JP2015038882A - Fuel battery system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To attain a good system efficiency while performing voltage control well.SOLUTION: A fuel battery system includes a fuel battery connected parallel to a motor and a power storage device, a first voltage transformer between the motor and the fuel battery, a second voltage transformer between the motor and the power storage device, a load circuit between the first and second voltage transformers and the motor, and a control part. Assuming that a terminal voltage of a fuel battery is V, a terminal voltage of the power storage device is V, and a load circuit input lower limit voltage which is an input terminal voltage of a load circuit that is set so that a system loss is minimum while a torque required for a motor is assured is V, the control part, in a case where conditions of V>Vand V>V+α are established, stops a step-up operation by the first voltage transformer while prohibits stopping of the step-up operation by the first voltage transformer in a case where the conditions are not met.

Description

  The present invention relates to a fuel cell system that supplies electric power output from a fuel cell to a motor.

  As this type of system, a fuel cell and a power storage device (secondary battery or the like) are connected in parallel to a motor, and a boost type DC-DC converter (hereinafter referred to as “FC boost converter”) is provided between the motor and the fuel cell. .) And a step-up DC-DC converter (hereinafter referred to as a “battery step-up converter”) between the motor and the power storage device are known (for example, Japanese Patent Application Laid-Open No. 2007-318938). No. publication etc.). Further, in such a system, there is known a configuration in which the boosting operation in the FC boost converter or the battery boost converter is stopped depending on the situation (for example, JP 2007-209161 A, JP 2010-45889 A). JP, 2010-124689, etc.).

  In the conventional system as described above, if the step-up operation in each of the above-described converters is stopped carelessly, the voltage control in the system may be broken, and in some cases, overcurrent or overvoltage may be generated. there were. The present invention has been made to solve such problems.

  The fuel cell system of the present invention includes a motor, a fuel cell, a power storage device, a first voltage converter, a second voltage converter, a load circuit, and a control unit. The electric power output from the power storage device is supplied to the motor via the load circuit (see, for example, Japanese Patent Application Laid-Open No. 2009-165244, which is a prior application of the present applicant). The fuel cell may be referred to as a “fuel cell stack”.

  The fuel cell and the power storage device are connected in parallel with the motor. The first voltage converter is a voltage converter (step-up DC-DC converter) provided between the motor and the fuel cell, and boosts and outputs the input terminal voltage of the fuel cell. It is supposed to be. Similarly, the second voltage converter is a voltage converter (step-up DC-DC converter) provided between the motor and the power storage device, and boosts the input terminal voltage of the power storage device. And output it. The load circuit is provided between the first voltage converter, the second voltage converter, and the motor. The control unit is provided to control the operation of each unit in the fuel cell system.

A feature of the present invention is that the control unit operates as follows:
V _FC , the terminal voltage of the fuel cell
V _BAT , the terminal voltage of the power storage device,
The load circuit input lower limit voltage, which is the input terminal voltage of the load circuit, set to minimize the loss in the fuel cell system while ensuring the necessary torque in the motor, is V _{M_min} ,
Condition (1): V _FC > V _{M_min}
Condition (2): V _FC > V _BAT + α (α is a constant whose sign is positive)
Then,
When the condition (1) is satisfied and the condition (2) is satisfied, the boost operation by the first voltage converter is stopped and the output of the fuel cell is directly transmitted to the load circuit. ,
When at least one of the condition (1) and the condition (2) is not satisfied, the boost operation by the first voltage converter is prohibited.

  When the condition (1) is satisfied, the terminal voltage of the fuel cell is such that the load circuit input lower limit voltage (a loss in the fuel cell system is minimized while ensuring a necessary torque in the motor). Higher than the set input terminal voltage of the load circuit. Therefore, in this case, it is possible to directly transmit the output of the fuel cell to the input terminal of the load circuit without increasing the terminal voltage of the fuel cell.

Here, “V _BAT + α” in the condition (2) is specifically capable of voltage control by the boost-type second voltage converter when the terminal voltage of the power storage device is V _BAT . The lower limit voltage (“minimum voltage that can be boosted” in Japanese Patent Application Laid-Open No. 2010-27396 related to the prior application of the present applicant). That is, when the condition (2) is satisfied, the voltage on the output side of the second voltage converter can be stably boosted with respect to the voltage on the input side of the second voltage converter. ing. Therefore, when the condition (2) is satisfied, even if the boosting operation by the first voltage converter is stopped (that is, the input terminal voltage of the load circuit is voltage controlled by the boosting first voltage converter). The input terminal voltage of the load circuit can be subjected to stable voltage control by the boost-type second voltage converter (even in a state where the voltage is not received).

  Therefore, in the fuel cell system of the present invention, when the condition (1) is satisfied and the condition (2) is satisfied, the boosting operation by the first voltage converter is stopped. Then, the input terminal of the load circuit is electrically connected directly to the fuel cell. Further, the output of the fuel cell can be directly transmitted to the input terminal of the load circuit without undergoing a switching operation in the first voltage converter.

  Therefore, according to the fuel cell system of the present invention having the above-described configuration, the switching loss in the first voltage converter is reduced, thereby improving the system efficiency. Further, the input terminal voltage of the load circuit and the terminal voltage in the fuel cell in a state of being directly connected to the input terminal can be stably controlled by the second voltage converter.

  On the other hand, if the condition (1) is not satisfied, it is necessary to boost the terminal voltage of the fuel cell. If the condition (1) is satisfied but the condition (2) is not satisfied, when the first voltage converter is stopped, the voltage on the input side of the second voltage converter is The voltage on the output side of the second voltage converter does not have a relationship that enables stable boost control. For this reason, if the first voltage converter is stopped when the condition (2) is not satisfied, the input terminal voltage of the load circuit and the fuel in a state of being directly connected to the input terminal The terminal voltage in the battery cannot be subjected to stable voltage control.

  Therefore, in the fuel cell system of the present invention, when at least one of the condition (1) and the condition (2) is not satisfied, the boosting operation by the first voltage converter is prohibited. At this time, the input terminal voltage of the load circuit is subjected to voltage control by the operation of the first voltage converter (and the second voltage converter).

  Thus, according to the fuel cell system of the present invention having such a configuration, the input terminal voltage of the load circuit and the terminal voltage of the fuel cell are well controlled and good system efficiency is achieved. Achieved.

FIG. 1 is a diagram showing a schematic configuration of an embodiment of a fuel cell system of the present invention and a vehicle equipped with the same. FIG. 2 is a diagram showing an electric circuit configuration of the FC boost converter shown in FIG. FIG. 3 is a diagram showing an outline of a map for obtaining the load circuit input lower limit voltage _{VM_min} stored in the ROM in the control unit shown in FIG. FIG. 4 is a reference diagram for explaining a map derivation process for obtaining the load circuit input lower limit voltage _{VM_min} shown in FIG. 3. FIG. 5 is a reference diagram for explaining a map derivation process for obtaining the load circuit input lower limit voltage _{VM_min} shown in FIG. 3. FIG. 6 is a flowchart showing a specific example of the operation control process of the FC boost converter, which is executed by the CPU in the control unit shown in FIG.

  Embodiments of the present invention will be described below with reference to the drawings. In addition, the description about the following embodiment is specific to the extent possible, merely an example of the embodiment of the present invention in order to satisfy the description requirement (description requirement / practicability requirement) of the specification required by law. It is only what is described in. Therefore, as will be described later, it is quite natural that the present invention is not limited to the specific configurations of the embodiments described below. Various modifications (modifications) that can be made to the present embodiment are inserted in the description of the embodiment, so that understanding of the consistent description of the embodiment is hindered. Has been.

[System configuration]
FIG. 1 is a diagram showing a schematic configuration of an embodiment of a fuel cell system of the present invention and a vehicle WV equipped with the same. A fuel cell system 10 according to the present embodiment is mounted on the vehicle WV. The fuel cell system 10 of this embodiment includes a fuel cell 11, an FC boost converter 12, a battery 13, a battery boost converter 14, an inverter 15, a motor 16, a hydrogen tank 17, a compressor 18, and a control unit. 20. The fuel cell system 10 drives the drive wheels W by supplying power output from the fuel cell 11 and / or the battery 13 to the motor 16 via the inverter 15.

  The fuel cell 11 is electrically connected to the inverter 15 via the FC boost converter 12. Further, the battery 13 is electrically connected to the inverter 15 via the battery boost converter 14. The inverter 15 corresponding to the load circuit of the present invention is provided between the FC boost converter 12 and the battery boost converter 14 and the motor 16. That is, the fuel cell 11 and the battery 13 are connected in parallel with the inverter 15 and the motor 16.

  The FC boost converter 12 corresponding to the first voltage converter of the present invention is a boost DC-DC converter and is configured to boost the input terminal voltage of the fuel cell 11 and output it to the inverter 15 side. Yes. FIG. 2 is a diagram showing an electric circuit configuration of the FC boost converter 12 shown in FIG. In FIG. 2, it is assumed that illustration of the battery 13 and the battery boost converter 14 is omitted for simplification of illustration.

  As shown in FIG. 2, the FC boost converter 12 can change the boost ratio by adjusting the switching duty ratio in the switching element S1 at the time of the boost operation, and the coil L1 and the diode when the boost operation is stopped. The terminal voltage of the fuel cell 11 can be directly transmitted to the inverter 15 via D5 (in addition, the electric circuit configuration shown in FIG. It is easy to understand how this operates, but if necessary, see JP 2009-165244 A).

  Referring to FIG. 1 again, the battery boost converter 14 corresponding to the second voltage converter of the present invention is a boost DC-DC converter, and boosts the input terminal voltage of the battery 13 to the inverter 15 side. It is configured to allow output.

  The control unit (ECU) 20 is a so-called microcomputer for controlling the operation of each unit in the fuel cell system 10, and includes a CPU, a ROM, a RAM, a backup RAM, an interface, and a connection between them. And a bus. In the ROM, a routine (program) executed by the CPU and a table (lookup table, map) referred to when the routine is executed are stored in advance.

  The control unit 20 is connected to the sensor (not shown) (current sensor, voltage sensor, etc.) provided in the fuel cell 11 and the battery 13 via the above-mentioned interface, the rotation speed sensor (not shown) provided in the motor 16, It is electrically connected to various sensors such as an accelerator pedal sensor 21. The control unit 20 is electrically connected to the FC boost converter 12, the battery boost converter 14, the inverter 15, and the motor 16 via the above-described interface, and controls these operations. . Furthermore, the control unit 20 is electrically connected to a fuel gas supply system (an on-off valve or the like not shown in the hydrogen tank 17) and an oxidizing gas supply system (such as the compressor 18), and controls these operations. It is like that.

[Overview of system operation]
The control unit 20 calculates the required power of the fuel cell system 10 as a whole (the total value of the vehicle running power and the auxiliary machine power) based on the motor speed, the accelerator pedal operation amount detected by the accelerator pedal sensor 21, and the like. At the same time, the distribution of the output power of the fuel cell 11 and the battery 13 is determined. The control unit 20 controls the fuel gas supply system and the oxidizing gas supply system so that the power generation amount of the fuel cell 11 matches the target power, and controls the FC boost converter 12 and the battery boost converter 14 to control the fuel cell. 11 operation points (terminal voltage and output current) are controlled. Furthermore, the control unit 20 controls the output torque and the rotation speed of the motor 16 via the inverter 15 so that a target torque corresponding to the accelerator pedal operation amount or the like can be obtained.

  In the fuel cell system 10 of the present embodiment, since the FC boost converter 12 is provided between the fuel cell 11 and the inverter 15, the terminal voltage of the fuel cell 11 is boosted and supplied to the inverter 15 (motor 16). Is possible. Therefore, the fuel cell 11 can be downsized by reducing the number of stacked cells. As a result, the weight of the vehicle WV is reduced, and thus the energy efficiency of the vehicle WV is further improved.

  By the way, in the fuel cell system 10, the fuel cell 11 that can generate power is the main power source of the motor 16. Therefore, it is considered that reducing the power loss in the FC boost converter 12 interposed between the fuel cell 11 and the inverter 15 greatly contributes to improving the efficiency of the entire fuel cell system 10. Therefore, in the fuel cell system 10 of the present embodiment, the switching loss in the FC boost converter 12 is suppressed as much as possible by performing intermittent operation control of the FC boost converter 12. This improves system efficiency.

Here, the condition for stopping the boost operation of the FC boost converter 12 in the intermittent operation control of the FC boost converter 12 is that both of the following conditions (1) and (2) are satisfied. On the other hand, when at least one of the condition (1) and the condition (2) is not satisfied, the stop of the boost operation by the FC boost converter 12 is prohibited (that is, the boost operation in the FC boost converter 12 is maintained). .)
Condition (1): V _FC > V _{M_min}
Condition (2): V _FC > V _BAT + α

In the above conditions, V _FC is a terminal voltage of the fuel cell 11, V _BAT is a terminal voltage of the battery 13, _{VM_min} is a load circuit input lower limit voltage, and α is a constant whose sign is positive. The load circuit input lower limit voltage _{VM_min} is an input terminal voltage of the inverter 15 that is set so as to minimize the loss in the fuel cell system 10 while securing a necessary torque in the motor 16.

FIG. 3 is a diagram showing an outline of a map for obtaining the load circuit input lower limit voltage _{VM_min} stored in the ROM in the control unit 20 shown in FIG. In FIG. 3, the horizontal axis indicates the motor rotational speed N, the vertical axis indicates the motor torque T, and the dotted line on the upper right side indicates the maximum torque line that can be generated in the motor 16. As shown in FIG. 3, the load circuit input lower limit voltage _{VM_min} is acquired from a map as shown in FIG. 3 based on the motor rotation speed N and the motor torque T.

4 and 5 are reference diagrams for explaining a map derivation process for obtaining the load circuit input lower limit voltage _{VM_min} shown in FIG. 3. The map as shown in FIG. 3 shows the upper limit torque that can be generated by the motor 16 according to the inverter voltage VM that is the input terminal voltage of the inverter 15 and the motor rotational speed N (see FIG. Is the same as the maximum torque line shown in FIG. 3) and the loss characteristics of the fuel cell system 10 at each operation point of the fuel cell 11.

  Here, the switching loss in the FC boost converter 12, the battery boost converter 14 and the inverter 15 is smaller as the applied voltage is lower. Moreover, the iron loss (hysteresis loss and eddy current loss) in the motor 16 is also smaller as the applied voltage is lower. That is, from the viewpoint of switching loss and iron loss, the lower the inverter voltage VM, the smaller the loss.

  On the other hand, FIG. 5 is a diagram showing the most efficient operating point at the value of each inverter voltage VM by contour lines. As is apparent from FIG. 5, when the inverter voltage VM is lowered, conversely, an increase in loss (for example, copper loss) due to an increase in current may occur. Further, there is a restriction of continuous rated current that is determined by each circuit element such as a relay provided in the FC boost converter 12.

Therefore, conceptually, the load circuit input lower limit voltage _{VM_min} is obtained as _follows . First, from the current motor rotation speed N and required torque T, the map shown in FIG. 4 and the above-described continuous rated current, the minimum inverter voltage VM1 (required torque T) required for driving the motor 16 is determined. And the inverter voltage VM) that can be minimized as long as the current does not exceed the continuous rated value is determined. Then, the load circuit input lower limit voltage _{VM_min} is calculated on the basis of the inverter voltage VM1 and the loss characteristic at each operation point (FIG. 5 and the like). Specifically, for example, when the system efficiency is improved when the voltage is increased by a minute amount ΔVM from the inverter voltage VM1 determined as described above, a new value obtained by adding the ΔVM is used as a new inverter. By setting the voltage VM1 and repeating this, the load circuit input lower limit voltage _{VM_min} is _finally obtained. That is, it can be said that the map of FIG. 3 is obtained by modifying the map of FIG. 4 with the map of FIG.

[Example of operation]
FIG. 6 is a flowchart showing a specific example of the operation control process of FC boost converter 12 executed by a CPU (hereinafter simply referred to as “CPU”) in control unit 20 shown in FIG. . In the flowchart of FIG. 6, “step” is abbreviated as “S”.

The CPU executes the FC boost converter control routine 600 shown in FIG. 6 every predetermined time. When execution of the routine 600 is started, first, in step 610, the CPU calculates the motor torque T based on the current motor rotation speed N, the accelerator pedal operation amount, and the like. Next, in step 620, the CPU obtains the load circuit input lower limit voltage _{VM_min} based on the motor rotational speed N and the motor torque T as described above. Subsequently, the CPU acquires the fuel cell terminal voltage V _FC in step 630 and acquires the battery terminal voltage V _BAT in step 640.

  Thereafter, in step 650, the CPU determines whether or not the above condition (1) is satisfied. When this condition (1) is satisfied (step 650 = Yes), the process proceeds to step 660, and the CPU determines whether or not the above-described condition (2) is satisfied. When this condition (2) is satisfied (step 660 = Yes), the process proceeds to step 670, the CPU stops the switching operation in the FC boost converter 12, and once ends this routine. On the other hand, if at least one of the condition (1) and the condition (2) is not satisfied, the process proceeds to step 680, and the CPU prohibits the stop of the switching operation in the FC boost converter 12 and temporarily executes this routine. finish.

[Action / Effect]
When the condition (1) is satisfied, that is, when the fuel cell terminal voltage V _FC is higher than the load circuit input lower limit voltage _{VM_min} , the boost operation by the FC boost converter 12 is stopped (that is, the terminal of the fuel cell 11). It is possible to transmit the output of the fuel cell 11 directly to the input terminal of the inverter 15 without increasing the voltage.

  Further, when the condition (2) is satisfied, the voltage on the output side of the battery boost converter 14 is in a relationship that enables stable boost control with respect to the voltage on the input side of the battery boost converter 14. Therefore, when the condition (2) is satisfied, even if the boost operation by the FC boost converter 12 is stopped (that is, the input terminal voltage of the inverter 15 is not under voltage control by the boost FC boost converter 12). However, the input terminal voltage of the inverter 15 can be subjected to stable voltage control by the boost type battery boost converter 14.

  Therefore, in the fuel cell system 10 of the present embodiment, when the condition (1) is satisfied and the condition (2) is satisfied, the boosting operation by the FC boost converter 12 is stopped, and the output of the fuel cell 11 is directly Is transmitted to the inverter 15. As a result, the switching loss that should originally occur in the FC boost converter 12 is reduced, thereby improving the system efficiency. Further, the input voltage of the inverter 15 and the terminal voltage of the fuel cell 11 in a state of being directly connected to the input terminal can be stably controlled by the battery boost converter 14.

  On the other hand, when the condition (1) is not satisfied, the terminal voltage of the fuel cell 11 needs to be boosted. If condition (2) is not satisfied even if condition (1) is satisfied, the voltage on the output side of battery boost converter 14 is stably boosted with respect to the voltage on the input side of battery boost converter 14. There is no controllable relationship. For this reason, if the FC converter 12 is stopped when the condition (2) is not satisfied, the input terminal voltage of the inverter 15 and the terminal voltage in the fuel cell 11 in a state of being directly connected to the input terminal are: Stable voltage control cannot be received.

  Therefore, in the fuel cell system 10 of the present embodiment, the stop of the boosting operation by the FC boost converter 12 is prohibited when at least one of the condition (1) and the condition (2) is not satisfied. Therefore, in this case, the input terminal voltage of the inverter 15 is subjected to voltage control by the operation of the FC boost converter 12 (and the battery boost converter 14).

  Thus, according to the fuel cell system 10 of the present embodiment, the input terminal voltage of the inverter 15 and the terminal voltage of the fuel cell 11 are well controlled, and good system efficiency is achieved. In addition, by using a step-up DC-DC converter as a voltage converter connected to the fuel cell 11 or the battery 13, the number of switching elements inside the device can be reduced as much as possible. Simplification and further improvement in system efficiency are achieved.

[Exemplary list of modifications]
Note that, as described above, the above-described embodiments are merely examples of typical embodiments of the present invention that the applicant has considered to be the best at the time of filing of the present application. Therefore, the present invention is not limited to the above-described embodiment. Therefore, it goes without saying that various modifications can be made to the above-described embodiment within the scope not changing the essential part of the present invention.

  Hereinafter, some typical modifications will be exemplified. Needless to say, the modifications are not limited to those listed below. In addition, a plurality of modified examples can be applied in a composite manner as appropriate within a technically consistent range.

  The present invention (especially those expressed functionally and functionally in the constituent elements constituting the means for solving the problems of the present invention) is based on the above-described embodiment and the description of the following modifications. Should not be interpreted as limited. Such a limited interpretation is unacceptable and improper for imitators, while improperly harming the applicant's interests (rushing to file under a prior application principle).

  The present invention is not limited to the specific apparatus configuration disclosed in the above embodiment. For example, the present invention is not limited to a vehicle equipped with a fuel cell system. Further, the “power storage device” of the present invention is not limited to a battery. Specifically, for example, a capacitor or the like can be used as the “power storage device”. Furthermore, the present invention can be suitably applied to a configuration in which a plurality of fuel cell stacks and a plurality of power storage devices are provided.

  The present invention is not limited to the specific processing disclosed in the above embodiment. For example, the parameter acquired by the map can be obtained by calculation.

  In order to satisfactorily control the operating point (particularly the terminal voltage) of the fuel cell 11, it is preferable that the battery boost converter 14 is always operating when an output is generated in the fuel cell 11. Thereby, the occurrence of problems such as sintering due to the terminal voltage of the fuel cell 11 rising to the vicinity of the open terminal voltage is avoided as much as possible. From this point of view, when output is generated in the fuel cell 11, the fuel cell system 10 is controlled so that the input terminal voltage of the inverter 15 does not become lower than the lower limit voltage capable of voltage control by the battery boost converter 14. It is preferred that

  Other modifications not specifically mentioned are naturally included in the scope of the present invention as long as they do not change the essential part of the present invention. In addition, in each element constituting the means for solving the problems of the present invention, elements expressed functionally and functionally include the specific structures disclosed in the above-described embodiments and modifications, It includes any structure that can realize this action / function. Furthermore, the contents (including the specification and drawings) of each publication cited in the present specification may be incorporated as part of the specification.

DESCRIPTION OF SYMBOLS 10 ... Fuel cell system WV ... Vehicle W ... Drive wheel 11 ... Fuel cell 12 ... FC boost converter 13 ... Battery 14 ... Battery boost converter 15 ... Inverter 16 ... Motor 17 ... Hydrogen tank 18 ... Compressor 20 ... Control part 21 ... Accelerator pedal Sensor

JP 2007-209161 A JP 2007-318938 A JP 2010-45889 A JP 2010-124689 A

A feature of the present invention is that the control unit operates as follows:
V _FC , the terminal voltage of the fuel cell
V _BAT , the terminal voltage of the power storage device,
Was set up to ensure the necessary torque in the motor, an input terminal voltage of the load circuit, the load circuit input lower limit voltage V _{m_min,}
Condition (1): V _FC > V _{M_min}
Condition (2): V _FC > V _BAT + α (α is a constant whose sign is positive)
Then,
When the second voltage converter is performing a boost operation and the condition (1) is satisfied and the condition (2) is satisfied, the boost operation by the first voltage converter is stopped. Connecting the output of the fuel cell directly to the load circuit and controlling the input terminal voltage of the load circuit by the second voltage converter ;
When at least one of the condition (1) and the condition (2) is not satisfied, the step-up operation by the first voltage converter is prohibited.

If the condition (1) is satisfied, towards the terminal voltage of the fuel cell, remote high by the load circuit input lower limit voltage. Therefore, in this case, it is possible to directly transmit the output of the fuel cell to the input terminal of the load circuit without increasing the terminal voltage of the fuel cell.

Here, "V _{BAT +} alpha" in the condition (2), specifically, a voltage can be controlled by put that before Symbol second voltage converter when the terminal voltage of the electric storage device is a V _BAT This corresponds to the lower limit voltage ("minimum voltage that can be boosted" in Japanese Patent Application Laid-Open No. 2010-27396 related to the prior application of the present applicant). That is, when the condition (2) is satisfied, the voltage on the output side of the second voltage converter can be stably boosted with respect to the voltage on the input side of the second voltage converter. ing. For this reason, when the condition (2) is satisfied, the input terminal voltage of the load circuit is stable by the boost type second voltage converter even if the boost operation by the first voltage converter is stopped. It is possible to receive proper voltage control.

Claims (2)

A motor,
A fuel cell and a power storage device connected in parallel with the motor;
A first voltage converter that is provided between the motor and the fuel cell and boosts and outputs the input terminal voltage of the fuel cell;
A second voltage converter which is provided between the motor and the power storage device and boosts and outputs the input terminal voltage of the power storage device;
A load circuit provided between the first voltage converter and the second voltage converter and the motor;
In the fuel cell system, the electric power output from the fuel cell and / or the power storage device is supplied to the motor via the load circuit.
The terminal circuit voltage of the fuel cell is V _FC , the terminal voltage of the power storage device is V _BAT , and the load circuit is set so that the loss in the fuel cell system is minimized while ensuring the necessary torque in the motor. _Assuming that the load circuit input lower limit voltage, which is the input terminal voltage of V _{M_min} , is V _FC > V _{M_min} and V _FC > V _BAT + α (α is a constant whose sign is positive: V _BAT + α When the voltage is V _BAT , this corresponds to the lower limit voltage that can be controlled by the step-up type second voltage converter.) When the condition is satisfied, the step-up operation by the first voltage converter is stopped. The output of the fuel cell is directly transmitted to the load circuit, and when the condition is not satisfied, the boosting operation by the first voltage converter is prohibited. A fuel cell system comprising a control unit. The fuel cell system according to claim 1,
V _BAT + α in the above condition is a lower limit voltage that can be controlled by the step-up type second voltage converter.

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