Patent Publication Number: US-2023133894-A1

Title: Fuel cell system and method for controlling power thereof

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of priority to Korean Patent Application No. 10-2021-0145783, filed in the Korean Intellectual Property Office on Oct. 28, 2021, the entire contents of which are incorporated herein by reference. 
     TECHNICAL FIELD 
     The present disclosure relates to a fuel cell system and a method for controlling power thereof. 
     BACKGROUND 
     A fuel cell system may generate electrical energy by using a fuel cell stack. For example, when hydrogen is used as fuel of the fuel cell stack, global environment problems may be solved. Accordingly, studies and researches have been consecutively performed on the fuel cell system. 
     A vehicle employing the fuel cell system may use, as a main power source, a fuel cell to generate electrical energy by using hydrogen fuel, and may include a hybrid power net employing a high voltage battery as a sub-power source, thereby switching an operating mode depending on a traveling situation such that the traveling efficiency is enhanced. 
     Recently, attempts have been made to apply the fuel cell system to a vehicle, such as an excavator, used in an industrial field. 
     A fuel cell system applied to the vehicle used in the industrial field includes a battery and a super capacitor in addition to a fuel cell. In this case, the fuel cell, the battery, and the super capacitor may be operated in a hybrid type, so the power efficiency may be enhanced. However, to operate each energy source in the hybrid type, at least three converters have to be provided in the power-net. The converter is a high-price part, so the costs may be increased. 
     SUMMARY 
     This Summary is provided to introduce a selection of concepts in simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter. 
     In one general aspect, a fuel cell system includes a first converter to convert power output from a fuel cell stack or a battery into power in a specific level, a second converter to convert power input to or output from the battery, a power relay assembly to control power flow between a super capacitor and the first converter, and a controller to control outputs of the first converter and the second converter depending on a starting state or an operating state of the fuel cell system, and to control the operation of the power relay assembly. 
     The first converter may be disposed on a main bus stage to connect the fuel cell stack to an inverter, and the second converter may have one end connected to the main bus stage between the fuel cell stack and the first converter, and may have an opposite end connected to the battery, and adjust bi-directional power flow. 
     The second converter may supply starting power of the fuel cell system and charging power of the super capacitor, by using power discharged from the battery, when the fuel cell system is started. 
     The first converter may supply the charging power, which is received from the second converter, to the super capacitor through the power relay assembly, when the fuel cell system is started. 
     The power relay assembly may adjust a voltage between an output stage of the first converter and the super capacitor by using a pre-charge relay before receiving the charging power through the first converter, and may supply the charging power to the super capacitor by using a main relay when the charging power is supplied through the first converter. 
     The controller may operate the first converter in a constant current mode and may operate the second converter in a constant voltage mode, when the fuel cell system is started. 
     The controller may set a starting voltage of the fuel cell stack as an output voltage of the second converter, and may set a limit current of the second converter or an allowable discharge current of the battery as a restriction current of the second converter, when the fuel cell system is started. 
     The controller may set a charging voltage of the super capacitor as the output voltage of the first converter, may set a value, which is obtained by subtracting a required current of an auxiliary device from the allowable discharge current of the battery, as an output current of the first converter, and may set a limit current of the first converter or an allowable charge current of the super capacitor as a restriction current of the first converter, when the fuel cell system is started. 
     The second converter may adjust and output power discharged from the battery, when the fuel cell system is operated, and the first converter may adjust power output through at least one of the fuel cell stack and the second converter and may output the adjusted power to the inverter, when the fuel cell system is operated 
     The controller may operate the first converter in the constant current mode and the second converter in the constant voltage mode, when the fuel cell system is operated. 
     The controller may set the output voltage of the first converter, based on a measured voltage of the super capacitor, may set an output current of the first converter, based on the ratio between added required power of the fuel cell stack and the battery, and the measured voltage of the super capacitor, and may set the restriction current of the first converter, based on the limit current of the first converter. 
     The controller may set the output voltage of the second converter, based on a target voltage of the fuel cell stack, when the fuel cell system is operated, may set the output current of the second converter, based on the ratio between the target power of the battery and the measured voltage of the battery, and may set the restriction current of the second converter, based on the allowable discharge current of the battery. 
     The power relay assembly may supply power, which is discharged from the super capacitor, to the inverter, when the fuel cell system is operated. 
     In another general aspect, a method for controlling power of a fuel cell system includes setting an output of a first converter, which adjusts power output from a fuel cell stack or a battery depending on a starting state or an operating state of the fuel cell system, and an output of a second converter, which adjusts power input to or output from the battery, controlling an operation of a power relay assembly depending on the starting state or the operating state of the fuel cell system, and controlling supplying of power of the fuel cell stack, the battery, and the super capacitor depending on the outputs of the first converter and the second converter and the operation of the power relay assembly. 
     The setting of the output may include setting a starting voltage of the fuel cell stack as an output voltage of the second converter, when the fuel cell system is started, and setting a limit current of the second converter or an allowable discharge current of the battery as a restriction current of the second converter. 
     The setting of the output may include setting a charging voltage of the super capacitor as the output voltage of the first converter, setting a value, which is obtained by subtracting a required current of an auxiliary device from an allowable discharge current of the battery, as an output current of the first converter, and setting a limit current of the first converter or an allowable charge current of the super capacitor as the restriction current of the first converter, when the fuel cell system is started. 
     The controlling of the supplying of the power may include supplying, by the second converter, starting power of the fuel cell system by using power discharged from the battery, when the fuel cell system is started. 
     The controlling of the supplying of the power may include supplying, by the second converter, charging power of the super capacitor by using power discharged from the battery, when the fuel cell system is started. 
     The method may further include adjusting, by the first converter, the charging power, which is received from the second converter, and supplying adjusted charging power to the super capacitor through the power relay assembly, when the fuel cell system is started. 
     The controlling of the supplying of the power may include adjusting, by a power relay assembly connected to the super capacitor, a voltage between an output stage of the first converter and the super capacitor by using a pre-charge relay before supplying the charging power to the super capacitor, and supplying, by the power relay assembly, the charging power to the super capacitor by using a main relay when the charging power is supplied through the first converter. 
     The setting of the output may include setting the output voltage of the first converter, based on a measured voltage of the super capacitor, when the fuel cell system is operated, setting an output current of the first converter, based on the ratio between added required power of the fuel cell stack and the battery, and the measured voltage of the super capacitor, and setting the restriction current of the first converter, based on the limit current of the first converter. 
     The setting of the output may include setting the output voltage of the second converter, based on a target voltage of the fuel cell stack, when the fuel cell system is operated, setting the output current of the second converter, based on the ratio between the target power of the battery and the measured voltage, and setting the restriction current of the second converter, based on the allowable discharge current of the battery. 
     The controlling of the supplying of the power may include adjusting and outputting, by the second converter, power discharged from the battery, when the fuel cell system is operated, and adjusting, by the first converter, power output through at least one of the fuel cell stack and the second converter and supplying the adjusted power to the inverter. 
     The controlling of the supplying of the power may include supplying, by the power relay assembly, the power discharged from the super capacitor to the inverter, when the fuel cell system is operated. 
     Other features and aspects will be apparent from the following detailed description, the drawings, and the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects, features and advantages of the present disclosure will be more apparent from the following detailed description taken in conjunction with the accompanying drawings: 
         FIG.  1    is a view illustrating a fuel cell system, according to an embodiment of the present disclosure; 
         FIG.  2 A  is a view illustrating energy flow when a fuel cell system is started, according to an embodiment of the present disclosure; 
         FIG.  2 B  is a view illustrating the operating state of a converter, when a fuel cell system is started, according to an embodiment of the present disclosure; 
         FIG.  3 A  is a view illustrating energy flow when a fuel cell system is operated, according to an embodiment of the present disclosure; 
         FIG.  3 B  is a view illustrating the operating state of a converter, when a fuel cell system is operated, according to an embodiment of the present disclosure; 
         FIGS.  4  and  5    are views illustrating the operating flow for a method for controlling power of a fuel cell system, according to an embodiment of the present disclosure; 
         FIG.  6 A  illustrates an operation of setting an output of a first converter; and 
         FIG.  6 B  illustrates an operation of setting an output of a second converter. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, some embodiments of the present disclosure will be described in detail with reference to the exemplary drawings. In adding the reference numerals to the components of each drawing, it should be noted that the identical or equivalent component is designated by the identical numeral even when they are displayed on other drawings. Further, in describing the embodiment of the present disclosure, a detailed description of well-known features or functions will be ruled out in order not to unnecessarily obscure the gist of the present disclosure. 
     In addition, in the following description of components according to an embodiment of the present disclosure, the terms ‘first’, ‘second’, ‘A’, ‘B’, ‘(a)’, and ‘(b)’ may be used. These terms are merely intended to distinguish one component from another component, and the terms do not limit the nature, sequence or order of the constituent components. In addition, unless otherwise defined, all terms used herein, including technical or scientific terms, have the same meanings as those generally understood by those skilled in the art to which the present disclosure pertains. Such terms as those defined in a generally used dictionary are to be interpreted as having meanings equal to the contextual meanings in the relevant field of art, and are not to be interpreted as having ideal or excessively formal meanings unless clearly defined as having such in the present application. 
       FIG.  1    is a view illustrating a fuel cell system, according to an embodiment of the present disclosure. 
     Referring to  FIG.  1   , according to an embodiment of the present disclosure, a fuel cell system may include a fuel cell stack  110 , an inverter  120 , a motor  130 , auxiliary device  140 , a battery  150 , and a super capacitor  160 , a first converter  170 , a second converter  180 , and a power relay assembly (PRA)  190 . In addition, the fuel cell system may further include a controller  200  to control the power flow of the fuel cell system. 
     The fuel cell stack  110  (or which may be referred to as a ‘fuel cell’) has a structure capable of producing electricity through a redox reaction between a fuel (e.g., hydrogen) and an oxidizing agent (e.g., air). For example, the fuel cell stack  110  may include a membrane electrode assembly (MEA) in which catalyst electrode layers making electrochemical reactions are attached to opposite sides of an electrolyte membrane for moving hydrogen ions, a gas diffusion layer (GDL), which uniformly distributes reaction gases and transfers generated electrical energy, a gasket and fastening mechanism to maintain airtightness and proper fastening pressure of the reaction gases and a first coolant, and a bipolar plate to move the reaction gases and the first coolant. 
     In the fuel cell stack  110 , hydrogen serving as fuel and air (oxygen) serving as an oxidizing agent are respectively supplied to an anode and cathode of the membrane electrode assembly through a fluid passage of a separator. In this case, the hydrogen may be supplied to the anode, and the air may be supplied to the cathode. The hydrogen supplied to the anode is decomposed into hydrogen ions (protons) and electrons (protons) by the catalyst of the electrode layers formed on opposite sides of the electrolyte membrane. Among them, only the hydrogen ion may be transmitted to the cathode through the electrolyte membrane which is a cation exchange membrane. In addition, the electron may be transmitted to the cathode through the gas diffusion layer and the separator which are conductors. In the cathode, hydrogen ions supplied through the electrolyte membrane and electrons transferred through the separator may meet with oxygen in the air supplied to the cathode by an air supply device to generate water. As electrons flow through an external conductive line due to the movement of the hydrogen ions, the electrical energy may be generated. 
     The fuel cell stack  110 , which serves as a main power source of a vehicle, that is, a fuel cell vehicle, having the fuel cell system supplies power necessary for the driving of the motor  130  by using the produced electrical energy. In this case, the fuel cell vehicle may include an industrial vehicle, such as an excavator, at a construction site. 
     Meanwhile, the fuel cell stack  110  may supply power to charge the battery  150  and/or the super capacitor  160 . 
     The output of the fuel cell stack  110  may be controlled by the controller  200 . 
     The inverter  120 , the motor  130 , the auxiliary device  140 , the battery  150 , the super capacitor  160 , the first converter  170 , the second converter  180 , and the power relay assembly (PRA)  190  may be connected to a main bus stage connected to an output stage of the fuel cell stack  110 . 
     The inverter  120  converts high-voltage direct current (DC) power, which is received from the fuel cell stack  110 , into alternating current (AC) power to drive the motor  130  and transmits the AC power to the motor  130 . 
     The inverter  120  may receive the high-voltage DC power from the battery  150  and/or the super capacitor  160  connected to the main bus stage. In this case, the inverter  120  may convert the high-voltage DC power, which is received from the battery  150  or the super capacitor  160 , into the AC power for driving the motor  130  and may provide the converted AC power to the motor  130 . 
     When the fuel cell vehicle operates in a fuel cell mode, the inverter  120  may receive power for driving the motor  130  from the fuel cell stack  110 . When the fuel cell vehicle operates in an electrical vehicle (EV) mode, the inverter  120  may receive the power for driving the motor  130  from the battery  150  and/or the super capacitor  160 . Meanwhile, when the fuel cell vehicle operates in a hybrid mode, the inverter  120  may receive the power for driving the motor  130  from the fuel cell stack  110 , the battery  150 , and the super capacitor  160 . 
     In this case, the inverter  120  may include a plurality of switching devices (not illustrated). A plurality of switching devices may be controlled through a pulse width modulation (PWM) scheme to generate the AC power. In this case, a scheme of controlling the plurality of switching devices is not limited to any one, and it is obvious that the plurality of switching devices may be controlled through different schemes according to embodiments. 
     The AC power generated from the inverter  120  is supplied to the motor  130 . Accordingly, the motor  130  is driven using the AC power supplied from the inverter  120 . The motor  130  may generate rotational force using the AC power supplied from the inverter  120 , and may apply the generated rotational force to a driving wheel of the fuel cell vehicle. 
     Meanwhile, the motor  130  generates electrical energy by using braking force generated during regenerative braking. In this case, the inverter  120  may convert power of the electrical energy generated from the motor  130  during the regenerative braking and may provide the converted power as charging power of the super capacitor  160 . 
     The auxiliary device  140  may include auxiliary devices necessary for driving the fuel cell stack  110 . For example, the auxiliary device  140  may include a blower, an air compressor, an injector, a cooling water circulation pump, and various control valves. 
     The auxiliary device  140  may operate by receiving driving power from the fuel cell stack  110 . In addition, the auxiliary device  140  may operate by receiving the driving power from the battery  150 , when the fuel cell system is started at an initial stage. 
     The battery  150  is an auxiliary power source of the fuel cell vehicle and is charged using electrical energy generated from the fuel cell stack  110 . 
     The battery  150  may discharge the charged electrical energy to supply power necessary for driving the motor  130 . 
     In addition, the battery  150  may discharge electrical energy at the initial start of the fuel cell system to supply power required to drive the auxiliary device  140 . In addition, the battery  150  may discharge the electrical energy charged at the initial start of the fuel cell system to supply power necessary for charging the super capacitor  160 . 
     In this case, a discharge amount of the battery  150  may be controlled by the controller  200 . 
     The super capacitor  160  is an auxiliary power source of the fuel cell vehicle like the battery  150 , and is charged using electrical energy generated from the fuel cell stack  110 . The super capacitor  160  may be charged using power supplied from the battery  150  at the initial start of the fuel cell system. In addition, the super capacitor  160  may be charged using power generated from the motor  130  during regenerative braking. 
     The super capacitor  160  may discharge the charged electrical energy to supply power necessary for driving the motor  130 . The discharge amount of the super capacitor  160  may be controlled by the controller  200 . 
     The first converter  170  may be disposed on the main bus stage between the fuel cell stack  110  and the inverter  120 . The first converter  170 , which is a power converter to adjust power output from the fuel cell stack  110  or the battery  150  and to output the adjusted power to the main bus stage, may include a uni-directional high voltage DC-DC converter (HDC). 
     For example, the first converter  170  may adjust power output from the fuel cell stack  110  or the battery  150  and may supply the adjusted power to the inverter  120  connected to the main bus stage. 
     In addition, the first converter  170  may adjust power output from the fuel cell stack  110  or the battery  150  and may supply the adjusted power to the super capacitor  160  connected to the main bus stage, such that the super capacitor  160  is charged with power. 
     In this case, the controller  200  may determine an output voltage, an output current, and a restriction current of the first converter  170 . Accordingly, the first converter  170  may adjust power output to the main bus stage, depending on the output voltage, the output current, and the restriction current determined by the controller  200 . 
     One end of the second converter  180  may be connected to the main bus stage between the fuel cell stack  110  and the first converter  170 , and an opposite end of the second converter  180  may be connected to the battery  150 . 
     The second converter  180 , which is a power converter that adjusts power input to or output from the battery  150 , may include a bi-directional high voltage DC-DC converter (BHDC) that controls bi-directional movement of a current. 
     For example, the second converter  180  may adjust the power supplied from the fuel cell stack  110  to supply the adjusted power as the charging power of the battery  150 . In addition, the second converter  180  may adjust power generated from the motor  130  during the regenerative braking to supply the power as the charging power of the battery  150 . 
     Meanwhile, the second converter  180  adjusts the power discharged from the battery  150  when the fuel cell system is started and outputs the power to the main bus stage. In this case, the power output to the main bus stage may be supplied as driving power of the auxiliary device  140 , and may be supplied as charging power of the super capacitor  160 . 
     In this case, the controller  200  may determine an output voltage, an output current, and a restriction current of the second converter  180 . Accordingly, the first converter  180  may adjust power output to the main bus stage or the battery  150  depending on the output voltage, the output current, and the restriction current determined by the controller  200 . 
     The power relay assembly  190  may include a main relay disposed on a line connecting the super capacitor  160  to the main bus stage, a pre-charge relay connected in parallel to the main relay, and a pre-charge resistor connected in series to one end of the pre-charge relay. 
     The power relay assembly  190  may apply or block power flowing between the super capacitor  160  and the main bus stage by opening and closing the main relay and the pre-charge relay. In this case, the opening and closing operations of the main relay and the pre-charge relay may be controlled by the controller  200 . 
     In this case, the power relay assembly  190  may prevent the first converter  170  and the inverter  120  from being damaged by the remaining voltage of the super capacitor  160  when the fuel cell system is initially started. 
     The power relay assembly  190  may further include a current sensor (not illustrated). The current sensor may detect a direction of a current flowing between the super capacitor  160  and the main bus stage. 
     The controller  200  may perform power control for each unit of the fuel cell system. In this case, the controller  200  may be an upper controller. 
     The controller  200  according to the present embodiment may be a hardware device, such as a processor or a central processing unit (CPU), or a program implemented by a processor. The controller  200  may be connected to each component of the fuel cell system to perform an overall function of the fuel cell system. 
     When the fuel cell system is started, the controller  200  may control a power flow for starting the fuel cell stack  110  and charging the super capacitor  160 . 
     In this case, the controller  200  may determine the outputs of the first converter  170  and the second converter  180  and may control the operation of the power relay assembly  190 . 
     For example, the controller  200  drives the second converter  180  in a constant voltage mode to start the fuel cell stack  110 . In this case, the controller  200  determines the output voltage of the second converter  180  to the starting voltage. In addition, the controller  200  sets a limit current of the second converter  180  or an allowable discharge current of the battery  150  to a restriction current of the second converter  180 . In this case, the controller  200  may determine a less value of the limit current of the second converter  180  and the allowable discharge current of the battery  150 , as the restriction current of the second converter  180 . 
     Accordingly, when the battery  150  discharges power, the second converter  180  may supply power discharged from the battery  150  to the auxiliary device  140  to start the fuel cell stack  110 . 
     Meanwhile, since the voltage of the super capacitor  160  naturally decreases due to self-discharge when left unattended, charging of the super capacitor  160  is required when the fuel cell system is started at the initial stage. Accordingly, when the fuel cell stack  110  is started, the controller  200  operates the power relay assembly  190  for charging the super capacitor  160  and drives the first converter  170  in a constant current mode. 
     In this case, the controller  200  determines the output voltage of the first converter  170  to the charging voltage of the super capacitor  160 . In addition, the controller  200  determine, as the output current of the first converter  170 , a value obtained by subtracting a required current of the auxiliary device  140  from the allowable discharge current of the battery  150 . In addition, the controller  200  determines, as the restriction current of the first converter  170 , the limit current of the first converter  170  or an allowable charge current of the super capacitor  160 . In this case, the controller  200  may determine a less value of the limit current of the first converter  170  and an allowable charge current of the super capacitor  160  as the restriction current of the first converter  170 . 
     Accordingly, when the fuel cell system is initially started, the first converter  170  and the second converter  180  may supply some of the power discharged from the battery  150  as power for charging the super capacitor  160 . For example, the first converter  170  and the second converter  180  may supply the remaining power of the power, which discharged from the battery  150  to the super capacitor  160 , except for the required power of the auxiliary device  140 . 
     In this case, the power relay assembly  190  induces a voltage of the super capacitor  160 , an output voltage of the first converter  170 , and an input voltage of the inverter  120  to be equal to each other, by using the pre-charge relay before driving the first converter  170 . Thereafter, when the first converter  170  is operated in a constant current mode, the power relay assembly  190  supplies charging power to the super capacitor  160  through the main relay. 
     Hereinafter, the operation of controlling power when the fuel cell system is started will be described in more detail with reference to  FIGS.  2 A and  2 B . 
       FIG.  2 A  is a view illustrating energy flow when a fuel cell system is started, according to an embodiment of the present disclosure.  FIG.  2 B  is a view illustrating the operating state of a converter when a fuel cell system is started, according to an embodiment of the present disclosure. 
     Referring to  FIGS.  2 A and  2 B , the controller  200  may determine the driving mode, the output voltage, and the restriction current of the second converter  180 , to provide power to the auxiliary device  140  along a first route R 11 , when the fuel cell system is started. 
     In this case, as illustrated in  FIG.  2 B , the controller  200  may set the driving mode of the second converter  180  to a constant voltage mode, set the output voltage of the second converter  180  by the starting voltage, and set the restriction current of the second converter  180  by the limit current of the second converter  180 , or the allowable discharge current of the battery  150 , for starting the fuel cell. 
     In addition, the controller  200  may set the driving mode, the output voltage, the output current, and the restriction current of the first converter  170  to provide charging power to the super capacitor  160  along a second route R 12 , when the fuel cell system is initially started. 
     In this case, as illustrated in  FIG.  2 B , the controller  200  may set the driving mode of the first converter  170  to a constant current mode, may set the output voltage of the first converter  170  to the voltage of the super capacitor  160 , set the output current of the first converter  170  to a value, which is obtained by subtracting the required current of the auxiliary device  140  from a dischargeable current of the battery  150 , and set a restriction current to the limit current of the first converter  170  or an allowable charge current of the super capacitor  160 , for charging of the super capacitor  160 . 
     As illustrated in  FIG.  2 B , when the outputs of the first converter  170  and the second converter  180  are determined, the battery  150  discharges electrical energy, and the second converter  180  adjusts power discharged from the battery  150  to supply starting power to the auxiliary device  140  along the first route R 11 . 
     Accordingly, the auxiliary device  140  completes starting by driving the fuel cell stack  110  by using power supplied from the second converter  180 . 
     In addition, the second converter  180  may adjust the power discharged from the battery  150  to output the adjusted power to the first converter  170  along a second route R 12 . In this case, the first converter  170  adjusts the power output from the second converter  180  to supply charging power to the super capacitor  160  along the second route R 12 . 
     In this case, the controller  200  may control the power relay assembly  190 , which is connected to the super capacitor  160  on the second route R 12 , to be turned on, before the charging power is supplied from the first converter  170 . 
     Accordingly, the power relay assembly  190  transmits the charging power supplied from the first converter  170  to the super capacitor  160  to charge the super capacitor  160 . 
     The controller  200  may control the operation of the power relay assembly  190  to be turned off, when the charging of the super capacitor  160  is completed. 
     Meanwhile, the controller  200  may control the power flow of the fuel cell stack  110 , the battery  150 , and the super capacitor  160  during operation after the starting of the fuel cell stack  110  is completed. 
     In this case, the controller  200  may determine the outputs of the first converter  170  and the second converter  180  and may control the operation of the power relay assembly  190 . 
     For example, the controller  200  may control the output of the first converter  170  to supply the output power of the fuel cell stack  110  to the inverter  120 , when the fuel cell system is operated. 
     When the fuel cell vehicle operates in the hybrid mode, the controller  200  may supply power from the battery  150  and/or the super capacitor  160  to the inverter  120  for a load variation, which exceeds the reference range, of the fuel cell stack  110 . 
     In this case, the controller  200  may control the output of the first converter  170 , based on the added required power of the fuel cell stack  110  and the battery  150 . In addition, the controller  200  may control the output of the second converter  180 , based on the target power of the battery  150  and the target voltage of the fuel cell stack  110  to supply power from the battery  150  to the inverter  120 . 
     In addition, the controller  200  may control the relay operation of the power relay assembly  190  to supply the power, which is charged in the super capacitor  160 , to the inverter  120 . 
     Hereinafter, the operation of controlling power when a fuel cell system is operated will be described with reference to  FIGS.  3 A and  3 B . 
       FIG.  3 A  is a view illustrating energy flow when a fuel cell system is operated, according to an embodiment of the present disclosure, and  FIG.  3 B  is a view illustrating the operating state of a converter when a fuel cell system is operated, according to an embodiment of the present disclosure. 
     Referring to  FIGS.  3 A and  3 B , when the fuel cell system is operated, the fuel cell stack  110  supplies power to the inverter  120  along a third route R 21 . In this case, the fuel cell stack  110  supplies, to the inverter  120 , power corresponding to the variation of the load of the inverter  120 , which is measured when the inverter  120  and the motor  130  are driven, and present in a first range of a static load consecutively measured. 
     In this case, the first converter  170  disposed on the third route R 21  may adjust the power supplied from the fuel cell stack  110  and may output the power to the inverter  120 . 
     The fuel cell stack  110  may supply driving power to the auxiliary device  140  along a fourth route R 22  connected to the third route R 21 . In addition, the fuel cell stack  110  may supply charging power to the battery  150  and/or the super capacitor  160  along a fifth route R 23  and/or a sixth route R 24  connected to the third route  21 . 
     When power is requested to be supplied to the inverter  120  during operation of the fuel cell system, the battery  150  may supply power to the inverter  120  along the fifth route R 23  and the third route R 21 . 
     In this case, the battery  150  may supply, to the inverter  120 , power, which corresponds to the variation of a load of the inverter  120  measured when the inverter  120  and the motor  130  are driven and present within a second range set for a band pass filter (BPF). The second range may correspond to an intermediate frequency range between the first range of a static load supplied by the fuel cell and a third range set for a high pass filter. The detailed range may be modified according to embodiments. 
     In this case, the second converter  180  disposed on the fifth rout R 23  and the first converter  170  disposed on the third route R 21  may adjust power supplied from the battery  150  and output the adjusted power to the inverter  120   
     In this case, the first converter  170  may adjust power, based on the sum of the power of the fuel cell stack  110  and the power supplied from the battery  150 , and may output the adjusted power to the inverter  120 , when the power is supplied from the fuel cell stack  110  and the battery  150 . 
     In addition, when power is requested to be supplied to the inverter  120  during operation of the fuel cell system, the super capacitor  160  may supply power to the inverter  120  along a sixth route R 24  and the fourth route R 22 . 
     In this case, the super capacitor  160  may supply, to the inverter  120 , power corresponding to the variation of a load of the inverter  120 , which is measured when the inverter  120  and the motor  130  are driven and present within a third range set to the HPF (High Pass Filter) of the load of the inverter  120 . In this case, the third range may be a load range that is rapidly fluctuated, and may correspond to a frequency range higher than the second range. The detailed range may be modified according to embodiments. 
     In this case, the power relay assembly  190  disposed on the sixth route R 24  may provide power supplied from the super capacitor  160  to the inverter  120 . 
     Accordingly, the controller  200  may control operations of the first converter  170 , the second converter  180 , and the power relay assembly  190  to supply power of the fuel cell stack  110 , the battery  150 , and the super capacitor  160  to the inverter  120 , when the fuel cell system is operated. 
     In this case, as illustrated in  FIG.  3 B , the controller  200  sets the driving mode of the first converter  170  to the constant current mode. In addition, the controller  200  may set the output voltage of the first converter  170  to the measured voltage of the super capacitor  160 , set the output current based on the ratio of the added required power of the fuel cell stack  110  and the battery  150  to the measured voltage of the super capacitor  160 , and set the restriction current to the limit current of the first converter  170 . 
     In addition, the controller  200  sets the driving mode of the second converter  180  to the constant voltage mode. In addition, the controller  200  may set the output voltage of the second converter  180  to the target voltage of the fuel cell stack  110 , set the output current, based on the ratio of the target power of the battery  150  to the measured voltage of the battery  150 , and set the restriction current to the allowable discharge current of the battery  150 . 
     As illustrated in  FIG.  3 B , when the outputs of the first converter  170  and the second converter  180  are determined, the fuel cell stack  110  may output power by the target power. In this case, the output power may be output to the first converter  170  and the auxiliary device  140 . The first converter  170  adjusts the power output from the fuel cell stack  110  and supplies the adjusted power to the inverter  120  along the third route R 21 . 
     In this case, the target power of the fuel cell stack  110  may be obtained by adding the required power of the auxiliary device  140  and the required power of the fuel cell stack  110 . The required power of the fuel cell stack  110  may be obtained by subtracting the target power of the battery  150  from the added required power of the fuel cell stack  110  and the battery  150 . 
     The added required power of the fuel cell stack  110  and the battery  150  may be obtained by subtracting the target power of the super capacitor  160  from the load measured when the inverter  120  is driven. The target power of the super capacitor  160  may be obtained by subtracting power, which is calculated corresponding to a voltage obtained by subtracting the measured voltage of the super capacitor  160  from the target voltage calculated based on the target SOC of the super capacitor  160 , from the required power of the super capacitor  160 , which is calculated through the HPF, of the load of the inverter  120 . 
     The target power of the battery  150  may be obtained by subtracting power, which is calculated corresponding to a voltage obtained by subtracting the measured voltage of the battery  150  from the target voltage calculated based on the target SOC of the battery  150 , from the required power of the battery  150  calculated through BPF, of the added required power of the fuel cell stack  110  and the battery  150 . 
     In addition, the battery  150  may discharge energy by the target power, and the second converter  180  may adjust the power discharged from the battery  150  to output the adjusted power to the first converter  170  along the fifth route R 23  and third route R 21 . In this case, the first converter  170  adjusts the power output from the second converter  180  to supply power to the inverter  120  along the second route R 21 . 
     In addition, the super capacitor  160  discharges energy by the target power, and the power relay assembly  190  supplies power, which is discharged from the super capacitor  160 , to the inverter  120 . 
     Accordingly, the inverter  120  operates while consecutively receiving power, which is within the first range, from the fuel cell stack  110 . In addition, the inverter  120  may receive power through the battery  150  with respect to a load variation within the second range, and power through the super capacitor  160  with respect to the rapid load variation within the third range, during the operation of the inverter  120 . 
     Hereinafter, the operating flow of the fuel cell system having the above structure according to the present disclosure will be described in more detail. 
       FIG.  4    is a view illustrating the operating flow for a method for controlling power when a fuel cell system is started, according to an embodiment of the present disclosure. 
     Referring to  FIG.  4   , when a power pack is started (S 110 ), the fuel cell system controls the second converter  180  to be turned on (S 120 ), and controls the power relay assembly (PRA)  190  to be turned on (S 150 ). 
     The fuel cell system starts the fuel cell stack  110 , as the power of the battery  150  is supplied to the auxiliary device  140  through the second converter  180  in S 120  (S 130 ). 
     Meanwhile, when the output voltage of the first converter  170  is equal to the voltage of the super capacitor  160  by the pre-charge relay of the power relay assembly PRA  190  in S 150  (S 160 ), the fuel cell system controls the first converter  170  to be turned on (S 170 ). 
     Thereafter, the fuel cell system starts charging the super capacitor  160  by supplying power of the battery  150  to the super capacitor  160  through the second converter  180  and the first converter  170  (S 180 ). 
     The fuel cell system determines whether the starting of the fuel cell stack  110  is completed (S 140 ). In addition, the fuel cell system determines whether charging of the super capacitor  160  is completed (S 190 ). 
     The fuel cell system terminates the starting of the power pack (S 210 ), when the start state of the fuel cell stack  110  is confirmed and the charge completion state of the super capacitor  160  is confirmed (S 200 ). 
     Thereafter, the fuel cell system may start the operation of the fuel cell system. 
       FIG.  5    is a view illustrating an operating flow of a method for controlling power during operation of a fuel cell system according to an embodiment of the present disclosure. 
     Referring to  FIG.  5   , when the inverter  120  is operated (S 310 ), the fuel cell system measures a load (S 320 ). 
     The fuel cell system calculates the variation of the load measured in S 320  based on the HPF (S 330 ), and supplies power corresponding to the load variation calculated in the S 330  from the super capacitor  160  to the inverter  120  (S 340 ). 
     In addition, the fuel cell system calculates the remaining part of load measured in S 320  except for the power supplied by the super capacitor  160  in S 340 , as the added required power of the fuel cell stack  110  and the battery  150 , and sets the output of the first converter  170  based on the added required power of the fuel cell stack  110  and the battery  150  (S 350 ). 
     A detailed operation of setting the output of the first converter  170  in step S 350  will be described with reference to  FIG.  6 A . 
     Referring to  FIG.  6 A , the fuel cell system may supply, through the first converter (HDC)  170 , the added required power (FC+BAT required power) of the fuel cell stack  110  and the battery  150 , which is obtained by subtracting the target power (Scap target power) of the super capacitor  160  from the load of the inverter. 
     In this case, the Scap target power may be obtained by subtracting specific Scap power from the Scap required power. The Scap required power may be calculated based on a high pass filter (HPF) among the loads of the inverter  120 . The specific Scap power may be obtained by applying a voltage value, which is obtained by subtracting the measured Scap voltage from the Scap target voltage calculated based on the target SOC of the Scap to the lookup table (LUT) 
     In addition, the fuel cell system may set the output current of the first converter HDC  170  based on the ratio of the FC+BAT required power to the Scap measured voltage. 
     In addition, the fuel cell system may set the output voltage of the first converter HDC  170  based on the measured Scap voltage. 
     In addition, the fuel cell system calculates a load variation, based on a band pass filter (BPF) of the added required power of the fuel cell stack  110  and the battery  150  (S 360 ). In this case, the fuel cell system may calculate the target power of the battery  150  based on the load variation calculated in S 360 . 
     The fuel cell system sets the output of the second converter  180  based on the target power of the battery  150  and the target power of the fuel cell stack  110  (S 370 ). 
     A detailed operation of setting the output of the second converter  180  in S 370  will be described with reference to  FIG.  6 B . 
     Referring to  FIG.  6 B , the fuel cell system may set an output current of the second converter BHDC  180  from a ratio of the target power (BAT target power) of the battery  150  to the measured voltage (BAT measurement voltage) of the battery  150 . 
     In this case, the BAT target power may be obtained by subtracting specific BAT power from the BAT required power. The BAT required power may be calculated from the added required power of FC+BAT, based on based on the BPF. The specific BAT power may be obtained by applying a voltage value, which is obtained by subtracting the BAT measured voltage from the BAT target voltage calculated based on the BAT target SOC, to the lookup table LUT. 
     In addition, the fuel cell system may set the output voltage of the second converter BHDC  180  based on the target voltage of the fuel cell stack  110 . 
     In this case, the FC target voltage may be obtained by applying the FC target power to the lookup table LUT. The FC target power may be obtained by adding the required power of the auxiliary device  140  to the FC required power. In addition, the FC required power may be obtained by subtracting the BAT target power from the BAT+FC required power. 
     When the outputs of the first converter  170  and the second converter  180  are determined through the above processes, the fuel cell stack  110  and the battery  150  supply power to the inverter  120  (S 380 ). In step S 380 , the fuel cell stack  110  may supply power to the inverter  120  through the first converter  170 , and the battery  150  may supply power to the inverter  120  through the second converter  180  and the first converter  170 . 
     Hereinabove, although the present disclosure has been described with reference to exemplary embodiments and the accompanying drawings, the present disclosure is not limited thereto, but may be variously modified and altered by those skilled in the art to which the present disclosure pertains without departing from the spirit and scope of the present disclosure claimed in the following claims. 
     According to the present disclosure, the converter applied to the power net may be substituted with the power relay assembly (PRA), thereby minimizing the number of the converters to reduce costs and efficiently operating several energy sources. 
     Therefore, the embodiments of the present disclosure are provided to explain the spirit and scope of the present disclosure, but not to limit them, so that the spirit and scope of the present disclosure is not limited by the embodiments. The scope of protection of the present disclosure should be construed by the attached claims, and all equivalents thereof should be construed as being included within the scope of the present disclosure. 
     Hereinabove, although the present disclosure has been described with reference to exemplary embodiments and the accompanying drawings, the present disclosure is not limited thereto, but may be variously modified and altered by those skilled in the art to which the present disclosure pertains without departing from the spirit and scope of the present disclosure claimed in the following claims.