Patent Publication Number: US-2023134153-A1

Title: Uninterruptible power supply system

Description:
TECHNICAL FIELD 
     The present invention relates to an uninterruptible power supply system. 
     BACKGROUND ART 
     Uninterruptible power supplies have been widely used as power supplies for stably supplying AC power to essential loads, such as computer systems. As is disclosed in Japanese Patent Laying-Open No. 10-257692 (PTL 1), for example, an uninterruptible power supply generally includes a converter for converting AC power into DC power, and an inverter for converting DC power into AC power. 
     While a commercial AC power supply is normal, the converter converts AC power from the commercial AC power supply into DC power, and supplies the DC power to the inverter while charging a power storage device such as a battery. The inverter converts the DC power into AC power and supplies the AC power to a load. When the commercial AC power supply fails, power from the power storage device is supplied to the inverter, so that the inverter continues to supply the AC power to the load. 
     The uninterruptible power supply described in PTL 1 is configured to implement, when the commercial AC power supply recovers after occurrence of failure, a soft start in which an AC input voltage that is input to the converter is gradually raised over a prescribed raising time, in order to suppress a sudden change in the commercial AC power supply due to a voltage drop and the like. In PTL 1, the converter is controlled so that the AC input voltage is gradually raised and a discharge current of the battery is gradually reduced, and accordingly, a current supplied to the load can be maintained at a constant value. 
     CITATION LIST 
     Patent Literature 
     PTL 1: Japanese Patent Laying-Open No. 10-257692 
     SUMMARY OF INVENTION 
     Technical Problem 
     In some uninterruptible power supplies, a converter has an AC power feeding function of converting AC power from a commercial AC power supply into DC power and supplying the DC power to an inverter, and a battery power feeding function of supplying DC power of a battery to the inverter. This type of uninterruptible power supply is configured to control the converter so as to selectively perform either the AC power feeding function or the battery power feeding function depending on whether or not the commercial AC power supply has failed. 
     With such a configuration in which the converter is used to serve both the AC power feeding function and the battery power feeding function, components such as a filter and a semiconductor module can be miniaturized. However, because the AC power feeding function and the battery power feeding function cannot be performed simultaneously, when the commercial AC power supply recovers, the converter cannot be controlled so that a discharge current of the battery is gradually reduced while an AC input voltage is gradually raised. 
     The present invention has been made to solve such a problem, and an object of the present invention is to provide an uninterruptible power supply system in which a soft start can be implemented when an AC power supply recovers, regardless of the configuration of an uninterruptible power supply. 
     Solution to Problem 
     In one aspect of the present invention, an uninterruptible power supply system includes a plurality of uninterruptible power supplies connected in parallel between an AC power supply and a load, and a control device that controls the plurality of uninterruptible power supplies. The plurality of uninterruptible power supplies each include a first switch connected between the AC power supply and an AC node, and a power conversion device configured to selectively receive power from one of the AC node and a power storage device, generate AC power, and supply the AC power to the load. The control device sets the plurality of uninterruptible power supplies to a first power feeding mode while the AC power supply is normal, and sets the plurality of uninterruptible power supplies to a second power feeding mode when the AC power supply fails. During the first power feeding mode, the control device turns on the first switch, and controls the power conversion device so as to receive AC power from the AC node and generate AC power. During the second power feeding mode, the control device turns off the first switch, and controls the power conversion device so as to receive DC power from the power storage device and generate AC power. When the AC power supply recovers during the second power feeding mode, the control device successively switches the plurality of uninterruptible power supplies to the first power feeding mode by successively turning on a plurality of the first switches each of which corresponds to each of the plurality of uninterruptible power supplies. 
     ADVANTAGEOUS EFFECTS OF INVENTION 
     According to the present invention, an uninterruptible power supply system can be provided in which a soft start can be implemented when an AC power supply recovers, regardless of the configuration of an uninterruptible power supply. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a circuit block diagram showing the configuration of an uninterruptible power supply system according to an embodiment. 
         FIG.  2    is a block diagram showing a configuration example of a UPS shown in  FIG.  1   . 
         FIG.  3    is a circuit diagram showing a configuration example of a converter shown in  FIG.  2   . 
         FIG.  4    is a circuit block diagram showing an AC power feeding mode (first power feeding mode). 
         FIG.  5    is a circuit block diagram showing a battery power feeding mode (second power feeding mode). 
         FIG.  6    is a timing chart showing a basic operation of the UPS when a commercial AC power supply recovers. 
         FIG.  7    is a timing chart showing an operation of the uninterruptible power supply system when the commercial AC power supply recovers. 
         FIG.  8    is a block diagram showing another configuration example of the UPS shown in  FIG.  1   . 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     An embodiment of the present invention will be described hereinafter in detail with reference to the drawings. The same or corresponding parts in the drawings are designated by the same characters and a description thereof will not be repeated in principle. 
     (Configuration of Uninterruptible Power Supply System) 
       FIG.  1    is a circuit block diagram showing the configuration of an uninterruptible power supply system according to an embodiment. 
     Referring to  FIG.  1   , an uninterruptible power supply system  100  according to the embodiment supplies AC power of a commercial frequency supplied from a commercial AC power supply  1  to a load  2 . Commercial AC power supply  1  may be a three-phase AC power supply or a single-phase AC power supply. 
     Uninterruptible power supply system  100  includes a plurality of uninterruptible power supplies (UPSs)  51  to  5 N (N being an integer equal to or greater than two), a voltage detector  3 , a current detector  4 , and a control device  6 . 
     The plurality of UPSs  51  to  5 N are connected in parallel between commercial AC power supply  1  and load  2 . The plurality of UPSs  51  to  5 N each have the same capacity in the present embodiment. The plurality of UPSs  51  to  5 N may have different capacities. In the following description, UPSs  51  to  5 N will also be collectively referred to as UPS  5 . A number N of UPSs may be any number equal to or greater than two, and can be set to any value depending on load  2 . 
     Voltage detector  3  detects an instantaneous value of an AC input voltage Vi supplied from commercial AC power supply  1 , and provides a signal Vi indicating the detected value to control device  6 . Current detector  4  detects an AC output current Io flowing from UPS  5  to load  2 , and provides a signal Io indicating the detected value to control device  6 . 
     Control device  6  can be implemented by, for example, a microcomputer or the like. As one example, control device  6  has a not-shown memory and a not-shown CPU (Central Processing Unit) embedded therein, and can execute the control operation described below by software processing by the CPU executing a program prestored in the memory. Alternatively, instead of software processing, some or all of the control operation can also be implemented by hardware processing using an embedded dedicated electronic circuit or the like. 
     Control device  6  controls UPSs  51  to  5 N based on AC input voltage Vi, AC output current Io, and the like. Specifically, control device  6  detects whether or not commercial AC power supply  1  has failed based on the detected value of AC input voltage Vi. Control device  6  also controls UPS  5  in synchronization with a phase of AC input voltage Vi. 
       FIG.  2    is a block diagram showing a configuration example of UPS  5  shown in  FIG.  1   . 
     As shown in  FIG.  2   , UPS  5  includes an AC input terminal T 1 , a battery terminal T 2 , an AC output terminal T 3 , a converter  7 , a DC line  8 , a capacitor  9 , a step-down chopper  10 , an inverter  11 , switches S 1 , S 2 , and a controller  12 . Converter  7 , DC line  8 , capacitor  9 , step-down chopper  10  and inverter  11  form a “power conversion device.” 
     AC input terminal T 1  receives AC power of a commercial frequency from commercial AC power supply  1 . Battery terminal T 2  is connected to a battery  13 . Battery  13  corresponds to an example of “power storage device” storing DC power. A capacitor may be connected instead of battery  13 . Battery  13  may be individually connected to each UPS  5 , or battery  13  may be commonly connected to the plurality of UPSs  51  to  5 N. AC output terminal T 3  is connected to load  2 . Load  2  is driven by AC power of a commercial frequency supplied from the plurality of UPSs  51  to  5 N. 
     Converter  7 , DC line  8  and inverter  11  are connected in series between AC input terminal T 1  and AC output terminal T 3 . Capacitor  9  is connected to DC line  8 , and smooths a DC voltage on DC line  8 . Step-down chopper  10  is connected between battery terminal T 2  and DC line  8 . 
     Switch S 1  has a first terminal connected to AC input terminal T 1 , and a second terminal connected to an AC node  7   a  of converter  7 . Switch S 2  has a first terminal connected to battery terminal T 2 , and a second terminal connected to a DC node  7   b  of converter  7 . Switch S 1  corresponds to an example of “first switch,” and switch S 2  corresponds to an example of “second switch.” Turning on (conduction)/off (non-conduction) of switches S 1  and S 2  is controlled by control device  6 . 
     Although an input filter is provided between the second terminal of switch S 1  and AC node  7   a  of converter  7  and an output filter is provided between inverter  11  and AC output terminal T 3  in actual UPS  5 , the input filter and the output filter are not illustrated for the sake of simplification of the drawing and the description. The input filter and the output filter are each a low-pass filter including a reactor and a capacitor, which allows a current of a commercial frequency to pass therethrough while blocking a current of a switching frequency. 
     The instantaneous value of AC input voltage Vi supplied from commercial AC power supply  1  and an instantaneous value of an AC input current Ii flowing through AC input terminal T 1  are detected by controller  12 . An instantaneous value of an AC output voltage Vo and an instantaneous value of AC output current Io supplied to load  2  are detected by controller  12 . An instantaneous value of a DC voltage VDC on DC line  8  is detected by controller  12 . An instantaneous value of an inter-terminal voltage VB of battery  13  is detected by controller  12 . 
     Controller  12  controls each of converter  7 , inverter  11  and step-down chopper  10  based on AC input voltage Vi, DC voltage VDC, inter-terminal voltage VB, AC output voltage Vo, and the like. 
     Converter  7  is controlled by controller  12 , and converts AC power from commercial AC power supply  1  into DC power and outputs the DC power to DC line  8 .  FIG.  3    is a circuit diagram showing a configuration example of converter  7  shown in  FIG.  2   . As shown in  FIG.  3   , converter  7  is a step-up high power factor converter, for example, having a rectifier  71  and a power factor correction (PFC) circuit  72 . 
     Rectifier  71  is a full-wave-rectifying diode rectifier having diodes D 1 , D 2 . Rectifier  71  rectifies an AC voltage that is input to AC node  7   a,  and outputs the rectified voltage to DC line  8  (a DC positive bus PL, a DC negative bus NL). Rectifier  71  may be a thyristor rectifier. 
     PFC circuit  72  has semiconductor switching elements Q 1 , Q 2 , reactors L 1 , L 2 , diodes D 3 , D 4 , and capacitors C 1 , C 2 . IGBTs (Insulated Gate Bipolar Transistors) are applied, for example, to semiconductor switching elements Q 1 , Q 2 . Reactor L 1 , semiconductor switching element Q 1  and diode D 3  form a step-up circuit. Reactor L 2 , semiconductor switching element Q 2  and diode D 4  form a step-up circuit. 
     Converter  7  further has diodes D 5 , D 6 , D 7 . Battery  13  is connected to PFC circuit  72  through switch S 2  and diodes D 5 , D 6 , D 7 . When switch S 2  is turned on, inter-terminal voltage VB of battery  13  is input to PFC circuit  72 . 
     When AC power is normally supplied from commercial AC power supply  1  (while commercial AC power supply  1  is normal), and switch S 1  is ON, converter  7  converts AC power from commercial AC power supply  1  into DC power and outputs the DC power to DC line  8  (DC positive bus PL and DC negative bus NL). At this time, converter  7  is controlled so as to set DC voltage VDC on DC line  8  to a reference DC voltage VDCr. 
     When commercial AC power supply  1  fails, and switch S 2  is ON, on the other hand, converter  7  performs a step-up operation of stepping up inter-terminal voltage VB of battery  13  and providing the stepped up voltage to DC line  8 . In other words, converter  7  selectively performs either the operation of converting AC power from commercial AC power supply  1  into DC power and outputting the DC power to DC line  8 , or the operation of stepping up inter-terminal voltage VB of battery  13  and outputting the stepped up voltage to DC line  8 . 
     Referring back to  FIG.  2   , step-down chopper  10  is controlled by controller  12 , and performs a step-down operation of stepping down DC voltage VDC on DC line  8  and providing the stepped down voltage to battery  13  when commercial AC power supply  1  fails. DC power is thus stored in battery  13 . Step-down chopper  10  is of well-known type including a plurality of sets of IGBTs and diodes, and a reactor. 
     Inverter  11  is controlled by controller  12 , and converts DC power supplied from converter  7  through DC line  8  into AC power of a commercial frequency and supplies the AC power to load  2 . Inverter  11  is of well-known type including a plurality of sets of IGBTs and diodes. 
     (Operation of Uninterruptible Power Supply System) 
     An operation of uninterruptible power supply system  100  according to the embodiment will now be described. 
     Uninterruptible power supply system  100  according to the embodiment has an alternating-current (AC) power feeding mode and a battery power feeding mode as operation modes. Control device  6  selects either the AC power feeding mode or the battery power feeding mode as the operation mode, based on whether or not commercial AC power supply  1  has failed, and performs the selected operation mode. The AC power feeding mode corresponds to a “first power feeding mode,” and the battery power feeding mode corresponds to a “second power feeding mode.” 
       FIG.  4    is a circuit block diagram showing the AC power feeding mode (first power feeding mode). 
     As shown in  FIG.  4   , while commercial AC power supply  1  is normal, control device  6  selects the AC power feeding mode. When the AC power feeding mode is selected, control device  6  turns on switch Si so that AC power is supplied from commercial AC power supply  1  to converter  7  through switch S 1 . Control device  6  also turns off switch S 2 . 
     In UPS  5 , controller  12  controls converter  7  so as to convert AC power supplied from commercial AC power supply  1  through switch Si into DC power. Controller  12  also controls inverter  11  so as to convert DC power from converter  7  into AC power and supply the AC power to load  2 . Controller  12  further controls step-down chopper  10  so as to store DC power from converter  7  in battery  13 . As shown in  FIG.  4   , when output power from commercial AC power supply  1  is represented by Pac, and output power from inverter  11  is represented by Po, output power Po is basically equal to output power Pac during the AC power feeding mode. 
       FIG.  5    is a circuit block diagram showing the battery power feeding mode (second power feeding mode). When commercial AC power supply  1  fails during the execution of the AC power feeding mode, control device  6  selects the battery power feeding mode. 
     As shown in  FIG.  5   , when the battery power feeding mode is selected, control device  6  turns on switch S 2  so that DC power is supplied from battery  13  to converter  7  through switch S 2 . Control device  6  also turns off switch S 1 . 
     In UPS  5 , controller  12  controls converter  7  so as to step up DC power supplied from battery  13  through switch S 2  and supply the stepped up power to inverter  11 . Controller  12  also controls inverter  11  so as to convert DC power from converter  7  into AC power and supply the AC power to load  2 . As shown in  FIG.  5   , when output power from battery  13  is represented by Pb, output power Po is equal to output power Pb during the battery power feeding mode. 
     When commercial AC power supply  1  recovers during the execution of the battery power feeding mode, control device  6  selects the AC power feeding mode. When the AC power feeding mode is selected, control device  6  turns on switch Si again, and turns off switch S 2  again, as shown in  FIG.  4   .  FIG.  6    is a timing chart showing a basic operation of UPS  5  when commercial AC power supply  1  recovers. 
     In  FIG.  6   , the ordinate represents output power Pb from battery  13  and output power Pac from commercial AC power supply  1 , and the abscissa represents time. For each of output powers Pb, Pac, output power Po from inverter  11  in one UPS  5  is defined as 100%. Output power Po from inverter  11  is equal to a power consumption P 1  of load  2  that has been divided by number N of UPSs  5  (Po=P 1 /N). 
     In  FIG.  6   , time to is the timing when commercial AC power supply  1  fails. Time t 0  is the timing when commercial AC power supply  1  recovers. Time tB is the timing when commercial AC power supply  1  is normal. 
     During a period prior to time tO (during failure of commercial AC power supply  1 ), UPS  5  is set to the battery power feeding mode. During the battery power feeding mode, control device  6  turns off switch S 1  and turns on switch S 2  in UPS  5 . 
     When commercial AC power supply  1  recovers at time t 0 , UPS  5  is switched from the battery power feeding mode to the AC power feeding mode. At time t 0 , control device  6  turns on switch S 1 , and turns off switch S 2 . Thus, during a period subsequent to time t 0  (while commercial AC power supply  1  is normal), UPS  5  is set to the AC power feeding mode. 
     When switch S 1  is turned on at the time of recovery of commercial AC power supply  1  (time t 0 ), as shown in  FIG.  6   , an inrush current may flow due to a rise of AC input voltage Vi, thus affecting commercial AC power supply  1  in terms of voltage variation and the like. It is concerned that this will cause a disturbance in another load (not shown) connected to commercial AC power supply  1 . 
     In order to suppress the effect on the commercial AC power supply at the time of recovery, a conventional UPS employs soft start control in which the rise of an AC input voltage to a converter is made gradual. For example, in PTL 1, when the commercial AC power supply recovers, the converter is controlled so that the AC input voltage to the converter is gradually raised over the prescribed voltage raising time (dVi/dt). With the AC input voltage being gradually raised, an AC input current to the converter can also be gradually raised. 
     Further, in PTL 1, the discharge current of the battery is gradually reduced, while the AC input current to the converter is gradually increased, and accordingly, the current supplied to the load (in other words, an inverter output current) is maintained at a constant value. Thus, the load can continue operating in a stable manner when the commercial AC power supply recovers. 
     In the configuration example of UPS  5  shown in  FIG.  3   , however, step-down chopper  10  can only charge battery  13 , and cannot discharge battery  13 . In addition, converter  7  is capable of selecting and performing either the AC/DC conversion operation of converting AC power from commercial AC power supply  1  into DC power (see  FIG.  4   ) or the discharge operation of stepping up inter-terminal voltage VB of battery  13  and providing the stepped up voltage to DC line  8  (see  FIG.  5   ). 
     Since UPS  5  is configured so that converter  7  is used to serve both the power feeding from commercial AC power supply  1  (AC power feeding) and the power feeding from battery  13  (battery power feeding) in this manner, the AC power feeding and the battery power feeding cannot be performed simultaneously. In UPS  5 , therefore, the discharge current of battery  13  cannot be gradually reduced while the AC input voltage to converter  7  is gradually raised, as in a conventional UPS. It is thus difficult to soft start UPS  5  when commercial AC power supply  1  recovers. 
     Therefore, uninterruptible power supply system  100  according to the present embodiment employs a configuration in which the plurality of UPSs  51  to  5 N are connected in parallel, and successively switches these plurality of UPSs  51  to  5 N to the AC power feeding mode when commercial AC power supply  1  recovers. Thus, a soft start is implemented in entire uninterruptible power supply system  100 . 
       FIG.  7    is a timing chart showing an operation of uninterruptible power supply system  100  when commercial AC power supply  1  recovers. In  FIG.  7   , the ordinate represents output power Pb from battery  13  and output power Pac from commercial AC power supply  1  in each UPS  5 , as well as a total value Pb_total of output powers Pb and a total value Pac_total of output powers Pac in entire uninterruptible power supply system  100 , and the abscissa represents time. 
     For each of output powers Pb, Pac, output power Po from inverter  11  in one UPS  5  is defined as 100%. Output power Po from inverter  11  is equal to power consumption P 1  of load  2  that has been divided by number N of UPSs  5  (Po=P 1 /N). For each of output powers Pb_total, Pac_total, on the other hand, power consumption P 1  of load  2  is defined as 100%. 
     In  FIG.  7   , time t 1  is the timing when commercial AC power supply  1  recovers. In other words, during a period prior to time t 1 , the plurality of UPSs  51  to  5 N are all set to the battery power feeding mode. In other words, switch S 1  is OFF and switch S 2  is ON in each UPS  5  (see  FIG.  5   ). Output power Pb_total is equal to a total of output powers Pb from the plurality of UPSs  51  to  5 N, and is equal to power consumption P 1 . 
     When commercial AC power supply  1  recovers at time t 1 , control device  6  successively switches the plurality of UPSs  51  to  5 N to the AC power feeding mode. Specifically, control device  6  successively turns on the plurality of switches S 1 , and successively turns off the plurality of switches S 2 . 
     In the example of  FIG.  7   , control device  6  first turns on switch S 1  and turns off switch S 2  in first UPS  51 . At this time, control device  6  maintains switches S 1  in the off state and maintains switches S 2  in the on state in remaining (N−1) UPSs  52  to  5 N. Thus, at time t 1 , only first UPS  51  is switched to the AC power feeding mode. 
     As a result of the switching of first UPS  51  to the AC power feeding mode at time t 1 , output power Pb_total is reduced by output power Pb, and output power Pac_total is increased by output power Pac. Since Pb=Pac=Po holds, the power supplied from uninterruptible power supply system  100  to load  2  is maintained at a constant value. 
     Then, at time t 2  when a prescribed time At has elapsed since time t 1 , control device  6  turns on switch S 1  and turns off switch S 2  in second UPS  52 . At this time, control device  6  maintains switches S 1  in the off state and maintains switches S 2  in the on state in remaining (N−2) UPSs  53  to  5 N. Thus, at time t 2 , only second UPS  52  is switched to the AC power feeding mode. As a result, first UPS  51  and second UPS  52  perform the AC power feeding mode, and third to Nth UPSs  53  to  5 N perform the battery power feeding mode. 
     As a result of the switching of second UPS  52  to the AC power feeding mode at time t 2 , output power Pb_total is further reduced by output power Pb, and output power Pac_total is further increased by output power Pac. Since Pb=Pac=Po holds, the power supplied from uninterruptible power supply system  100  to load  2  is maintained at a constant value. 
     During a period between time t 3  when prescribed time Δt has elapsed since time t 2  and time tN, control device  6  successively switches remaining (N−2) UPSs  53  to  5 N to the AC power feeding mode. Each time UPS  5  is switched to the AC power feeding mode, output power Pb_total is reduced by output power Pb, and output power Pac_total is increased by output power Pac. Since Pb=Pac=Po holds, the power supplied from uninterruptible power supply system  100  to load  2  is maintained at a constant value. 
     When Nth UPS  5 N is switched to the AC power feeding mode at time tN, the plurality of UPSs  51  to  5 N are all set to the AC power feeding mode. Output power Pac_total is equal to a total of output powers Pac from the plurality of UPSs  51  to  5 N, and is equal to power consumption P 1 . 
     In this manner, control device  6  successively switches the plurality of UPS s  51  to  5 N to the AC power feeding mode by successively turning on switches S 1  and successively turning off switches S 2  in the plurality of UPSs  51  to  5 N. Thus, gradual switching from the battery power feeding to the AC power feeding takes place in entire uninterruptible power supply system  100  when commercial AC power supply  1  recovers. After time t 1 , in response to the successive switching of the plurality of UPSs  51  to  5 N to the AC power feeding mode, a soft start is implemented in which AC input voltage Vi and AC input current Ii from commercial AC power supply  1  are gradually raised. Thus, the effect on commercial AC power supply  1  at the time of recovery can be suppressed. In addition, since AC input voltage Vi and AC input current Ii are gradually raised and the discharge current of battery  13  is gradually reduced, it is possible to continue to supply the constant power to load  2  at the time of recovery as well. 
     Although  FIG.  7    illustrates a configuration in which the plurality of UPSs  51  to  5 N are successively switched one by one to the AC power feeding mode, the plurality of UPSs  51  to  5 N may be divided into a plurality of groups each including one or a plurality of UPSs, and successively switched to the AC power feeding mode on a group-by-group basis. 
     Although  FIG.  7    illustrates a configuration in which the plurality of UPSs  51  to  5 N are successively switched to the AC power feeding mode with a constant time difference (prescribed time Δt), time difference Δt does not necessarily need to be constant. 
     Other Configuration Examples 
     Although UPS  5  is described in the embodiment above as being configured so that converter  7  is used to serve both the AC power feeding and the battery power feeding, the present invention is also applicable to a UPS configured so that the AC power feeding and the battery power feeding are performed independently of each other in a converter and a chopper.  FIG.  8    is a block diagram showing another configuration example of UPS  5  shown in  FIG.  1   . In UPS  5  according to the present variation, switch S 2  (second switch) is removed from UPS  5  shown in  FIG.  2   , and converter  7  and step-down chopper  10  are replaced by a converter  7 A and a bidirectional chopper  10 A, respectively. Converter  7 A, DC line  8 , capacitor  9 , bidirectional chopper  10 A and inverter  11  form a “power conversion device.” 
     Converter  7 A is controlled by controller  12 , and converts AC power from commercial AC power supply  1  into DC power and outputs the DC power to DC line  8 . Converter  7 A is a PWM (Pulse Width Modulation) converter, for example. Converter  7  is of well-known type including a plurality of sets of IGBTs and diodes. 
     While commercial AC power supply  1  is normal, and switch S 1  is ON, converter  7  converts AC power from commercial AC power supply  1  into DC power and outputs the DC power to DC line  8 . At this time, converter  7  is controlled so as to set DC voltage VDC on DC line  8  to reference DC voltage VDCr. When commercial AC power supply  1  fails, on the other hand, controller  12  causes converter  7  to stop operating. 
     Bidirectional chopper  10 A is controlled by controller  12 , and supplies and receives DC power between DC line  8  and battery  13 . Bidirectional chopper  10 A is of well-known type including a plurality of sets of IGBTs and diodes, and a reactor. Bidirectional chopper  10 A selectively performs either a step-down operation of stepping down DC voltage VDC on DC line  8  and providing the stepped down voltage to battery  13 , or a step-up operation of stepping up inter-terminal voltage VB of battery  13  and providing the stepped up voltage to DC line  8 . 
     While commercial AC power supply  1  is normal, bidirectional chopper  10 A supplies DC power from DC line  8  to battery  13  so as to set inter-terminal voltage VB of battery  13  to a reference battery voltage VBr. When commercial AC power supply  1  fails, on the other hand, bidirectional chopper  10 A supplies DC power from battery  13  to DC line  8  so as to set DC voltage VDC on DC line  8  to reference DC voltage VDCr. Thus, DC power corresponding to output power Po from inverter  11  is supplied only from battery  13 . 
     According to UPS  5  in the present variation, the AC power feeding is performed by converter  7 A, and the battery power feeding is performed by bidirectional chopper  10 A. Accordingly, when commercial AC power supply  1  recovers, controller  12  can implement a soft start by performing control so as to gradually increase the AC input current to converter  7 A and gradually reduce the discharge current of battery  13 , as in a conventional UPS. 
     According to uninterruptible power supply system  100  in the present embodiment, however, the plurality of UPSs  51  to  5 N connected in parallel with respect to commercial AC power supply  1  are successively switched to the AC power feeding mode, thus allowing a soft start to be implemented in entire uninterruptible power supply system  100 , without the need for each UPS  5  to perform the conventional soft start control. 
     Specifically, control device  6  successively switches the plurality of UPSs  51  to  5 N to the AC power feeding mode by providing a difference in timing to turn on switch S 1  among the plurality of UPSs  51  to  5 N. In each UPS  5 , when switch S 1  is turned on, controller  12  causes bidirectional chopper  10 A to stop its step-up operation. Thus, as in the timing chart of  FIG.  7   , output power Pac_total can be gradually increased, and output power Pb_total can be gradually reduced. 
     It should be understood that the embodiment disclosed herein is illustrative and non-restrictive in every respect. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims. 
     REFERENCE SIGNS LIST 
       1  commercial AC power supply;  2  load;  5 ,  51  to  5 N UPS;  7  converter;  8  DC line;  9  capacitor;  10  step-down chopper;  10 A bidirectional chopper;  11  inverter;  12  controller;  13  battery (power storage device);  100  uninterruptible power supply system; Si switch (first switch); S 2  switch (second switch).