Uninterruptible power supply device

Abstract

An uninterruptible power supply device (100) comprises: a converter (4) that converts AC power supplied from an AC power supply (12) to DC power; an inverter (8) that converts the DC power to AC power and supplies the converted AC power to a load (14); a DC link (5) connected between the converter (4) and the inverter (8); and a bidirectional chopper (7) that performs DC voltage conversion between the DC link (5) and an energy storage device (13). The bidirectional chopper (7) executes a charging operation of storing DC power at the DC link (5) in the energy storage device (13) when the AC power supply (12) is healthy. When executing the charging operation in a situation where an end-of-charge voltage of the energy storage device (13) is higher than a DC link voltage of the DC link (5) and an end-of-discharge voltage of the energy storage device (13) is lower than the DC link voltage, the bidirectional chopper (7) switches between a first buck operation for bucking the DC link voltage and a first boost operation for boosting the DC link voltage, depending on the voltage of the energy storage device (13).

Description

uninterruptible power supply

[0001] The present disclosure relates to uninterruptible power supplies.

[0002] For example, International Publication No. 2021 / 130981 (Patent Document 1) discloses an uninterruptible power supply including a power converter connected between an AC power source and a load. The power converter includes a converter that converts AC power supplied from the AC power source into DC power and outputs the DC power to a DC link, an inverter that converts DC power received from the DC link into AC power and supplies the AC power to the load, and a bidirectional chopper that exchanges DC power between the DC link and a power storage device.

[0003] In this uninterruptible power supply, the storage device is used as a power storage device that stores DC power to be used in the event of an AC power outage. When the AC power supply is normal, the storage device is charged, and when the AC power supply is out of service, the bidirectional chopper is controlled to discharge the storage device.

[0004] International Publication No. 2021 / 130981

[0005] Many conventional uninterruptible power supplies use lead-acid batteries as their power storage devices. In conventional uninterruptible power supplies, the DC link voltage input to the inverter is increased so that the inverter output voltage is directly converted into the AC output voltage of the uninterruptible power supply. On the other hand, in order to minimize the number of lead-acid batteries used and thereby reduce the size of the power storage device, a power storage device with an operating voltage range lower than the DC link voltage is used. Therefore, the bidirectional chopper is configured to step down the DC link voltage and output it to the power storage device when charging the power storage device, and to step up the voltage of the power storage device and output it to the DC link when discharging the power storage device.

[0006] In recent years, the use of lithium-ion batteries has been increasing in various products such as mobile devices, electric vehicles, and industrial robots. Compared to lead-acid batteries, lithium-ion batteries have many advantages, including i) high energy density, which allows for small and lightweight designs; ii) high voltage and large current, which allows for a reduction in the number of batteries used; iii) long life; iv) high-speed charging; and v) a wide usable temperature range.

[0007] By using this lithium-ion battery in the power storage device of an uninterruptible power supply, it is possible to expand the operating voltage range of the power storage device while suppressing the increase in size of the power storage device. Furthermore, using lithium-ion batteries in the power storage device offers advantages such as a long life, high-speed charging, and a wide operating temperature range. However, in uninterruptible power supplies, there are cases where the upper limit of the operating voltage range of the power storage device is higher than the DC link voltage. To address such cases, improvements to the bidirectional chopper are required.

[0008] The present disclosure has been made to solve such problems, and an object of the present disclosure is to provide an uninterruptible power supply that can also be used with a power storage device that has a wide operating voltage range.

[0009] According to an aspect of the present disclosure, an uninterruptible power supply is connected between an AC power supply and a load. The uninterruptible power supply includes: a converter that converts AC power supplied from the AC power supply into DC power; an inverter that converts the DC power into AC power and supplies the DC power to the load; a DC link connected between the converter and the inverter for inputting the DC power to the inverter; and a bidirectional chopper that performs DC voltage conversion between the DC link and a power storage device. The bidirectional chopper is configured to perform a charging operation to store DC power from the DC link in the power storage device when the AC power supply is healthy. When the charging end voltage of the power storage device is higher than the DC link voltage of the DC link and the discharging end voltage of the power storage device is lower than the DC link voltage, the bidirectional chopper switches between a first step-down operation that steps down the DC link voltage and a first step-up operation that steps up the DC link voltage, depending on the voltage of the power storage device.

[0010] According to the present disclosure, it is possible to provide an uninterruptible power supply that can also be used with a power storage device that operates over a wide range of voltages.

[0011] FIG. 1 is a circuit block diagram showing the configuration of an uninterruptible power supply according to a first embodiment. FIG. 2 is a block diagram showing an example of the hardware configuration of a control device. FIG. 3 is a diagram explaining the operation of the uninterruptible power supply when the AC power supply is healthy. FIG. 4 is a diagram explaining the charging operation of a battery by a bidirectional chopper. FIG. 5 is a diagram explaining the charging operation of a battery by a bidirectional chopper. FIG. 6 is a diagram explaining the operation of the uninterruptible power supply during a power outage of the AC power supply. FIG. 7 is a diagram explaining the discharging operation of a battery by a bidirectional chopper. FIG. 8 is a diagram explaining the discharging operation of a battery by a bidirectional chopper. FIG. 9 is a flowchart explaining control of a bidirectional chopper by a control device. FIG. 10 is a circuit diagram showing a first example configuration of a bidirectional chopper. FIG. 11 is a diagram explaining the operation of a bidirectional chopper according to the first example configuration. FIG. 12 is a circuit diagram showing a second example configuration of a bidirectional chopper. FIG. 13 is a diagram explaining the operation of a bidirectional chopper according to the second example configuration. FIG. 14 is a circuit block diagram showing a third example configuration of a bidirectional chopper. FIG. 15 is a diagram explaining the operation of a bidirectional chopper according to the third example configuration.

[0012] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present disclosure will be described in detail with reference to the accompanying drawings. In the drawings, the same or corresponding parts are designated by the same reference numerals and the description thereof will not be repeated.

[0013] [Embodiment 1] <Configuration of Uninterruptible Power Supply> Fig. 1 is a circuit block diagram showing the configuration of an uninterruptible power supply according to embodiment 1. As shown in Fig. 1, uninterruptible power supply 100 is connected between AC power supply 12 and load 14. Note that uninterruptible power supply 100 receives a three-phase AC voltage from AC power supply 12 and supplies the three-phase AC voltage to load 14, but for simplicity of the drawing and explanation, Fig. 1 shows only a circuit for one phase.

[0014] The uninterruptible power supply 100 includes an input terminal T1, a DC terminal T2, an output terminal T3, switches S1 to S3, a converter 4, current detectors CD1 to CD3, a DC link 5, a capacitor 6, an inverter 8, an operation unit 9, and a control device 10.

[0015] The input terminal T1 receives AC power of a predetermined frequency (e.g., commercial frequency) from an AC power source 12. The AC power source 12 may be a commercial AC power source or a generator. The instantaneous value of the AC input voltage VI is detected by the control device 10. The presence or absence of a power outage is determined based on the instantaneous value of the AC input voltage VI. The current detector CD1 detects the AC input current Ii flowing through the input terminal T1 and provides a signal Iif indicating the detected value to the control device 10.

[0016] The output terminal T3 is connected to the load 14. The load 14 is driven by AC power of a predetermined frequency (for example, a commercial frequency) supplied from the uninterruptible power supply 100.

[0017] The DC terminal T2 is connected to a battery 13. The battery 13 constitutes a "power storage device" that stores DC power. However, a capacitor may be connected instead of the battery 13. The instantaneous value of the terminal voltage VB of the battery 13 is detected by the control device 10. In the following description, the terminal voltage VB of the battery 13 is also referred to as the "battery voltage VB."

[0018] The switch S1 is connected between the input terminal T1 and the AC node of the converter 4, and is controlled by the control device 10. When AC power is being supplied normally from the AC power supply 12 (when the AC power supply 12 is operating normally), the switch S1 is turned on, and AC power is supplied from the AC power supply 12 to the converter 4 via the switch S1. When AC power is not being supplied normally from the AC power supply 12 (when the AC power supply 12 is experiencing a power outage), the switch S1 is turned off, and the AC power supply 12 and the converter 4 are disconnected.

[0019] The converter 4 is controlled by the control device 10, and when the AC power supply 12 is healthy, converts AC power from the AC power supply 12 into DC power and outputs it to the DC link 5. The converter 4 is a well-known device that includes multiple sets of semiconductor switching elements and diodes.

[0020] The capacitor 6 is connected to the DC link 5 and smooths and stabilizes the DC link voltage VD. The instantaneous value of the DC voltage VD of the DC link 5 is detected by the control device 10. In the following description, the DC voltage VD of the DC link 5 is also referred to as the "DC link voltage VD."

[0021] When the AC power supply 12 is normal, the control device 10 controls the converter 4 so that the DC link voltage VD becomes equal to the reference DC voltage VDR. When the AC power supply 12 experiences a power outage, the control device 10 stops the operation of the converter 4.

[0022] The DC link 5 is connected to the DC terminal T2 via the bidirectional chopper 7 and the switch S2. The switch S2 is controlled by the control device 10. When the uninterruptible power supply 100 is in use, the switch S2 is turned on. When maintenance of the battery 13 and the bidirectional chopper 7 is performed, the switch S2 is turned off.

[0023] The bidirectional chopper 7 is controlled by the control device 10, and performs DC voltage conversion between the DC link 5 and the battery 13, thereby exchanging DC power between the DC link 5 and the battery 13. The bidirectional chopper 7 is configured to be capable of selectively performing a charging operation for storing DC power from the DC link 5 in the battery 13, and a discharging operation for supplying the DC power stored in the battery 13 to the DC link 5. The current detector CD2 detects a DC current IB flowing between the battery 13 and the bidirectional chopper 7, and provides a signal IBf indicating the detected value to the control device 10.

[0024] When the AC power supply 12 is healthy, the control device 10 controls the bidirectional chopper 7 so that the battery voltage VB becomes the reference DC voltage VBR. When a power outage occurs in the AC power supply 12, the control device 10 controls the bidirectional chopper 7 so that the DC link voltage VD becomes the reference DC voltage VDR. The bidirectional chopper 7 will be described in detail later.

[0025] Furthermore, the DC link 5 is connected to a DC node of the inverter 8, and the AC node of the inverter 8 is connected to the output terminal T3 via a switch S3. The switch S3 is controlled by the control device 10. When the uninterruptible power supply 100 is in use, the switch S3 is turned on. When maintenance of the inverter 8 is performed, the switch S3 is turned off.

[0026] The current detector CD3 detects the AC output current IO of the inverter 8 and provides a signal IOf indicating the detected value to the control device 10. The instantaneous value of the AC output voltage VO applied to the load 14 is detected by the control device 10.

[0027] The inverter 8 is controlled by a control device 10, and converts DC power supplied from the converter 4 and the bidirectional chopper 7 via the DC link 5 into AC power of a predetermined frequency (for example, a commercial frequency), and supplies the AC power to a load 14. The inverter 8 is a well-known device that includes multiple sets of semiconductor switching elements and diodes.

[0028] The operation unit 9 includes a plurality of buttons, a plurality of switches, and an image display unit. By operating the operation unit 9, a user of the uninterruptible power supply 100 can turn the power of the uninterruptible power supply 100 on and off, and operate the uninterruptible power supply 100 automatically or manually. The operation unit 9 outputs signals and information indicating the content of operations performed by the user to the control device 10.

[0029] The control device 10 controls the switches S1 to S3, the converter 4, the bidirectional chopper 7, and the inverter 8 based on signals from the operation unit 9, the AC input voltage VI, the AC output voltage VO, the DC link voltage VD, the battery voltage VB, the AC input current Ii, the DC current IB, and the AC output current IO.

[0030] 2 is a block diagram showing an example of the hardware configuration of the control device 10. Typically, the control device 10 can be configured by a microcomputer in which a predetermined program is stored in advance.

[0031] 2, the control device 10 includes a CPU (Central Processing Unit) 102, a memory 104, and an input / output (I / O) circuit 106. The CPU 102, the memory 104, and the I / O circuit 106 can exchange data with one another via a bus 108. Programs are stored in a partial area of ​​the memory 104, and the CPU 102 executes these programs to realize various functions described below. The I / O circuit 106 inputs and outputs signals and data to and from the outside of the control device 10.

[0032] 2, at least a part of the control device 10 may be configured using a circuit such as an FPGA (Field Programmable Gate Array) or an ASIC (Application Specific Integrated Circuit), or at least a part of the control device 10 may be configured using an analog circuit.

[0033] In the uninterruptible power supply 100 shown in Fig. 1, for example, a lithium-ion battery is used as the battery 13. Compared to lead-acid batteries, lithium-ion batteries have the following advantages: i) high energy density allows for size and weight reduction; ii) high voltage and large current are available, allowing for a reduction in the number of batteries used; iii) long life; iv) high-speed charging; and v) usable environmental temperature range. For these reasons, lithium-ion batteries have been increasingly used in various products in recent years, such as mobile devices, electric vehicles, and industrial robots.

[0034] The battery 13 has a usable voltage range. In this specification, the lower limit of the usable voltage range is referred to as the "discharge end voltage V1," and the upper limit of the usable voltage range is referred to as the "charge end voltage V2." The discharge end voltage V1 is the lower limit of the discharge voltage at which safe discharging can be performed. The charge end voltage V2 is the upper limit of the charge voltage at which safe charging can be performed. A state in which the battery is discharged to a voltage below the discharge end voltage V1 is called "over-discharge," and a state in which the battery is charged to a voltage above the charge end voltage V2 is called "over-charge," both of which can cause deterioration in the performance of the battery 13.

[0035] In this embodiment, the relationship V1<VD<V2 is established among the discharge end voltage V1, the charge end voltage V2, and the DC link voltage VD. That is, the charge end voltage V2 is higher than the DC link voltage VD, and the discharge end voltage V1 is lower than the DC link voltage VD.

[0036] In a conventional uninterruptible power supply using a lead-acid battery, the DC link voltage VD input to the inverter 8 is increased so that the output voltage of the inverter 8 becomes the AC output voltage of the uninterruptible power supply. On the other hand, in order to minimize the number of lead-acid batteries used and thereby reduce the size of the battery 13, a step-up / step-down chopper is inserted between the DC link 5 and the battery 13, and the battery voltage VB is increased when the battery 13 is discharged, and the DC link voltage VD is decreased when the battery 13 is charged. In this configuration, the relationship V1<V2<VD holds between the battery voltage VB and the DC link voltage VD. That is, the discharge end voltage V1 and the charge end voltage V2 are lower than the DC link voltage VD.

[0037] By using a lithium ion battery for the battery 13 of the uninterruptible power supply 100, it is possible to meet the demand for a smaller battery 13 while widening the operating voltage range of the battery 13 compared to conventional uninterruptible power supplies. Therefore, as in this embodiment, it is possible to use a battery whose end-of-charge voltage V2 is higher than the DC link voltage VD as the battery 13. Furthermore, by using a lithium ion battery for the battery 13, it is possible to enjoy the advantages of a long life, high-speed charging, and a wide usable environmental temperature range.

[0038] <Operation of Uninterruptible Power Supply> Next, the operation of uninterruptible power supply 100 according to the first embodiment will be described.

[0039] 3 is a diagram illustrating the operation of the uninterruptible power supply 100 when the AC power supply 12 is normal. The arrows in the diagram indicate the flow of power exchanged between the AC power supply 12, the load 14, and the battery 13.

[0040] 3 , the converter 4 converts AC power supplied from the AC power source 12 into DC power and outputs it to the DC link 5. The inverter 8 converts DC power input from the DC link 5 into AC power and supplies it to the load 14.

[0041] The bidirectional chopper 7 stores the DC power supplied from the converter 4 via the DC link 5 in the battery 13. The control device 10 controls the bidirectional chopper 7 so that the battery voltage VB becomes equal to the reference DC voltage VBR. The reference DC voltage VBR is set to the end-of-charge voltage V2 of the battery 13. When the battery voltage VB increases and reaches the end-of-charge voltage V2, the control device 10 stops the bidirectional chopper 7, thereby stopping the charging of the battery 13.

[0042] 4A and 4B are diagrams illustrating the charging operation of the battery 13 by the bidirectional chopper 7. Fig. 4A shows how DC power is transmitted from the DC link 5 to the battery 13 by the bidirectional chopper 7. In this case, the input voltage of the bidirectional chopper 7 is the DC link voltage VD, and the output voltage of the bidirectional chopper 7 is the battery voltage VB.

[0043] 4B shows an example of the change over time of the battery voltage VB during charging. In the example of FIG. 4B, the battery voltage VB is equal to the discharge cut-off voltage V1 at time t1 when charging of the battery 13 starts. As charging progresses, the battery voltage VB gradually increases from V1. Then, at time t3, the battery voltage VB reaches the charge cut-off voltage V2, and charging of the battery 13 is stopped.

[0044] Here, there is a relationship V1<VD<V2 among the discharge end voltage V1, the charge end voltage V2, and the DC link voltage VD. Therefore, when charging the battery 13, the bidirectional chopper 7 switches between a step-down operation for stepping down the DC link voltage VD, which is the input voltage, and a step-up operation for stepping up the DC link voltage VD, depending on the battery voltage VB.

[0045] Specifically, the bidirectional chopper 7 performs a step-down operation during a period in which the battery voltage VB is lower than the DC link voltage VD (a period from time t1 to t2), and performs a step-up operation during a period in which the battery voltage VB is higher than the DC link voltage VD (a period from time t2 to t3).

[0046] 5 is a diagram illustrating the operation of the uninterruptible power supply 100 during a power outage of the AC power supply 12. The arrows in the diagram indicate the flow of power exchanged between the AC power supply 12, the load 14, and the battery 13. When a power outage occurs in the AC power supply 12, the switch S1 provided between the input terminal T1 and the AC terminal of the converter 4 is turned off, thereby disconnecting the AC power supply 12 from the uninterruptible power supply 100. The operation of the converter 4 is stopped.

[0047] The bidirectional chopper 7 supplies DC power from the battery 13 to the DC link 5. The inverter 8 converts the DC power input from the DC link 5 into AC power and supplies it to the load 14. The control device 10 controls the bidirectional chopper 7 so that the DC link voltage VD becomes equal to the reference DC voltage VDR. When the battery voltage VB decreases due to the discharge of the battery 13 and reaches the discharge end voltage V1, the control device 10 stops the bidirectional chopper 7, thereby stopping the discharge of the battery 13.

[0048] 6A and 6B are diagrams illustrating the discharge operation of the battery 13 by the bidirectional chopper 7. Fig. 6A shows how DC power is transmitted from the battery 13 to the DC link 5 by the bidirectional chopper 7. In this case, the input voltage of the bidirectional chopper 7 is the battery voltage VB, and the output voltage of the bidirectional chopper 7 is the DC link voltage VD.

[0049] 6B shows an example of the change over time of the battery voltage VB during a discharging operation. In the example of FIG. 6B, the battery voltage VB is equal to the charge cut-off voltage V2 at time t4 when discharging of the battery 13 starts. As discharging progresses, the battery voltage VB gradually decreases from V2. Then, at time t6, the battery voltage VB reaches the discharge cut-off voltage V1, and discharging of the battery 13 is stopped.

[0050] During the discharging operation, the bidirectional chopper 7 switches between a step-down operation for stepping down the battery voltage VB, which is an input voltage, and a step-up operation for stepping up the battery voltage VB, depending on the battery voltage VB. Specifically, the bidirectional chopper 7 performs the step-down operation during the period when the battery voltage VB is higher than the DC link voltage VD (the period from time t4 to t5). The bidirectional chopper 7 performs the step-up operation during the period when the battery voltage VB is lower than the DC link voltage VD (the period from time t5 to t6).

[0051] Fig. 7 is a flowchart for explaining the control of the bidirectional chopper 7 by the control device 10. The flowchart of Fig. 7 is repeatedly executed by the control device 10 when the uninterruptible power supply 100 is in operation.

[0052] 7, the control device 10 determines whether or not a power outage has occurred in the AC power supply 12 based on the detected value of the AC input voltage VI in step (hereinafter simply referred to as "S") 01. If the AC input voltage VI is within the normal range, a NO determination is made in S01, and if the AC input voltage VI is lower than the normal range, a YES determination is made in S01.

[0053] If the AC power supply 12 is healthy (NO in S01), the control device 10 proceeds to S02 and controls the bidirectional chopper 7 to store the DC power of the DC link 5 in the battery 13 (see FIG. 3).

[0054] While the battery 13 is being charged, the control device 10 compares the battery voltage VB with the DC link voltage VD in S03. If VB<VD (YES in S03), the control device 10 controls the bidirectional chopper 7 in S04 to step down the DC link voltage VD and output it to the battery 13. If VB>VD (NO in S03), the control device 10 controls the bidirectional chopper 7 in S05 to step up the DC link voltage VD and output it to the battery 13.

[0055] Returning to S01, if a power outage has occurred in the AC power supply 12 (YES in S01), the control device 10 proceeds to S06 and controls the bidirectional chopper 7 to supply DC power from the battery 13 to the DC link 5 (see FIG. 5 ).

[0056] During the discharge of the battery 13, the control device 10 compares the battery voltage VB with the DC link voltage VD in S07. If VB > VD (YES in S07), the control device 10 controls the bidirectional chopper 7 in S08 to step down the battery voltage VB and output it to the DC link 5. If VB < VD (NO in S07), the control device 10 controls the bidirectional chopper 7 in S09 to step up the battery voltage VB and output it to the DC link 5.

[0057] As described above, when charging the battery 13, the bidirectional chopper 7 switches between stepping down and stepping up the DC link voltage VD in accordance with the battery voltage VB. When discharging the battery 13, the bidirectional chopper 7 switches between stepping down and stepping up the battery voltage VB in accordance with the battery voltage VB. This makes it possible to charge and discharge the battery 13, whose charge end voltage V2 is higher than the DC link voltage VD.

[0058] <Configuration Example of Bidirectional Chopper> Next, a configuration example of the bidirectional chopper 7 shown in FIG. 1 will be described.

[0059] (First Configuration Example) Fig. 8 is a circuit diagram showing a first configuration example of the bidirectional chopper 7. While Fig. 1 shows only the positive side DC link 5, Fig. 8 also shows the negative side DC link 5n.

[0060] 8, the bidirectional chopper 7 according to the first configuration example includes a pair of DC terminals T11, T12, a pair of DC terminals T13, T14, semiconductor switching elements (hereinafter also simply referred to as "switching elements") Q1, Q2, diodes D1, D2, capacitors C1 to C3, and reactors L1, L2. In FIG. 8, IGBTs (Insulated Gate Bipolar Transistors) are used as the switching elements Q1, Q2, but any semiconductor elements such as MOSFETs (Metal Oxide Semiconductor Field Effect Transistors) can be used.

[0061] A pair of DC terminals T11, T12 are connected to a battery 13. The positive DC terminal T11 is connected to the positive electrode of the battery 13, and the negative DC terminal T12 is connected to the negative electrode of the battery 13. A pair of DC terminals T13, T14 are connected to DC links 5, 5n. The positive DC terminal T13 is connected to the positive DC link 5, and the negative DC terminal T14 is connected to the negative DC link 5n.

[0062] The collector of the switching element Q1 is connected to the positive DC terminal T11, and the emitter of the switching element Q1 is connected to a first terminal of a capacitor C2. A second terminal of the capacitor C2 is connected to a first terminal of a reactor L2. A second terminal of the reactor L2 is connected to the positive DC terminal T13. The switching element Q1 corresponds to an example of a "first switching element."

[0063] The capacitor C1 is connected between the positive side DC terminal T11 and the negative side DC terminal T12. A first terminal of the reactor L1 is connected to the emitter of the switching element Q1 and the first terminal of the capacitor C1, and a second terminal of the reactor L1 is connected to the negative side DC terminals T12 and T14.

[0064] The collector of the switching element Q2 is connected to the second terminal of the capacitor C2 and the first terminal of the reactor L2, and the emitter of the switching element Q2 is connected to the negative DC terminals T12 and T14. The capacitor C3 is connected between the positive DC terminal T13 and the negative DC terminal T14. The switching element Q2 corresponds to an example of a "second switching element."

[0065] Diodes D1 and D2 are connected in antiparallel to switching elements Q1 and Q2, respectively. Diodes D1 and D2 are provided to allow a freewheeling current to flow when the corresponding switching element Q is turned off. If switching element Q is a MOSFET, diodes D1 and D2 may be configured as parasitic diodes (body diodes).

[0066] When charging the battery 13, the bidirectional chopper 7 according to the first configuration example switches between stepping down and stepping up the DC link voltage VD applied across the pair of DC terminals T13 and T14, depending on the battery voltage VB. At this time, the bidirectional chopper 7 switches the switching element Q2 and fixes the switching element Q1 in the off state. If the duty factor of the switching element Q2 is α2, the relationship expressed by the following equation (1) holds between the DC link voltage VD, which is the input voltage of the bidirectional chopper 7, and the battery voltage VB, which is the output voltage of the bidirectional chopper 7. The duty factor is the ratio of the time during which the switching element is on to the switching period of the switching element. As shown in equation (1), when α2<0.5, the DC link voltage VD is stepped down, and when α2>0.5, the DC link voltage VD is stepped up. VB = VD × α2 / (1 - α2) (1) When discharging the battery 13, the bidirectional chopper 7 according to the first configuration example switches between stepping down and stepping up the battery voltage VB applied across the pair of DC terminals T11 and T12 depending on the battery voltage VB. At this time, the bidirectional chopper 7 switches the switching element Q1 and fixes the switching element Q2 in the off state. If the duty factor of the switching element Q1 is α1, the relationship shown in the following equation (2) holds between the battery voltage VB, which is the input voltage of the bidirectional chopper 7, and the DC link voltage VD, which is the output voltage of the bidirectional chopper 7. As shown in equation (2), when α1 < 0.5, the battery voltage VB is stepped down, and when α1 > 0.5, the battery voltage VB is stepped up. VD = VB × α1 / (1 - α1) (2) FIG. 9 is a diagram illustrating the operation of the bidirectional chopper 7 according to the first configuration example. 9, the DC link voltage VD can be switched between stepping down and stepping up by the duty ratio α2 of the switching element Q2. The DC link voltage VD can be switched between stepping down and stepping up by the duty ratio α1 of the switching element Q1.

[0067] When the AC power supply 12 is healthy, the control device 10 controls the bidirectional chopper 7 so that, when VB<VD, the DC link voltage VD is stepped down and output to the battery 13 by setting the duty ratio α2 of the switching element Q2 to α2<0.5 (first step-down operation).When VB>VD, the control device 10 controls the bidirectional chopper 7 so that, when VB>VD, the DC link voltage VD is stepped up and output to the battery 13 by setting α2>0.5 (first step-up operation).

[0068] During a power outage of the AC power supply 12, when VB > VD, the control device 10 controls the bidirectional chopper 7 to lower the battery voltage VB and output it to the DC link 5 by setting the duty ratio α1 of the switching element Q1 to α1 < 0.5 (second step-down operation).When VB < VD, the control device 10 controls the bidirectional chopper 7 to boost the battery voltage VB and output it to the DC link 5 by setting α1 > 0.5 (second step-up operation).

[0069] (Second Configuration Example) Fig. 10 is a circuit diagram showing a second configuration example of the bidirectional chopper 7. As shown in Fig. 10, the bidirectional chopper 7 according to the second configuration example includes a pair of DC terminals T11, T12, a pair of DC terminals T13, T14, switching elements Q1, Q2, diodes D1, D2, capacitors C1, C2, and a reactor L1. In Fig. 10, IGBTs are used as the switching elements Q1, Q2, but any semiconductor elements such as MOSFETs can be used.

[0070] A pair of DC terminals T11, T12 are connected to a battery 13. The positive side DC terminal T11 is connected to the positive electrode of the battery 13, and the negative side DC terminal T12 is connected to the negative electrode of the battery 13. A pair of DC terminals T13, T14 are connected to DC links 5, 5n. The negative side DC terminal T14 is connected to the negative side DC link 5n, and the positive side DC terminal T13 is connected to the positive side DC link 5. In the second configuration example, the polarities of the input voltage and the output voltage are reversed.

[0071] The collector of the switching element Q1 is connected to the positive DC terminal T11, and the emitter of the switching element Q1 is connected to the collector of the switching element Q2. The emitter of the switching element Q2 is connected to the negative DC terminal T14. Diodes D1 and D2 are connected in anti-parallel to the switching elements Q1 and Q2, respectively.

[0072] Capacitor C1 is connected between positive side DC terminal T11 and negative side DC terminal T12. A first terminal of reactor L1 is connected to the emitter of switching element Q1 and the collector of switching element Q2, and a second terminal of reactor L1 is connected to negative side DC terminal T12 and positive side DC terminal T13. Capacitor C2 is connected between positive side DC terminal T13 and negative side DC terminal T14.

[0073] When charging the battery 13, the bidirectional chopper 7 according to the second configuration example switches between stepping down and stepping up the DC link voltage VD applied across the pair of DC terminals T13 and T14 in accordance with the battery voltage VB. In this case, the bidirectional chopper 7 switches the switching element Q2 and fixes the switching element Q1 in the off state. If the duty ratio of the switching element Q2 is α2, the relationship expressed by the above formula (1) holds between the DC link voltage VD, which is the input voltage of the bidirectional chopper 7, and the battery voltage VB, which is the output voltage of the bidirectional chopper 7. As shown in formula (1), when α2<0.5, the DC link voltage VD is stepped down, and when α2>0.5, the DC link voltage VD is stepped up.

[0074] When discharging the battery 13, the bidirectional chopper 7 according to the second configuration example switches between stepping down and stepping up the battery voltage VB applied across the pair of DC terminals T11 and T12 depending on the battery voltage VB. In this case, the bidirectional chopper 7 switches the switching element Q1 and fixes the switching element Q2 in the off state. If the duty ratio of the switching element Q1 is α1, the relationship expressed by the above formula (2) holds between the battery voltage VB, which is the input voltage of the bidirectional chopper 7, and the DC link voltage VD, which is the output voltage of the bidirectional chopper 7. As shown in formula (2), when α1<0.5, the battery voltage VB is stepped down, and when α1>0.5, the battery voltage VB is stepped up.

[0075] 11 is a diagram illustrating the operation of the bidirectional chopper 7 according to the second configuration example. As shown in FIG. 11, the DC link voltage VD can be switched between stepping down and stepping up by the duty ratio α2 of the switching element Q2. The DC link voltage VD can be switched between stepping down and stepping up by the duty ratio α1 of the switching element Q1.

[0076] When the AC power supply 12 is healthy, the control device 10 controls the bidirectional chopper 7 so that, when VB<VD, the DC link voltage VD is stepped down and output to the battery 13 by setting the duty ratio α2 of the switching element Q2 to α2<0.5 (first step-down operation).When VB>VD, the control device 10 controls the bidirectional chopper 7 so that, when VB>VD, the DC link voltage VD is stepped up and output to the battery 13 by setting α2>0.5 (first step-up operation).

[0077] During a power outage of the AC power supply 12, when VB > VD, the control device 10 controls the bidirectional chopper 7 to lower the battery voltage VB and output it to the DC link 5 by setting the duty ratio α1 of the switching element Q1 to α1 < 0.5 (second step-down operation).When VB < VD, the control device 10 controls the bidirectional chopper 7 to boost the battery voltage VB and output it to the DC link 5 by setting α1 > 0.5 (second step-up operation).

[0078] (Third Configuration Example) Fig. 12 is a circuit diagram showing a third configuration example of the bidirectional chopper 7. As shown in Fig. 12, the bidirectional chopper 7 according to the third configuration example includes a pair of DC terminals T11, T12, a pair of DC terminals T13, T14, switching elements Q1 to Q6, diodes D1 to D6, capacitors C1 to C4, and reactors L1, L2. In Fig. 12, IGBTs are used as the switching elements Q1 to Q6, but any semiconductor elements such as MOSFETs can be used.

[0079] A pair of DC terminals T11, T12 are connected to a battery 13. The positive DC terminal T11 is connected to the positive electrode of the battery 13, and the negative DC terminal T12 is connected to the negative electrode of the battery 13. A pair of DC terminals T13, T14 are connected to DC links 5, 5n. The positive DC terminal T13 is connected to the positive DC link 5, and the negative DC terminal T14 is connected to the negative DC link 5n.

[0080] The collector of switching element Q1 is connected to the collector of switching element Q5, and the emitter of switching element Q1 is connected to a first terminal of reactor L1. The second terminal of reactor L1 is connected to positive DC terminal T13. The emitter of switching element Q5 is connected to positive DC terminal T11. The collector of switching element Q2 is connected to the emitter of switching element Q1 and the first terminal of reactor L1, and the emitter of switching element Q2 is connected to negative DC terminals T12 and T14. Diodes D1, D2, and D5 are connected in anti-parallel to switching elements Q1, Q2, and Q5, respectively. Capacitor C1 is connected between the positive DC terminal T11 and the negative DC terminal T12. Capacitor C2 is connected between the positive DC terminal T13 and the negative DC terminal T14.

[0081] A first terminal of reactor L2 is connected to the positive DC terminal T11, and a second terminal of reactor L2 is connected to the emitter of switching element Q4. The collector of switching element Q4 is connected to the collector of switching element Q6. The emitter of switching element Q6 is connected to the positive DC terminal T13. The collector of switching element Q3 is connected to the emitter of switching element Q4 and the second terminal of reactor L2, and the emitter of switching element Q3 is connected to the negative DC terminals T12 and T14. Diodes D3, D4, and D6 are connected in anti-parallel to switching elements Q3, Q4, and Q6, respectively. Capacitor C3 is connected between the positive DC terminal T11 and the negative DC terminal T12. Capacitor C4 is connected between the positive DC terminal T13 and the negative DC terminal T14.

[0082] In the third configuration example, the switching elements Q1, Q2, and Q5, the diodes D1, D2, and D5, the reactor L1, and the capacitors C1 and C2 constitute a “first bidirectional chopper.” The first bidirectional chopper is configured to be able to exchange power in both directions when VB > VD.

[0083] Specifically, when the battery voltage VB is the input voltage and the DC link voltage VD is the output voltage, the first bidirectional chopper operates as a step-down chopper that steps down the battery voltage VB. At this time, power flows from the battery 13 to the DC links 5, 5n, and the battery 13 discharges. The relationship between the battery voltage VB and the DC link voltage VD is expressed by the following equation (3): α1 is the duty ratio of the switching element Q1, and 0<α1<1: VD = VB × α1 (3) On the other hand, when the DC link voltage VD is the input voltage and the battery voltage VB is the output voltage, the first bidirectional chopper operates as a step-up chopper that steps up the DC link voltage VD. At this time, power flows from the DC links 5, 5n to the battery 13, and the battery 13 is charged. The relationship between the battery voltage VB and the DC link voltage VD is expressed by the following equation (4). α2 is the conduction ratio of switching element Q2, and 0<α2<1. VB=VD×1 / (1-α2) (4) In the third configuration example, switching elements Q3, Q4, and Q6, diodes D3, D4, and D6, reactor L2, and capacitors C3 and C4 form a "second bidirectional chopper." The second bidirectional chopper is configured to be able to exchange power in both directions when VB<VD.

[0084] Specifically, when the battery voltage VB is the input voltage and the DC link voltage VD is the output voltage, the second bidirectional chopper operates as a boost chopper that boosts the battery voltage VB. At this time, power flows from the battery 13 to the DC links 5, 5n, and the battery 13 is discharged. The relationship between the battery voltage VB and the DC link voltage VD is expressed by the following equation (5): α3 is the duty ratio of the switching element Q3, and 0<α3<1: VD=VB×1 / (1−α3) (5) On the other hand, when the DC link voltage VD is the input voltage and the battery voltage VB is the output voltage, the second bidirectional chopper operates as a buck chopper that bucks the DC link voltage VD. At this time, power flows from the DC links 5, 5n to the battery 13, and the battery 13 is charged. The relationship between the battery voltage VB and the DC link voltage VD is expressed by the following equation (6): where α4 is the duty ratio of the switching element Q4, and 0<α4<1. VB = VD × α4 (6) FIG. 13 is a diagram illustrating the operation of the bidirectional chopper 7 according to the third configuration example. As shown in FIG. 13 , when charging the battery 13, the DC link voltage VD can be switched between being stepped down and being stepped up depending on the duty ratio α2 of the switching element Q2 and the duty ratio α4 of the switching element Q4. Specifically, the DC link voltage VD is stepped down depending on the duty ratio α4 of the switching element Q4, and the DC link voltage VD is stepped up depending on the duty ratio α2 of the switching element Q2. Note that when stepping down the DC link voltage VD, turning the switching element Q5 off prevents current from flowing from the DC links 5, 5n to the battery 13 via the reactor L1. When stepping up the DC link voltage VD, the switching element Q5 is turned on so as not to interfere with the step-up operation of the first bidirectional chopper.

[0085] Furthermore, when the battery 13 is being discharged, the battery voltage VB can be switched between being stepped down and being stepped up by the duty ratio α1 of the switching element Q1 and the duty ratio α3 of the switching element Q3. Specifically, the battery voltage VB is stepped down according to the duty ratio α1 of the switching element Q1, and the battery voltage VB is stepped up according to the duty ratio α3 of the switching element Q3. When the battery voltage VB is being stepped down, the switching element Q6 is turned off, thereby preventing current from flowing from the battery 13 to the DC links 5, 5n via the reactor L2. When the battery voltage VB is being stepped up, the switching element Q6 is turned on so as not to interfere with the step-up operation of the second bidirectional chopper.

[0086] When the AC power supply 12 is healthy, and VB<VD, the control device 10 controls the bidirectional chopper 7 to fix the switching elements Q1 to Q3, Q5, and Q6 to the OFF state and control the duty ratio α4 of the switching element Q4 to step down the DC link voltage VD and output it to the battery 13 (first step-down operation).When VB>VD, the control device 10 controls the bidirectional chopper 7 to fix the switching elements Q1 to Q3, Q5, and Q6 to the OFF state, fix the switching element Q5 to the ON state, and control the duty ratio α2 of the switching element Q2 to step up the DC link voltage VD and output it to the battery 13 (first step-up operation).

[0087] During a power outage of the AC power supply 12, when VB > VD, the control device 10 controls the bidirectional chopper 7 to fix the switching elements Q2 to Q6 to the OFF state and control the duty ratio α1 of the switching element Q1 to step down the battery voltage VB and output it to the DC link 5 (second step-down operation).When VB < VD, the control device 10 controls the bidirectional chopper 7 to fix the switching elements Q1, Q2, Q4, and Q5 to the OFF state, fix the switching element Q6 to the ON state, and control the duty ratio α3 of the switching element Q3 to step up the battery voltage VB and output it to the DC link 5 (second step-up operation).

[0088] In the above-described embodiment, the operation of the bidirectional chopper 7 when the relationship V1<VD<V2 holds among the charge end voltage V2, discharge end voltage V1, and DC link voltage VD of the battery 13 has been described, but when the relationship V1<V2<VD holds, VB<VD always holds when charging the battery 13, and therefore the bidirectional choppers 7 according to the first to third configuration examples only perform a step-down operation to step down the DC link voltage VD. Also, when discharging the battery 13, VB<VD always holds, and therefore the bidirectional choppers 7 according to the first to third configuration examples only perform a step-up operation to step up the battery voltage VB.

[0089] As described above, the uninterruptible power supply according to the first embodiment can accommodate both a battery whose end-of-charge voltage V2 is greater than the DC link voltage VD and a battery whose end-of-charge voltage V2 is lower than the DC link voltage VD. This broadens the operating voltage range of batteries that can be connected to the uninterruptible power supply compared to conventional uninterruptible power supplies, thereby increasing the number of usable battery types. This makes it possible to propose to users of the uninterruptible power supply the combination of the uninterruptible power supply and battery that is optimal in terms of the size and cost of the equipment.

[0090] [Embodiment 2] In the above-described embodiment 1, the uninterruptible power supply 100 is described as including a power converter (converter 4, inverter 8, and bidirectional chopper 7) connected between the AC power supply 12 and the load 14. However, the uninterruptible power supply 100 according to the present disclosure may include at least the bidirectional chopper 7 that transfers power between the battery 13 and the DC link 5. For example, the uninterruptible power supply 100 may have a configuration shown in Fig. 14 .

[0091] [Correction based on Rule 91 29.10.2024] Figure 14 is a circuit block diagram showing the configuration of an uninterruptible power supply according to embodiment 2. Note that uninterruptible power supply 100 according to embodiment 2 receives a three-phase AC voltage from AC power supply 12 and supplies the three-phase AC voltage to load 14, but for the sake of simplicity of the drawing and explanation, only the circuit for one phase is shown in Figure 14.

[0092] 14, uninterruptible power supply 100 according to the second embodiment includes input terminal T1, DC terminal T2, output terminal T3, VCBs (Vacuum Circuit Breakers) 15 to 17, a high-speed switch (HSS; High Speed ​​Switch) 20, a bidirectional converter 11, a bidirectional chopper 7, a reactor 21, capacitors 6 and 22, current detectors CD1 to CD3, and a control device 10. Uninterruptible power supply 100 according to the second embodiment is also called a multiple power compensator.

[0093] VCB 15, HSS 20, and VCB 16 are connected in series between input terminal T1 and output terminal T3. VCB 15 and VCB 16 are turned on during normal operation of uninterruptible power supply 100, and are turned off, for example, during maintenance of HSS 20 (during maintenance bypass power supply).

[0094] The HSS 20 is configured by, for example, a semiconductor switching element, and is controlled by the control device 10. The HSS 20 is turned on when the AC power supply 12 is normal, and is turned off when the AC power supply 12 experiences a power outage.

[0095] The VCB 17 is connected between the input terminal T1 and the output terminal T3. The VCB 17 is turned off during normal operation of the uninterruptible power supply 100 and turned on, for example, during maintenance bypass power supply. When the VCB 17 is turned on, the AC input voltage VI is supplied from the AC power supply 12 to the load 14 via the VCB 17, and the load 14 operates.

[0096] An AC terminal of the bidirectional converter 11 is connected to a node N1 between the HSS 20 and the VCB 16 via a reactor 21. A DC terminal of the bidirectional converter 11 is connected to the DC link 5. The bidirectional converter 11 is a well-known device including a plurality of switching elements and a plurality of diodes, and is controlled by the control device 10 using, for example, pulse width modulation (PWM). By turning on and off each switching element included in the bidirectional converter 11 at a predetermined switching frequency, it is possible to convert AC power to DC power, and conversely, convert DC power to AC power.

[0097] The reactor 21 and the capacitor 22 constitute an AC filter. The AC filter is a low-pass filter that passes a current of the commercial frequency and blocks a current of the switching frequency generated by the bidirectional converter 11. In other words, the AC filter converts the output voltage of the bidirectional converter 11 into a sinusoidal AC voltage.

[0098] A capacitor 6 is connected to the DC link 5 to smooth and stabilize the DC link voltage VD. The instantaneous value of the DC link voltage VD is detected by a control device 10.

[0099] The bidirectional chopper 7 is connected between the DC link 5 and the DC terminal T2. The bidirectional chopper 7 is controlled by the control device 10, and exchanges DC power between the DC link 5 and the battery 13. The bidirectional chopper 7 is configured to be capable of selectively performing a charging operation for storing DC power from the DC link 5 in the battery 13, and a discharging operation for supplying the DC power stored in the battery 13 to the DC link 5. Any of the bidirectional choppers according to the first to third configuration examples described above can be applied to the bidirectional chopper 7.

[0100] The current detector CD1 detects the AC input current Ii flowing through the HSS 20 and provides a signal Iif indicating the detected value to the control device 10. The current detector CD2 detects the DC current IB flowing between the battery 13 and the bidirectional chopper 7 and provides a signal IBf indicating the detected value to the control device 10. The current detector CD3 detects the AC output current IL flowing through the reactor 21 and provides a signal ILf indicating the detected value to the control device 10.

[0101] The control device 10 controls the entire uninterruptible power supply 100 based on the AC voltages VI and VO, the DC voltages VD and VB, and the output signals Iif, IBf, and IOf of the current detectors CD1 to CD3.

[0102] In the uninterruptible power supply 100 shown in FIG. 14, when the AC power supply 12 is healthy, the HSS 20 is turned on, and AC power is supplied from the AC power supply 12 to the load 14 via the VCB 15, the HSS 20, and the VCB 16, thereby driving the load 14.

[0103] Furthermore, AC power is supplied from the AC power source 12 to the bidirectional converter 11 via the VCB 15 and the HSS 20, and the AC power is converted into DC power by the bidirectional converter 11 and supplied to the DC link 5. This DC power is stored in the battery 13 by the bidirectional chopper 7. At this time, the control device 10 controls the bidirectional converter 11 so that the DC link voltage VD becomes the reference DC voltage VDR. The control device 10 also controls the bidirectional chopper 7 so that the battery voltage VB becomes the reference DC voltage VBR.

[0104] When the relationship V1<VD<V2 exists among the discharge end voltage V1, charge end voltage V2, and DC link voltage VD of the battery 13, the bidirectional chopper 7 switches between a step-down operation for stepping down the DC link voltage VD, which is an input voltage, and a step-up operation for stepping up the DC link voltage VD, depending on the battery voltage VB. As described in the first embodiment, the bidirectional chopper 7 performs the step-down operation during a period when the battery voltage VB is lower than the DC link voltage VD. The bidirectional chopper 7 performs the step-up operation during a period when the battery voltage VB is higher than the DC link voltage VD.

[0105] In the event of a power outage of the AC power supply 12, the HSS 20 is instantly turned off, electrically disconnecting the AC power supply 12 from the load 14. At the same time, the DC power of the battery 13 is supplied to the DC link 5 by the bidirectional chopper 7. The DC power is then converted to AC power by the bidirectional converter 11 and supplied to the load 14, allowing the load 14 to continue operating. The control device 10 controls the bidirectional chopper 7 so that the DC link voltage VD becomes equal to the reference DC voltage VDR. When the battery voltage VB decreases due to the discharge of the battery 13 and reaches the discharge end voltage V1, the control device 10 stops the bidirectional chopper 7, thereby stopping the discharge of the battery 13.

[0106] When the relationship V1<VD<V2 is satisfied, the bidirectional chopper 7 switches between a step-down operation for stepping down the battery voltage VB, which is an input voltage, and a step-up operation for stepping up the battery voltage VB, depending on the battery voltage VB. As described in the first embodiment, the bidirectional chopper 7 performs the step-down operation during a period in which the battery voltage VB is higher than the DC link voltage VD. The bidirectional chopper 7 performs the step-up operation during a period in which the battery voltage VB is lower than the DC link voltage VD.

[0107] As a result, the uninterruptible power supply according to the second embodiment can also achieve the same effects as the uninterruptible power supply according to the first embodiment.

[0108] The embodiments disclosed herein should be considered to be illustrative and not restrictive in all respects. The scope of the present disclosure is defined by the scope of the claims, and it is intended to include all modifications within the meaning and scope of the claims.

[0109] 4 Converter, 5, 5n DC link, 6, 22, C1 to C4 Capacitors, 7 Bidirectional chopper, 8 Inverter, 9 Operation unit, 10 Control device, 11 Bidirectional converter, 12 AC power supply, 13 Battery, 14 Load, 15 to 17 VCB, 20 HSS, 21, L1, L2 Reactor, 100 Uninterruptible power supply, 102 CPU, 104 Memory, 106 I / O circuit, 108 Bus, T1 Input terminal, T2, T11, T12, T13, T14 DC terminal, T3 Output terminal, S1 to S3 Switches, VB Battery voltage, VD DC link voltage, CD1 to CD3 Current detectors, Q1 to Q6 Switching elements, D1 to D6 Diodes, S1 to S3 Switches.

Claims

1. An uninterruptible power supply connected between an AC power source and a load, comprising: a converter that converts AC power supplied from the AC power source into DC power; an inverter that converts the DC power into AC power and supplies the DC power to the load; a DC link connected between the converter and the inverter for inputting the DC power to the inverter; and a bidirectional chopper that performs DC voltage conversion between the DC link and a power storage device, wherein the bidirectional chopper is configured to execute a charging operation of storing the DC power of the DC link in the power storage device when the AC power source is healthy, and when a charging end voltage of the power storage device is higher than a DC link voltage of the DC link and a discharging end voltage of the power storage device is lower than the DC link voltage, the bidirectional chopper switches between a first step-down operation of stepping down the DC link voltage and a first step-up operation of stepping up the DC link voltage in accordance with a voltage of the power storage device.

2. An uninterruptible power supply as described in claim 1, wherein the bidirectional chopper performs the first voltage step-down operation when the voltage of the power storage device is lower than the DC link voltage, and performs the first voltage step-up operation when the voltage of the power storage device is higher than the DC link voltage.

3. The uninterruptible power supply device according to claim 1 or 2, wherein the bidirectional chopper is configured to execute a discharging operation for supplying DC power stored in the power storage device to the DC link when a power outage occurs in the AC power supply, and when the charging end voltage of the power storage device is higher than the DC link voltage and the discharging end voltage is lower than the DC link voltage, the bidirectional chopper switches between executing a second step-down operation for stepping down the voltage of the power storage device and a second step-up operation for stepping up the voltage of the power storage device depending on the voltage of the power storage device when the discharging operation is executed.

4. [Correction under Rule 91 29.10.2024] The uninterruptible power supply of claim 3, wherein the bidirectional chopper performs the second voltage step-down operation when the voltage of the storage device is higher than the DC link voltage, and performs the second voltage step-up operation when the voltage of the storage device is lower than the DC link voltage.

5. An uninterruptible power supply as described in claim 3, wherein, when the charging end voltage of the storage device is lower than the DC link voltage, the bidirectional chopper performs the first voltage step-down operation when performing the charging operation, and performs the second voltage step-up operation when performing the discharging operation.

6. The uninterruptible power supply of claim 3, wherein the bidirectional chopper includes: a first switching element that is turned on and off when the discharging operation is performed and that is turned off when the charging operation is performed; and a second switching element that is turned on and off when the charging operation is performed and that is turned on and off when the discharging operation is performed, wherein the bidirectional chopper switches between the first voltage step-down operation and the first voltage step-up operation by controlling a second conduction rate of the second switching element in accordance with the voltage of the storage device, and switches between the second voltage step-down operation and the second voltage step-up operation by controlling a first conduction rate of the first switching element in accordance with the voltage of the storage device.

7. An uninterruptible power supply as described in claim 3, wherein the bidirectional chopper includes: a first bidirectional chopper configured to switch between the first voltage step-up operation and the second voltage step-down operation when the voltage of the power storage device is higher than the DC link voltage; and a second bidirectional chopper configured to switch between the first voltage step-down operation and the second voltage step-up operation when the voltage of the power storage device is lower than the DC link voltage.

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