximum power point voltage value of the solar cell.

9. A control method for a power supply system according to any one of claims 1 to 8, wherein charging and discharging of the first storage device are controlled so that the acquired terminal voltage value becomes a predetermined voltage value within the allowable fluctuation range when the first storage device is being charged with power from the solar cell and when the first storage device is being discharged to which the power to be converted is supplied.

10. A control method for a power supply system according to any one of claims 1 to 9, wherein when the acquired terminal voltage value reaches the limit value, the charge / discharge state of the first storage device is switched from one state to the other, between a charge state in which the first storage device is charged by power from the solar cell and a discharge state in which the first storage device supplies the power to be converted.

11. A control method for a power supply system according to any one of claims 1 to 10, wherein the limit value is a threshold value for switching between a first charging mode in which the first storage device is charged with power from the solar cell and a second charging mode in which the second storage device is charged with power obtained by converting the power of the first storage device, based on the acquired terminal voltage value.

12. A control device for a power supply system that charges a first storage device connected to a solar cell with power from the solar cell, and charges a second storage device with power after conversion from the power of the first storage device by a power conversion unit, comprising: a first acquisition unit that acquires a power generation value of the solar cell; a second acquisition unit that acquires a terminal voltage value of the first storage device; and a control unit that controls the power conversion operation of the power conversion unit based on the terminal voltage value acquired by the second acquisition unit, wherein the control unit determines a limit value of the allowable fluctuation range of the terminal voltage value due to charging and discharging of the first storage device, based on the generated power value and the terminal voltage value acquired by the first acquisition unit and the second acquisition unit, so that the allowable fluctuation range of the terminal voltage value due to charging and discharging of the first storage device follows the maximum power point voltage of the solar cell.

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