Patent Publication Number: US-2015064587-A1

Title: Apparatus and method for controlling fuel cell system using sub-power conditioning system

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of Korean Patent Application No. 10-2013-0106026, filed Sep. 4, 2013, entitled “APPARATUS AND METHOD OF CONTROLLING FUEL CELL SYSTEM USING SUB-POWER CONDITIONING SYSTEM”, which is hereby incorporated by reference in its entirety into this application. 
     BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present disclosure relates generally to apparatuses and methods for controlling fuel cell systems and, more particularly, to an apparatus and method for controlling a fuel cell system using a sub-power conditioning system. 
     2. Description of the Related Art 
     Fuel cell stacks refer to a structure in which several tens through several hundreds of unit fuel cells are stacked on top of one another and connected to each other to obtain desired electric power output. To produce a large capacity fuel cell system, a plurality of fuel cell stacks must be connected to each other. Typically, the fuel cell stacks are connected in series, parallel or series-parallel to each other. If a problem occurs in at least one stack of the connected fuel cell stacks and the performance thereof deteriorates, current is concentrated on the remaining normal stacks, thus affecting the lifetime and durability of the normal stacks. 
       FIG. 1  is a graph showing the effect of deterioration in performance of a deteriorated stack on a normal stack. If a normal stack having operating voltage of 50V at 20 A and a deteriorated stack having operating voltage of 40V at 20 A are connected in parallel to each other and a load of 40 A is applied thereto, current is concentrated to the normal stack that has comparatively high power efficiency. 
     Thereby, as shown in  FIG. 1 , the normal stack is operated in an area of 22 A, greater than 40A/2=20 A, and 45V. Thus, a heat generation rate is increased, whereby the lifetime of the normal stack is reduced. On the other hand, the deteriorated stack is operated in an area of 18 A, less than 20 A by an increment in current of the normal stack, and 45V. That is, when the stacks are connected in parallel to each other, voltages applied to the stacks become the same, but a phenomenon of current being biased to one side along the current-voltage curve of the stacks is caused. As such, if at least one among the stacks that are connected in parallel to each other is deteriorated, a current load is concentrated to the remaining normal stack, thus having a disastrous effect on the lifetime and durability of the normal stack. 
     In the patent document of the following prior art document, power equipment was introduced, which has a fuel cell system array which is configured such that when a fuel cell system is defective or malfunctioned, the fuel cell system is disconnected from a power bus by a switch and an auxiliary fuel cell system is connected to the power bus so that power can be reliably supplied to a load. However, the following patent document introduces only the structure in which the fuel cell system that has been defective or malfunctioned is disconnected from and the auxiliary fuel cell system is connected to the power bus, but does not propose any structure for controlling deteriorated fuel cell stacks to use them as a power supply for a sub-load. 
     Therefore, an apparatus and method for controlling a fuel cell system are required, which separate deteriorated fuel cell stacks from a main power conditioning system to prevent deterioration in performance of some stacks from affecting the other normal stacks, thus minimizing a reduction in the lifetime and durability of the entirety of the fuel cell system, and which separately control the deteriorated fuel cell stacks to provide power to a sub-load through a sub-power conditioning system (sub-PCS), whereby the fuel cell system can be efficiently used. 
     PRIOR ART DOCUMENT 
     Patent Document 
     (Patent document 1) Japanese Unexamined Patent Publication No. 2005-526363 
     SUMMARY OF THE INVENTION 
     The present disclosure has been made in an effort to provide an apparatus for controlling a fuel cell system which separates deteriorated fuel cell stacks from a main power conditioning system to prevent deterioration in performance of some stacks from affecting the other normal stacks, thus minimizing a reduction in the lifetime and durability of the entirety of the fuel cell system, and which separately controls the deteriorated fuel cell stacks to provide power to a sub-load through a sub-power conditioning system (sub-PCS), whereby the fuel cell system can be efficiently used. 
     The present disclosure also provides a method for controlling a fuel cell system which separates deteriorated fuel cell stacks from a main power conditioning system to prevent deterioration in performance of some stacks from affecting the other normal stacks, thus minimizing a reduction in the lifetime and durability of the entirety of the fuel cell system, and which separately controls the deteriorated fuel cell stacks to provide power to a sub-load through a sub-power conditioning system (sub-PCS), whereby the fuel cell system can be efficiently used. 
     In an apparatus for controlling a fuel cell system according to an embodiment of the present disclosure, a main power conditioning system supplies output of normal stacks among a plurality of fuel cell stacks to a load. A sub-power conditioning system supplies output of one or more deteriorated stacks among the fuel cell stacks to a load. A switching unit changes a target to which output of each of the fuel cell stacks is connected. A control unit senses conditions of each of the fuel cell stacks and controls the switching unit. 
     Furthermore, a stack condition sensing unit may sense conditions of the fuel cell stacks and transmit information about the conditions of the fuel cell stacks to the control unit. 
     When at least one deteriorated stack is detected based on the transmitted information about the conditions of the fuel cell stacks, the control unit may separate output of the at least one deteriorated stack from the main power conditioning system and connect the output to the sub-power conditioning system. 
     When at least one inoperative stack is detected based on the transmitted information about the conditions of the fuel cell stacks, the control unit may separate the inoperative stack from the main power conditioning system. 
     The normal stacks that provide the output thereof to the main power conditioning system may be re-connected in series, parallel or series-parallel to each other, other than the at least one deteriorated stack. 
     The one or more deteriorated stacks may be re-connected in series. 
     The switching unit may include: a positive voltage switching unit for switching a target to which positive voltage of each of the fuel cell stacks is supplied; a negative voltage switching unit for switching a target to which negative voltage of each of the fuel cell stacks is supplied; and a deteriorated-stack connection switching unit connecting the deteriorated stacks in series to each other and providing output of the deteriorated stacks to the sub-power conditioning system. 
     When n is an integer of 2 or more, the stacks may include an n number of stacks, the stack condition sensing unit may include an n number of shunt resistors, and first ends of the first through n-th shunt resistors may be respectively connected to positive electrodes of the first through n-th stacks. 
     The positive voltage switching unit may include first through n-th positive voltage switches. The negative voltage switching unit may include first through n-th negative voltage switches. The deteriorated-stack connection switching unit may include first through n-th deteriorated-stack connection switches. 
     Each of the first through n-th positive voltage switches may include a common terminal and first through third terminals. Each of the first through n-th negative voltage switches may include a common terminal and first through third terminals. Each of the first through (n−1)th deteriorated-stack connection switches may include a common terminal and first through third terminals. The n-th deteriorated-stack connection switch may include a common terminal, a first terminal and a second terminal. 
     Each of the first through n-th positive voltage switches, the first through n-th negative voltage switches and the first through (n−1)th deteriorated-stack connection switches may connect the corresponding common terminal to one of the corresponding first through third terminals in response to a control signal of the control unit. The n-th deteriorated-stack connection switch may connect the corresponding common terminal to the corresponding first or second terminal in response to a control signal of the control unit. 
     The common terminals of the first through n-th positive voltage switches may be respectively connected to second ends of the first through n-th shunt resistors. The first terminals of the first through n-th positive voltage switches may be respectively connected to a positive voltage input terminal of the main power conditioning system. The second terminals of the first through n-th positive voltage switches may be open. 
     The common terminals of the first through n-th negative voltage switches may be respectively connected to negative electrodes of the first through n-th stacks. The first terminals of the first through n-th negative voltage switches may be connected to a negative voltage input terminal of the main power conditioning system. The second terminals of the first through n-th negative voltage switches may be open. 
     The common terminals of the first through (n−1)th deteriorated-stack connection switches may be respectively connected to the third terminals of the second through n-th positive voltage switches. The first terminals of the first through n-th deteriorated-stack connection switches may be respectively connected to the third terminals of the first through n-th negative voltage switches. The second terminals of the second through n-th deteriorated-stack connection switches may be respectively connected to the common terminals of the first through (n−1)th deteriorated-stack connection switches. The third terminals of the first through (n−1)th deteriorated-stack connection switches and the common terminal of the n-th deteriorated-stack connection switch may be connected to a negative voltage input terminal of the sub-power conditioning system. The second terminal of the first deteriorated-stack connection switch and the third terminal of the first positive voltage switch may be connected to a positive voltage input terminal of the sub-power conditioning system. 
     The fuel cell stacks may be arranged in an array of n1×1, n1×n2 or n1×n2×n3, when each of n1, n2 and n3 is an integer of 2 or more. 
     The stacks of the stack array may be connected in series, parallel or series-parallel to each other. 
     The fuel cell stacks may form a plurality of fuel cell stack modules, each of which includes a predetermined number of fuel cell stacks. 
     The control unit may sense conditions of the fuel cell stack modules, and when at least one deteriorated stack module is detected, the at least one deteriorated stack module may be separated from the main power conditioning system. 
     The control unit may connect output of the at least one deteriorated stack module to the sub-power conditioning system. 
     A method for controlling a fuel cell system according to an embodiment of the present disclosure includes determining, by a control unit, whether at least one deteriorated stack or inoperative stack is present using information about conditions of fuel cell stacks; and controlling a switching unit when it is determined that at least one deteriorated stack or inoperative stack is present, and separating output of the at least one deteriorated stack or inoperative stack from a main power conditioning system. 
     The control unit may control the switching unit and connects the output of the at least one deteriorated stack to a sub-power conditioning system. 
     The normal stacks that provide output to the main power conditioning system may be re-connected in series, parallel or series-parallel to each other, other than the at least one deteriorated stack. 
     Deteriorated stacks that provide output to the sub-power conditioning system may be connected in series to each other. 
     The switching unit may include: a positive voltage switching unit for switching a target to which positive voltage of each of the fuel cell stacks is supplied; a negative voltage switching unit for switching a target to which negative voltage of each of the fuel cell stacks is supplied; and a deteriorated-stack connection switching unit connecting the deteriorated stacks in series to each other and providing the output of the deteriorated stacks to the sub-power conditioning system. 
     The fuel cell stacks may form a plurality of fuel cell stack modules, each of which includes a predetermined number of fuel cell stacks. 
     The control unit may sense conditions of the fuel cell stack modules, and when at least one deteriorated stack module is detected, the at least one deteriorated stack module may be separated from the main power conditioning system. 
     The control unit may connect output of the at least one deteriorated stack module to the sub-power conditioning system. 
     In the present disclosure, when some fuel cell stacks are deteriorated, the deteriorated fuel cell stacks are separated from a main power conditioning system to prevent deterioration in performance of some stacks from affecting the other normal stacks. Thereby, a reduction in the lifetime and durability of the entirety of the fuel cell system can be minimized. Furthermore, the deteriorated fuel cell stacks are separately controlled to provide power to a sub-load through a sub-power conditioning system. Therefore, the fuel cell system can be efficiently used. Moreover, inoperative stacks are separated from the fuel cell system, whereby a reduction in the lifetime and durability of the entirety of the fuel cell system can be more reliably minimized. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects, features and advantages of the present disclosure will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a graph showing the effect of deterioration in performance of a deteriorated stack on a normal stack; 
         FIG. 2  is a block diagram illustrating an apparatus for controlling a fuel cell system using a sub-power conditioning system which is in a normal generation state according to an embodiment of the present disclosure; 
         FIG. 3  is a block diagram illustrating the fuel-cell-system control apparatus using the sub-power conditioning system which is in a generation state when a deteriorated stack occurs according to the present disclosure; 
         FIG. 4  is a block diagram illustrating the fuel-cell-system control apparatus using the sub-power conditioning system which is in a generation state when a deteriorated stack or an inoperative stack occurs according to the present disclosure; and 
         FIG. 5  is a flowchart showing a method for controlling the fuel cell system using the sub-power conditioning system according to the present disclosure. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The objects, features and advantages of the present disclosure will be more clearly understood from the following detailed description of the preferred embodiments taken in conjunction with the accompanying drawings. 
     Throughout the accompanying drawings, the same reference numerals are used to designate the same or similar components, and redundant descriptions thereof are omitted. 
     Further, in the following description, the terms “first”, “second”, “one side”, “the other side” and the like are used to differentiate a certain component from other components, but the configuration of such components should not be construed to be limited by the terms. 
     Further, in the description of the present disclosure, when it is determined that the detailed description of the related art would obscure the gist of the present disclosure the description thereof will be omitted. 
     Hereinafter, preferred embodiments of the present disclosure will be described in detail with reference to the attached drawings. 
       FIG. 2  is a block diagram illustrating an apparatus for controlling a fuel cell system using a sub-power conditioning system which is in a normal generation state according to an embodiment of the present disclosure.  FIG. 3  is a block diagram illustrating the fuel-cell-system control apparatus using the sub-power conditioning system which is in a generation state when a deteriorated stack occurs according to the present disclosure.  FIG. 4  is a block diagram illustrating the fuel-cell-system control apparatus using the sub-power conditioning system which is in a generation state when a deteriorated stack or an inoperative stack occurs according to the present disclosure.  FIG. 5  is a flowchart showing a method for controlling the fuel cell system apparatus using the sub-power conditioning system according to the present disclosure. 
     Hereinafter, the apparatus and method for controlling the fuel cell system using the sub-power conditioning system according to the present disclosure will be described with reference to  FIGS. 1 through 5 . 
     As shown in  FIG. 2 , the fuel-cell-system control apparatus using the sub-power conditioning system according to the present disclosure includes a main power conditioning system  200  which supplies output of normal stacks among first through sixth fuel cell stacks STK 1  through STK 6  to a main load, the sub-power conditioning system  202  which supplies output of deteriorated stacks among the first through sixth fuel cell stacks STK 1  through STK 6  to a sub-load, a plurality of switching units  206 ,  208  and  210  which change a target to which output of each of the first through sixth fuel cell stacks STK 1  through STK 6  is transmitted, a control unit  204  which senses conditions of the first through sixth fuel cell stacks STK 1  through STK 6  and controls the switching units  206 ,  208  and  210 , and first through sixth shunt resistors R 1  through R 6  which function as a stack condition sensing unit to sense conditions of the first through sixth fuel cell stacks STK 1  through STK 6  and transmit information about the conditions of the stacks to the control unit. Furthermore, the first through sixth shunt resistors R 1  through R 6  respectively sense current that flows through the first through sixth fuel cell stacks STK 1  through STK 6 . 
     The switching units  206 ,  208  and  210  include a positive voltage switching unit  206  which switches a target to which positive voltage of the first through sixth fuel cell stacks SKT 1  through SKT 6  is supplied, a negative voltage switching unit  208  which switches a target to which negative voltage of the first through sixth fuel cell stacks SKT 1  through SKT 6  is supplied, and a deteriorated-stack connection switching unit  210  which connects the deteriorated stacks in series and provides output of the deteriorated stacks to the sub-power conditioning system  202 . 
     First ends of the first through sixth shunt resistors R 1  through R 6  are respectively connected to positive electrodes of the first through sixth fuel cell stacks SKT 1  through SKT 6 . 
     The positive voltage switching unit  206  includes first through sixth positive voltage switches PSW 1  through PSW 6 . The negative voltage switching unit  208  includes first through sixth negative voltage switches NSW 1  through NSW 6 . The deteriorated-stack connection switching unit  210  includes first through sixth deteriorated-stack connection switches DSW 1  through DSW 6 . 
     The first through sixth positive voltage switches PSW 1  through PSW 6  include respective common terminals P 1 _ 0  through P 6 _ 0 , respective first terminals P 1 _ 1  through P 6 _ 1 , respective second terminals P 1 _ 2  through P 6 _ 2  and respective third terminals P 1 _ 3  through P 6 _ 3 . 
     The first through sixth negative voltage switches NSW 1  through NSW 6  include respective common terminals N 1 _ 0  through N 6 _ 0 , respective first terminal N 1 _ 1  through N 6 _ 1 , respective second terminals N 1 _ 2  through N 6 _ 2  and respective third terminals N 1 _ 3  through N 63 . 
     The first through fifth deteriorated-stack connection switches DSW 1  through DSW 5  include respective common terminals D 1 _ 0  through D 5 _ 0 , respective first terminals D 1 _ 1  through D 5 _ 1 , respective second terminals D 1 _ 2  through D 5 _ 2  and respective third terminals D 1 _ 3  through D 5 _ 3 . The sixth deteriorated-stack connection switch DSW 6  includes a common terminal D 6 _ 0 , a first terminal D 6 _ 1  and a second terminal D 6 _ 2 . 
     Each of the first through sixth positive voltage switches PSW 1  through PSW 6 , first through sixth negative voltage switches NSW 1  through NSW 6  and first through fifth deteriorated-stack connection switches DSW 1  through DSW 5  is an SPTT (single pole triple throw) switch which electrically connects a common terminal to one of first through third terminals in response to a control signal. The sixth deteriorated-stack connection switch DSW 6  is an SPDT (single pole double throw) switch which electrically connects a common terminal to the first or second terminal. However, the first through sixth positive voltage switches PSW 1  through PSW 6 , the first through sixth negative voltage switches NSW 1  through NSW 6  and the first through sixth deteriorated-stack connection switches DSW 1  through DSW 6  according to the prevent disclosure are not limited to the above-mentioned SPTT or SPDT switch. In other words, they may be embodied by other kinds of switches, so long as these switches can have the same function as that of the SPTT or SPDT switch. 
     Each of the first through sixth positive voltage switches PSW 1  through PSW 6 , first through sixth negative voltage switches NSW 1  through NSW 6  and first through fifth deteriorated-stack connection switches DSW 1  through DSW 5  connects a common terminal to one of first through third terminals in response to a corresponding one of control signals PC 1  through PC 6 , NC 1  through NC 6  and DC 1  through DC 5  of the control unit  204 . The sixth deteriorated-stack connection switch DSW 6  connects a common terminal D 6 _ 0  to the first common terminal D 6 _ 1  or second common terminal D 6 _ 2  in response to a control signal DC 6  of the control unit  204 . 
     The common terminals P 1 _ 0  through P 6 _ 0  of the first through sixth positive voltage switches PSW 1  through PSW 6  are respectively connected to second ends of the first through sixth shunt resistors R 1  through R 6 . The first terminals P 1 _ 1  through P 6 _ 1  of the first through sixth positive voltage switches PSW 1  through PSW 6  are connected to a positive voltage input terminal MPI of the main power conditioning system  200 . The second terminals P 1 _ 2  through P 6 _ 2  of the first through sixth positive voltage switches PSW 1  through PSW 6  are open. 
     The common terminals N 1 _ 0  through N 6 _ 0  of the first through sixth negative voltage switches NSW 1  through NSW 6  are respectively connected to negative electrodes of the first through sixth stacks STK 1  through STK 6 . The first terminals N 1 _ 1  through N 6 _ 1  of the first through sixth negative voltage switches NSW 1  through NSW 6  are connected to a negative voltage input terminal MNI of the main power conditioning system  200 . The second terminals N 1 _ 2  through N 6 _ 2  of the first through sixth negative voltage switches NSW 1  through NSW 6  are open. 
     The common terminals D 1 _ 0  through D 5 _ 0  of the first through fifth deteriorated-stack connection switches DSW 1  through DSW 5  are respectively connected to the third terminals P 2 _ 3  through P 6 _ 3  of the second through sixth positive voltage switches PSW  2  through PSW  6 . The first terminals D 1 _ 1  through D 6 _ 1  of the first through sixth deteriorated-stack connection switches DSW 1  through DSW 6  are respectively connected to the third terminals N 1 _ 3  through N 6 _ 3  of the first through sixth negative voltage switches NSW 1  through NSW 6 . 
     The second terminals D 2 _ 2  through D 6 _ 2  of the second through sixth deteriorated-stack connection switches DSW 2  through DSW 6  are respectively connected to the common terminals D 1 _ 0  through D 5 _ 0  of the first through fifth deteriorated-stack connection switches DSW 1  through DSW 5 . The third terminals D 1 _ 3  through D 5 _ 3  of the first through fifth deteriorated-stack connection switches DSW 1  through DSW 5  and the common terminal D 6 _ 0  of the sixth deteriorated-stack connection switch DSW 6  are connected to a negative voltage input terminal SNI of the sub-power conditioning system  202 . The second terminal D 1 _ 2  of the first deteriorated-stack connection switch DSW 1  and the third terminal P 1 _ 3  of the first positive voltage switch PSW 1  are connected to a positive voltage input terminal SPI of the sub-power conditioning system  202 . 
     While the term “connection” refers to electrical connection, the present disclosure is not limited to this. Also, in the embodiment of the present disclosure shown in  FIG. 2 , although the fuel-cell-system control apparatus has been illustrated as controlling the six stacks, that is, the first through sixth stacks STK 1  through STK 6 , the present disclosure is not limited to this. If n is an integer of 2 or more, the fuel-cell-system control apparatus controls n number of stacks including the first through n-th stacks STK 1  through STKn (not shown). In this case, the construction of the switching units  206 ,  208  and  210  of  FIG. 2  may be expanded to change a target to which output of each of the first through n-th stacks STK 1  through STKn is transmitted. 
     In other words, the positive voltage switching unit  206  includes first through n-th positive voltage switches PSW 1  through PSWn, the negative voltage switching unit  208  includes first through n-th negative voltage switches NSW 1  through NSWn, and the deteriorated-stack connection switching unit  210  includes first through n-th deteriorated-stack connection switches DSW 1  through DSWn, thus making it possible to change a target to which output of each of the first through n-th stacks STK 1  through STKn is transmitted. 
     The first through n-th positive voltage switches PSW 1  through PSWn, the first through n-th negative voltage switches NSW 1  through NSWn and the first through n-th deteriorated-stack connection switches DSW 1  through DSWn may be respectively controlled by the control signals PC 1  through PCn, NC 1  through NCn, DC 1  through DCn of the control unit  204 . 
     The fuel-cell-system control apparatus according to the embodiment of  FIG. 2  may be configured such that it controls a plurality of fuel cell stacks, which are arranged in an array of n1×1, n1×n2 or n1×n2×n3 when each of n1, n2 and n3 is an integer of 2 or more, and are connected in series, parallel or series-parallel (not shown) to each other. In this case, the construction of the switching units  206 ,  208  and  210  shown in  FIG. 2  may be expanded to change a target to which output of each of n1×1 number of stacks, n1×n2 number of stacks and n1×n2×n3 number of stacks which are connected in series, parallel or series-parallel to each other is transmitted. 
     The fuel-cell-system control apparatus according to the embodiment of  FIG. 2  may further include first-b through sixth-b stacks STK 1   b  through STK 6   b  (not shown) which are respectively connected in series between the first through sixth stacks STK 1  through STK 6  and the first through sixth negative voltage switches NSW 1  through NSW 6 . 
     In  FIG. 2 , the control unit  204  and the switching unit  206 ,  208  and  210  control the connection of the fuel cell stacks, but the present disclosure is not limited to this. That is, the apparatus according to the embodiment may be configured such that the fuel cell stacks form a plurality of fuel cell stack modules each of which includes a predetermined number of fuel cell stacks, and the control unit  204  and the switching unit  206 ,  208  and  210  control the connection of the fuel cell stack modules. In the case, the control unit  204  senses conditions of the fuel cell stack modules. If at least one deteriorated stack module is detected, the deteriorated stack module is separated from the main power conditioning system  200  and then connected to the sub-power conditioning system  202 . In other words, the control unit  204  divides the fuel cell stacks by modules and controls connection between the modules. 
     Hereinafter, the operation of the fuel-cell-system control apparatus according to the embodiment of the present disclosure having the above-mentioned construction will be explained. 
     Operation of Apparatus When All Stacks Are Normal. 
     In  FIG. 2 , the control unit  204  determines whether a deteriorated stack is present or not based on current that flows through the first through sixth stacks STK 1  through STK 6  from the first through sixth shunt resistors R 1  through R 6 . 
     As shown in  FIG. 2 , when all of the stacks are normally operated, the control unit  204  outputs the control signals PC 1  through PC 6 , NC 1  through NC 6 , DC 1  through DC 6  to the switching unit  206 ,  208  and  210  such that the first through sixth stacks STK 1  through STK 6  are connected in parallel to each other, thus supplying the output of the first through sixth stacks STK 1  through STK 6  to the main power conditioning system  200 . 
     Operation of Apparatus When Deteriorated Stack is Present. 
       FIG. 3  illustrates the operation of the apparatus when a deteriorated stack is present. The control unit  204  determines whether at least one deteriorated stack is present or not based on current that flows through the first through sixth stacks STK 1  through STK 6  from the first through sixth shunt resistors R 1  through R 6 . In the case of  FIG. 3 , because the level of current which flows through the first and fifth stacks STX 1  and STX 5  is smaller than that of the normal stacks or deviation in current between the stacks is caused, the control unit  204  determines that the first and fifth stacks STX 1  and STX 5  are deteriorated. 
     The control unit  204  thus controls the first positive voltage switch PSW 1  and the first negative voltage switch NSW 1  and separates the first stack STK 1  that has deteriorated from the main power conditioning system  200 . The control unit  204  controls the fifth positive voltage switch PSW 5  and the fifth negative voltage switch NSW 5  and separates the fifth stack STK 5  that has deteriorated from the main power conditioning system  200 . Thereby, deterioration in performance of the first stack STK 1  and fifth stack STK 5  that have deteriorated can be prevented from being affected to the second through fourth stacks STK 2  through STK 4  and sixth stack STK 6  that are normal. 
     The above-mentioned operation will be explained in detail. The control unit  204  transmits an appropriate control signal PC 1  to the positive voltage switch PSW 1  and electrically connects the common terminal P 1 _ 0  of the positive voltage switch PSW 1  to the third terminal P 1 _ 3 . Further, the control unit  204  transmits an appropriate control signal NC 1  to the negative voltage switch NSW 1  and electrically connects the common terminal N 1 _ 0  of the negative voltage switch NSW 1  to the third terminal N 1 _ 3 . 
     Also, the control unit  204  transmits an appropriate control signal PC 5  to the fifth positive switch PSW 5  and electrically connects the common terminal P 5 _ 0  of the fifth positive switch PSW 5  to the third terminal P 5 _ 3 . The control unit  204  transmits an appropriate control signal NC 5  to the fifth negative switch NSW 5  and electrically connects the common terminal N 5 _ 0  of the fifth negative voltage switch NSW 5  to the third terminal N 5 _ 3 . 
     Thereby, the first and fifth stacks STX 1  and STX 5  that have deteriorated are separated from the positive input terminal MPI of the main power conditioning system  200 , thus preventing the deterioration in performance of the first and fifth stacks STX 1  and STX 5  that have deteriorated from being affected to the second through fourth stacks STX 2  through STX 4  and sixth stack STX 6  that are normal. 
     The control unit  204  controls the first through sixth deteriorated-stack connection switches DSW 1  through DSW 6  and connects the first stack STK 1  to the fifth stack STX 5  in series. The control unit  204  also connects the first and fifth stacks STX 1  and STX 5  that are connected in series to each other to the sub-power conditioning system  202  so that the output of the first and fifth stacks STX 1  and STX 5  that have deteriorated can be supplied to the sub-power conditioning system  202 . 
     The above-mentioned operation will be explained in detail. The control unit  204  transmits appropriate control signals DC 1  and DC 5  to the first and fifth deteriorated-stack connection switches DSW 1  and DSW 5  and respectively electrically connects the common terminals D 1 _ 0  and D 5 _ 0  of the first and fifth deteriorated-stack connection switches DSW 1  and DSW 5  to the first terminals D 1 _ 1  and D 5 _ 1 . The control unit  204  further transmits appropriate control signals DC 2  through DC 4  to the second through fourth deteriorated-stack connection switches DSW 2  through DSW 4  and respectively electrically connects the common terminals D 2 _ 0  through D 4 _ 0  of the second through fourth deteriorated-stack connection switches DSW 2  through DSW 4  to the second terminals D 2 _ 2  through D 4 _ 2 . The control unit  204  transmits an appropriate control signal DC 6  to the sixth deteriorated-stack connection switch DSW 6  and electrically connects the common terminal D 6 _ 0  of the sixth deteriorated-stack connection switch DSW 6  to the second terminal D 6 _ 2 . 
     Thereby, the first and fifth stacks STK 1  and STK 5  that have deteriorated are connected in series to the sub-power conditioning system  202 . 
     When the target to which the output of the stacks is transmitted is changed in the above-mentioned manner, the main power conditioning system  200  can normally receive power from the second through fourth stacks STX 2  through STX 4  and sixth stack STX 6  that are normal and are connected in parallel to each other, and the sub-power conditioning system  202  can receive power from the first and fifth stacks STK 1  and STK 5  that have deteriorated and been connected in series to each other. 
     The output of the second through fourth stacks STK 2  through STK 4  and sixth stack STK 6  that are normal and are connected in parallel to each other is used to operate a main load  212  through the main power conditioning system  200 . 
     The output of the first and fifth stacks STX 1  and STX 5  that are deteriorated and are connected in series to each other is used to operate a sub-load  214 , that is, a partial load, through the sub-power conditioning system  202  in consideration of voltage. Therefore, the first and fifth stacks STX 1  and STX 5  that are deteriorated and are connected in parallel to each other operate at constant load current and do not affect to each other with regard to deterioration in performance, so that the speed of deterioration can be minimized. 
     Operation of Apparatus When Deteriorated Stack and Inoperative Stack Are Present. 
       FIG. 4  illustrates the operation of the apparatus when a deteriorated stack and inoperative stack are present. The control unit  204  determines whether at least one of deteriorated stack and an inoperative stack is present based on currents that flow through the first through sixth stacks STX 1  through STX 6  from the first through sixth shunt resistors R 1  through R 6 . In the case of  FIG. 4 , because the level of currents which flow through the first and fourth stacks STX 1  and STX 4  is smaller than that of the normal stacks or deviation in currents between the stacks is caused, the control unit  204  determines that the first and fourth stacks and STX 1  and STX 4  are deteriorated. Also, because the level of current which flows through the fifth stack STX 5  is less than a predetermined level of current, the control unit  204  determines that the fifth stack STX 5  is inoperative. 
     The control unit  204  controls the first positive voltage switch PSW 1  and the first negative voltage switch NSW 1  and separates the first stack STX 1  that has deteriorated from the main power conditioning system  200 . The control unit  204  controls the fourth positive voltage switch PSW 4  and the fourth negative voltage switch NSW 4  and separates the fourth stack STX 4  that has deteriorated from the main power conditioning system  200 . Thereby, deterioration in performance of the first and fourth stacks STX 1  and STX 4  that have deteriorated can be prevented from affecting the second stack STX 2 , the third stack STX 3  and the sixth stack STX 6  that are normal. The control unit  204  controls the fifth positive voltage switch PSW 5  and the fifth negative voltage switch NSW 5  and separates the fifth stack STX 5  that is inoperative from the main power conditioning system  200 . 
     The above-mentioned operation will be explained in detail. The control unit  204  transmits an appropriate control signal PC 1  to the first positive voltage switch PSW 1  and electrically connects the common terminal P 1 _ 0  of the first positive voltage switch PSW 1  to the third terminal P 1 _ 3 . The control unit  204  transmits an appropriate control signal NC 1  to the first negative voltage switch NSW 1  and electrically connects the common terminal N 1 _ 0  of the first negative voltage switch NSW 1  to the third terminal N 1 _ 3 . 
     The control unit  204  transmits an appropriate control signal PC 4  to the fourth positive voltage switch PSW 4  and electrically connects the common terminal P 4 _ 0  of the fourth positive voltage switch PSW 4  to third terminal P 4 _ 3 . The control unit  204  transmits an appropriate control signal NC 4  to the fourth negative voltage switch NSW 4  and electrically connects the common terminal N 4 _ 0  of the fourth negative voltage switch NSW 4  to the third terminal N 4 _ 3 . 
     Thereby, the first stack STX 1  and fourth stack STX 4  that have deteriorated are separated from the main power conditioning system  200 , thus preventing the deterioration in performance of the first and fourth stacks STX 1  and STX 4  that have deteriorated from affecting the second, third and sixth stacks STK 2 , STK 3  and STK 6 . 
     Furthermore, the control unit  204  transmits an appropriate control signal PC 5  to the fifth positive voltage switch PSW 5  and electrically connects the common terminal P 5 _ 0  of the fifth positive voltage switch PSW 5  to the second terminal P 5 _ 2 . The control unit  204  transmits an appropriate control signal NC 5  to the fifth negative voltage switch NSW 5  and electrically connects the common terminal N 5 _ 0  of the fifth negative voltage switch NSW 5  to the second terminal N 5 _ 2 . The second terminal P 5 _ 2  of the fifth positive voltage switch PSW 5  and the second terminal N 5 _ 2  of the fifth negative voltage switch NSW 5  are open rather than being connected to any element. Thus, the fifth stack STX 5  that is inoperative is separated from not only the main power conditioning system  200  but also the sub-power conditioning system  202 . Thereby, the fifth stack STX 5  that is inoperative is prevented from affecting the entirety of the fuel cell system. As a result, a reduction in the lifetime and durability of the fuel cell system can be minimized. 
     The control unit  204  controls the first through sixth deteriorated-stack connection switches DSW 1  through DSW 6  and connects the first and fourth stacks STX 1  and STX 4  that have deteriorated in series to each other. The control unit  204  also connects the first and fourth stacks STX 1  and STX 4 , that have deteriorated and are connected in series to each other, to the sub-power conditioning system  202  so that the output of the first and fourth stacks STX 1  and STX 4  that have deteriorated is applied to the sub-power conditioning system  202 . 
     The above-mentioned operation will be explained in detail. The control unit  204  respectively transmits appropriate control signals DC 1  and DC 4  to the first and fourth deteriorated-stack connection switches DSW 1  and DSW 4  and respectively electrically connects the common terminals D 1 _ 0  and D 4 _ 0  of the first and fourth deteriorated-stack connection switches DSW 1  and DSW 4  to the first terminals D 1 _ 1  and D 4 _ 1 . 
     The control unit  204  respectively transmits appropriate control signals DC 2 , DC 3  and DC 5  to the second, third and fifth deteriorated-stack connection switches DSW 2 , DSW 3  and DSW 5  and respectively electrically connects the common terminals D 2 _ 0 , D 3 _ 0  and D 5 _ 0  of the second, third, and fifth deteriorated-stack connection switches DSW 2 , DSW 3 , DSW 5  to the second terminals D 2 _ 2 , D 3 _ 2  and D 5 _ 2 . 
     The control unit  204  also transmits an appropriate control signal DC 6  to the sixth deteriorated-stack connection switch DSW 6  and electrically connects the common terminal D 6 _ 0  of the sixth deteriorated-stack connection switch DSW 6  to the second terminal D 6 _ 2 . 
     Therefore, the first and fourth stacks STX 1  and STX 4  that have deteriorated are connected in series to the sub-power conditioning system  202 . 
     When a target to which the output of the stacks is transmitted is changed in the above-mentioned manner, the main power conditioning system  200  can normally receive power from the second stack STX 2 , third stack STX 3  and sixth stack STX 6  that are normal and are connected in parallel to each other. The sub-power conditioning system  202  can receive power from the first stack STX 1  and fourth stack STX 4  that are deteriorated and connected in series to each other. 
     Also, output of the second stack STX 2 , third stack STX 3  and sixth stack STX 6  that are normal and are connected in parallel to each other is used to operate the main load  212  through the main power conditioning system  200 . 
     The output of the first and fourth stacks STX 1  and STX 4  that are deteriorated and are connected in series to each other is used to operate the sub-load  214 , that is, a partial load, through the sub-power conditioning system  202  in consideration of voltage. Therefore, the first and fourth stacks STX 1  and STX 4  that are deteriorated and are connected in series to each other operate at constant load current and do not affect to each other with regard to deterioration in performance, so that the speed of deterioration can be minimized. 
     Method for Controlling Fuel Cell System Using Sub-Power Conditioning System 
       FIG. 5  is a flowchart showing a method for controlling the fuel cell system using the sub-power conditioning system according to the present disclosure. 
     Referring to  FIG. 5 , at step S 500 , the control unit  204  senses conditions of the first through sixth stacks STK 1  through STK 6  and determines whether at least one deteriorated or inoperative stack is present from information about conditions of the first through sixth stacks STK 1  through STK 6   
     If, at step S 500 , it is determined that at least one deteriorated stack is present, the control unit  204  controls the switching units  206 ,  208  and  210  such that the deteriorated stack is separated from the main power conditioning system  200  and connected in series to the sub-power conditioning system  202 , at step  502 . 
     If, at step S 500 , it is determined that at least one inoperative stack is present, the control unit  204  controls the switching units  206 ,  208  and  210  such that the inoperative stack is separated and isolated both from the main power conditioning system  200  and from the sub-power conditioning system  202 , at step  504 . 
     As a result, the deteriorated or inoperative stack can be prevented from affecting the normal stacks, whereby a reduction in the lifetime and durability of the fuel cell system can be minimized. Furthermore, because deteriorated stacks are connected in series to each other and the output thereof is supplied to the sub-power conditioning system, the fuel cell system can be more efficiently used. 
     The methods discussed in this specification can be embodied by a variety of means depending on application examples. For instance, the methods may be embodied in a form of hardware, firmware, software or a combination of them. In an application example accompanying hardware, a control circuit or a control unit may be embodied by one or more application specific integrated circuits (ASICs), digital signal processors (DSPs), field programmable gate arrays (FPGAs), processors, controllers, micro-controllers, microprocessors, electronic devices, other electronic units designed to carry out the functions discussed in this specification, or a combination of them. 
     Although the embodiments of the present disclosure have been disclosed for illustrative purposes, it will be appreciated that the present disclosure is not limited thereto, and those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention. 
     Accordingly, any and all modifications, variations or equivalent arrangements should be considered to be within the scope of the invention, and the detailed scope of the invention will be disclosed by the accompanying claims.