Uninterruptible power supply device

Abstract

This uninterruptible power supply device (10) is provided with a converter (12), a bidirectional chopper (18), and an inverter (20). Each of the converter (12) and the inverter (20) includes a leg circuit having a plurality of converter cells (26) that are cascade connected. The bidirectional chopper (18) includes a plurality of chopper cells (30) connected in series between a DC positive bus (14p) and a DC negative bus (14n). A power storage device includes a plurality of power storage cells that are provided so as to respectively correspond to the plurality of chopper cells, and that are connected in parallel to each other. Each of the plurality of chopper cells (30) includes: a first input / output terminal and a second input / output terminal; a capacitor connected between the first input / output terminal and the second input / output terminal; and a charge / discharge circuit for transmitting / receiving DC power between the capacitor and a corresponding power storage cell.

Description

Uninterruptible power supply device

[0001] The present disclosure relates to an uninterruptible power supply device.

[0002] For example, International Publication No. 2014 / 132452 (Patent Document 1) discloses an uninterruptible power supply device for supplying power to a plurality of servers installed in a data center or the like. This uninterruptible power supply device is connected to a low-voltage (400V system) AC power supply. The uninterruptible power supply device includes a converter that converts low-voltage AC power (AC400V) from the AC power supply into DC power, and an inverter that converts DC power from the converter or DC power stored in a battery into low-voltage AC power (AC400V).

[0003] International Publication No. 2014 / 132452

[0004] In order to supply AC power to a large-capacity load ranging from several tens of MVA to several hundreds of MVA, a power supply system is constructed by connecting a plurality of uninterruptible power supply devices in parallel to the load. This power supply system supplies low-voltage AC power (AC400V) to the load via a transmission line.

[0005] In the above power supply system, since a large current flows through the transmission line, there are concerns about an increase in power loss and voltage drop. In addition, since a large number of transmission lines are required to carry a large current, there is a problem that the power supply system becomes larger and the equipment cost increases.

[0006] Therefore, the main object of the present disclosure is to provide an uninterruptible power supply device that can be miniaturized and made more efficient.

[0007] An uninterruptible power supply (UPS) according to this disclosure is connected between an AC power source and an AC transmission line. The UPS comprises a converter, a bidirectional chopper, and an inverter. The converter converts AC power supplied from the AC power source into DC power and supplies it between a DC positive busbar and a DC negative busbar. The bidirectional chopper converts the DC voltage between the DC positive busbar and the DC negative busbar to the DC voltage of the energy storage device. The inverter converts the DC power received from the DC positive busbar and the DC negative busbar into AC power and supplies it to the AC transmission line. Each of the converter and the inverter includes a leg circuit having a plurality of cascaded converter cells. The bidirectional chopper includes a plurality of chopper cells connected in series between the DC positive busbar and the DC negative busbar. The energy storage device includes a plurality of energy storage cells provided corresponding to each of the plurality of chopper cells and connected in parallel to one another. Each of the multiple chopper cells includes a first input / output terminal and a second input / output terminal, a capacitor connected between the first input / output terminal and the second input / output terminal, and a charge / discharge circuit for transferring DC power between the capacitor and the corresponding energy storage cell.

[0008] According to this disclosure, it is possible to provide an uninterruptible power supply that can be miniaturized and made more efficient.

[0009] This is a circuit block diagram showing an uninterruptible power supply according to this embodiment. This is a circuit diagram showing the main circuit configuration of the converter, inverter, and bidirectional chopper. This is a circuit diagram showing a first configuration example of a converter cell. This is a circuit diagram showing a second configuration example of a converter cell. This is a circuit diagram showing a first configuration example of a chopper cell. This is a flowchart showing the control of the chopper cell by the control circuit. This is a circuit block diagram for explaining the initial charging of a capacitor. This is a circuit diagram showing a second configuration example of a chopper cell. This is a circuit diagram showing a third configuration example of a chopper cell by the control circuit. This is a diagram showing an example of the configuration of a power supply system using an uninterruptible power supply according to this embodiment. This is a diagram showing the main circuit configuration of the converter, inverter, and bidirectional chopper shown in Figure 11.

[0010] Embodiments of this disclosure will be described in detail below with reference to the drawings. In the following, the same or corresponding parts in the drawings will be denoted by the same reference numerals, and their descriptions will not be repeated in principle.

[0011] Figure 1 is a circuit block diagram showing an uninterruptible power supply (UPS) according to this embodiment. As shown in Figure 1, the UPS 10 is connected between the AC power source 1 and the AC transmission line 4. The UPS 10 receives a three-phase AC voltage from the AC power source 1 and supplies a three-phase AC voltage to the AC transmission line 4, but for the sake of simplicity in the drawing and explanation, only a single-phase circuit is shown in Figure 1.

[0012] In this specification, in accordance with IEC (International Electrotechnical Commission) standards, AC voltages are classified as follows: voltages of 230 kV or higher are classified as extra-high voltage (EHVAC: Extra-high Voltage Alternating Current), voltages of 35 kV to 230 kV are classified as high voltage (HVAC: High Voltage Alternating Current), voltages of 1 kV to 35 kV are classified as medium voltage (MVAC: Medium Voltage Alternating Current), and voltages of less than 1 kV are classified as low voltage (LVAC: Low Voltage Alternating Current).

[0013] Furthermore, for direct current, voltage classes exceeding 100kV are classified as high voltage (HVDC: High Voltage Direct Current), voltage classes between 1.5kV and 100kV are classified as medium voltage (MVDC: Medium Voltage Direct Current), and voltage classes below 1.5kV are classified as low voltage (LVDC: Low Voltage Direct Current).

[0014] In Japan, AC voltages are currently classified as follows: 7kV and above are classified as extra-high voltage (EHVAC), 600V to 7kV as high voltage (HVAC), and below 600V as low voltage (LVAC). DC voltages are classified as follows: over 7kV as extra-high voltage (EHVDC), 750V to 7kV as high voltage (HVDC), and below 1.5kV as low voltage (LVDC).

[0015] AC power source 1 is a power system that supplies medium-voltage AC power, or a generator that generates medium-voltage AC power. AC transmission line 4 transmits medium-voltage AC power.

[0016] The uninterruptible power supply 10 includes an input terminal T1, a DC terminal T2, an output terminal T3, switches S1 and S2, a converter 12, current detectors CD1 and CD2, a DC bus 14, a bidirectional chopper 18, an inverter 20, and a control device 22.

[0017] Input terminal T1 receives AC power of a predetermined frequency from AC power source 1 via transformer 2. The AC input voltage VI is a medium-voltage AC voltage, and its instantaneous value is detected by control device 22. Based on the instantaneous value of the AC input voltage VI, it is determined whether or not a power outage has occurred. Current detector CD1 detects the AC input current Ii flowing through input terminal T1 and provides a signal Iif indicating the detected value to control device 22.

[0018] The output terminal T3 is connected to the AC transmission line 4. The AC transmission line 4 transmits medium-voltage AC power. Multiple loads 6 are electrically connected to the AC transmission line 4. A switch S3 and a transformer 5 are connected in series between the AC transmission line 4 and each load 6. The load capacity of the uninterruptible power supply 10 changes when the switch S3 connected to each load 6 is turned on or off.

[0019] When switch S3 is ON, transformer 5 steps down the medium-voltage AC power supplied from AC transmission line 4 to low-voltage AC power and supplies it to load 6. Load 6 is driven by the low-voltage AC power supplied from transformer 5.

[0020] The DC terminal T2 is connected to the battery 3. The battery 3 constitutes an "energy storage device" that stores DC power. However, a capacitor may be connected instead of the battery 3. The instantaneous value of the terminal voltage VB of the battery 3 is detected by the control device 22. When the battery 3 is fully charged, the terminal voltage VB is, for example, several kV.

[0021] The battery 3 is composed of multiple battery cells 3a connected in parallel to each other. As will be described later, the multiple battery cells 3a are provided corresponding to the multiple chopper cells included in the bidirectional chopper 18. The battery cells 3a constitute an "energy storage cell".

[0022] Switch S1 is connected between the input terminal T1 and the AC node of the converter 12 and is controlled by the control device 22. When AC power is supplied normally from the AC power supply 1 (when the AC power supply 1 is healthy), switch S1 is turned on, and AC power is supplied from the AC power supply 1 to the converter 12 via switch S1. When AC power is not supplied normally from the AC power supply 1 (when the AC power supply 1 fails), switch S1 is turned off, and the connection between the AC power supply 1 and the converter 12 is interrupted.

[0023] The converter 12 is controlled by the control device 22 and, when the AC power supply 1 is functioning properly, converts the medium-voltage AC power from the AC power supply 1 into medium-voltage DC power and outputs it to the DC bus 14. The converter 12 will be explained in more detail later. The instantaneous value of the DC voltage VD of the DC bus 14 is detected by the control device 22.

[0024] When AC power supply 1 is functioning properly, the control device 22 controls the converter 12 so that the DC voltage VD becomes the reference DC voltage VDR. When AC power supply 1 fails, the control device 22 stops the operation of the converter 12.

[0025] The DC bus 14 is connected to the high-voltage side node 18a of the bidirectional chopper 18, and the low-voltage side node 18c of the bidirectional chopper 18 is connected to the DC terminal T2. The DC bus 14 includes a DC positive bus 14p and a DC negative bus 14n (see Figure 2).

[0026] The bidirectional chopper 18 is controlled by the control device 22 and performs DC voltage conversion between the DC bus 14 and the battery 3, thereby transferring DC power between the DC bus 14 and the battery 3. When the AC power supply 1 is functioning properly, the control device 22 controls the bidirectional chopper 18 so that the terminal voltage VB of the battery 3 becomes the reference DC voltage VBR. In the event of a power outage of the AC power supply 1, the control device 22 controls the bidirectional chopper 18 so that the DC voltage VD becomes the reference DC voltage VDR. The bidirectional chopper 18 will be explained in more detail later.

[0027] The DC bus 14 is connected to the DC node of the inverter 20, and the AC node of the inverter 20 is connected to the output terminal T3 via switch S2. Switch S2 is controlled by the control device 22. When the uninterruptible power supply 10 is in use, switch S2 is turned on. When the inverter 20 is being maintained, switch S2 is turned off.

[0028] The current detector CD2 detects the AC output current Io of the inverter 20 and provides the control device 22 with a signal Iof indicating the detected value. The instantaneous value of the AC output voltage VO applied to the AC transmission line 4 is detected by the control device 22.

[0029] The inverter 20 is controlled by the control device 22 and converts the DC power supplied from the converter 12 or bidirectional chopper 18 via the DC bus 14 into AC power of a predetermined frequency and supplies it to the AC transmission line 4. The inverter 20 will be described in more detail later.

[0030] The control device 22 controls switches S1 to S3, converter 12, bidirectional chopper 18, and inverter 20 based on the AC input voltage VI, AC output voltage VO, DC voltage VD, terminal voltage VB of battery 3, AC input current Ii, DC current IB, and AC output current Io.

[0031] The control device 22 can typically be configured as a microcomputer with a predetermined program pre-stored in it. For example, the control circuit 54 includes a CPU (Central Processing Unit), memory, and input / output circuits. A program is pre-stored in a portion of the memory, and the CPU can execute this program to realize various functions described later.

[0032] Figure 2 is a circuit diagram showing the main circuit configuration of the converter 12, inverter 20, and bidirectional chopper 18. As shown in Figure 2, each of the converter 12 and inverter 20 is composed of a modular multilevel converter (MMC) in which multiple converter cells are connected in a cascade.

[0033] The converter 12 includes multiple leg circuits 12r, 12s, and 12t. The multiple leg circuits 12r, 12s, and 12t are connected in parallel to each other between the DC positive bus 14p and the DC negative bus 14n. In the following description, when referring to the multiple leg circuits 12r, 12s, and 12t collectively, they will also be referred to as "leg circuit 12x".

[0034] The leg circuits 12x are provided for each of the multiple phases that make up the AC. Figure 2 shows the case where the AC power supply 1 is a three-phase AC power supply, and three leg circuits 12r, 12s, and 12t are provided corresponding to the R phase, S phase, and T phase, respectively. The AC input terminals Nr, Ns, and Nt provided for the leg circuits 12r, 12s, and 12t are connected to the input terminal T1 via the switch S1.

[0035] The leg circuit 12r includes an upper arm 24 from the DC positive busbar 14p to the AC input terminal Nr, and a lower arm 25 from the DC negative busbar 14n to the AC input terminal Nr. Leg circuits 12s and 12t have a similar configuration, so the configuration of leg circuit 12r will be described as representative below.

[0036] The upper arm 24 includes a plurality of cascaded transducer cells 26 and a reactor L1. The plurality of transducer cells 26 and reactor L1 are connected in series. Similarly, the lower arm 25 includes a plurality of cascaded transducer cells 26 and a reactor L2. The plurality of transducer cells 26 and reactor L2 are connected in series.

[0037] Reactors L1 and L2 are buffer reactors for suppressing short-circuit current. Reactor L1 may be inserted at any position on the upper arm 24, and reactor L2 may be inserted at any position on the lower arm 25. Multiple reactors L1 and L2 may be provided. Alternatively, only reactor L1 on the upper arm 24 or only reactor L2 on the lower arm 25 may be provided.

[0038] The inverter 20 includes multiple leg circuits 20u, 20v, and 20w. These multiple leg circuits 20u, 20v, and 20w are connected in parallel to each other between the DC positive busbar 14p and the DC negative busbar 14n. In the following description, when referring to the multiple leg circuits 20u, 20v, and 20w collectively, they will also be referred to as "leg circuit 20x".

[0039] The leg circuits 20x are provided for each of the multiple phases that make up the AC. Figure 2 shows the case where the load 6 is a three-phase AC load, and three leg circuits 20u, 20v, and 20w are provided corresponding to the U phase, V phase, and W phase, respectively. The AC output terminals Nu, Nv, and Nw provided for the leg circuits 20u, 20v, and 20w are connected to the output terminal T3 via switch S3. The leg circuits 20u, 20v, and 20w have the same configuration as the leg circuit 12r, so their explanation is omitted.

[0040] Figure 3 is a circuit diagram showing a first configuration example of the converter cell 26 shown in Figure 2. As shown in Figure 3, the converter cell 26 according to the first configuration example includes a series unit formed by connecting two switching elements SW1 and SW2 in series, a capacitor 28, and input / output terminals 26a and 26b.

[0041] The converter cell 26 of the leg circuit 12r will be typically described. In the upper arm 24, the input / output terminals 26a of the uppermost converter cell 26 are connected to the DC positive bus 14p. The input / output terminals 26b of each converter cell 26 other than the lowermost one are connected to the input / output terminals 26a of the adjacent converter cell 26. The input / output terminals 26b of the lowermost converter cell 26 are connected to the AC input terminal Nr.

[0042] In the lower arm 25, the input / output terminals 26a of the uppermost converter cell 26 are connected to the AC input terminal Nr. The input / output terminals 26b of each converter cell 26 other than the lowermost one are connected to the input / output terminals 26a of the adjacent converter cell 26. The input / output terminals 26b of the lowermost converter cell 26 are connected to the DC negative bus 14n.

[0043] The series body of the switching elements SW1 and SW2 and the capacitor 28 are connected in parallel. The series body of the switching elements SW1 and SW2 constitutes a half-bridge circuit. The capacitor 28 constitutes a "power storage element" for storing DC power.

[0044] Both terminals of the switching element SW2 are connected to the input / output terminals 26a and 26b, respectively. The converter cell 26 outputs the voltage Vc between the terminals of the capacitor 28 or zero voltage between the input / output terminals 26a and 26b by the switching operation of the switching elements SW1 and SW2. When the switching element SW1 is on and the switching element SW2 is off, the voltage Vc between the terminals of the capacitor 28 is output from the converter cell 26. When the switching element SW1 is off and the switching element SW2 is on, the converter cell 26 outputs zero voltage.

[0045] FIG. 4 is a circuit diagram showing a second configuration example of the converter cell 26 shown in FIG. 2. As shown in FIG. 4, the converter cell 26 according to the second configuration example includes a first series body formed by connecting two switching elements SW1 and SW2 in series, a second series body formed by connecting two switching elements SW3 and SW4 in series, a capacitor 28, and input / output terminals 26a and 26b. The first series body, the second series body, and the capacitor 38 are connected in parallel. The first series body and the second series body constitute a full-bridge circuit.

[0046] The connection point of the switching element SW1 and the switching element SW2 is connected to the input / output terminal 26a. The connection point of the switching element SW3 and the switching element SW4 is connected to the input / output terminal 26b. The converter cell 26 outputs the voltage Vc, -Vc, or zero voltage between the terminals of the capacitor 28 between the input / output terminals 26a and 26b by the switching operations of the switching elements SW1 to SW4.

[0047] The switching elements SW1 to SW4 are configured by connecting a freewheeling diode (FWD) in antiparallel to a self-extinguishing semiconductor switching element such as an IGBT (Insulated Gate Bipolar Transistor) or a MOSFET (Metal Oxide Semiconductor Field Effect Transistor).

[0048] Returning to FIG. 2, the bidirectional chopper 18 includes a plurality of chopper cells (BCELL) 30 connected in series between the DC positive bus 14p and the DC negative bus 14n. Each of the plurality of chopper cells 30 is configured to control the charge and discharge of the corresponding battery cell 3a, and is therefore also called a "battery circuit cell".

[0049] Figure 5 is a circuit diagram showing a first configuration example of the chopper cell 30 shown in Figure 2. As shown in Figure 5, the chopper cell 30 according to the first configuration example includes input / output terminals 30a, 30b, DC terminals 30c, 30d, switching elements SW5, SW6, capacitor 40, bypass switch S4, charge / discharge circuit 45, switches S5, S6, resistor element 46, voltage detectors 42, 48, current detector CD3, fuse 50, power supply 52, and control circuit 54.

[0050] In the bidirectional chopper 18, the input / output terminal 30a of the uppermost chopper cell 30 is connected to the DC positive bus 14p. The input / output terminal 30b of each chopper cell 30, except for the bottommost one, is connected to the input / output terminal 30a of the adjacent chopper cell 30. The input / output terminal 30b of the bottommost chopper cell 30 is connected to the DC negative bus 14n.

[0051] DC terminal 30c is connected to the positive terminal of the corresponding battery cell 3a, and DC terminal 30d is connected to the negative terminal of the corresponding battery cell 3a.

[0052] The positive terminal of capacitor 40 is connected to input / output terminal 30a via DC line PL, and the negative terminal of capacitor 40 is connected to input / output terminal 30b via DC line NL. In other words, the same number of capacitors 40 as the number of chopper cells 30 included in the bidirectional chopper 18 are connected in series between the DC positive busbar 14p and the DC negative busbar 14n.

[0053] Multiple capacitors 40 arranged in series form a capacitor for stabilizing and smoothing the DC voltage VD between DC buses 14p and 14n. Ideally, the terminal voltage VDc of each chopper cell 30 is the value obtained by dividing the DC voltage VD by the number of chopper cells 30 included in the bidirectional chopper 18. The voltage detector 42 detects the instantaneous value of the terminal voltage VDc of the capacitors 40 and provides a signal indicating the detected value to the control circuit 54.

[0054] The bypass switch S4 is connected between input / output terminals 30a and 30b. The bypass switch S4 is configured to short-circuit input / output terminals 30a and 30b by being turned on in response to a command from the control circuit 54. In some cases, the bypass switch S4 can be used to short-circuit the chopper cell 30 when the corresponding battery cell 3a fails. In this case, even if the battery cell 3a fails in some of the chopper cells 30, the operation of the uninterruptible power supply 10 can be continued by using the other chopper cells 30.

[0055] The charge / discharge circuit 45 is configured to exchange DC power between the capacitor 40 and the battery cell 3a by performing bidirectional DC voltage conversion between the capacitor 40 and the battery cell 3a. Specifically, the charge / discharge circuit 45 is composed of switching elements SW5 and SW6 and a reactor 44. Each of the switching elements SW5 and SW6 is composed of a self-extinguishing semiconductor switching element such as an IGBT or MOSFET connected in antiparallel to a freewheeling diode.

[0056] Switching elements SW5 and SW6 are connected in series between DC line PL and DC line NL. The connection point between switching element SW5 and switching element SW6 is connected to the first terminal of reactor 44. The second terminal of reactor 44 is connected to DC terminal 30c via switch S5.

[0057] Note that while Figure 5 illustrates an example in which the charge / discharge circuit 45 includes a bidirectional chopper, it is not limited to this. The charge / discharge circuit 45 can include a well-known non-isolated DC / DC converter.

[0058] The charge / discharge circuit 45 is controlled by the control circuit 54. When the AC power supply 1 is functioning properly, the charge / discharge circuit 45 stores DC power supplied from the capacitor 40 via the DC line PL in the battery cell 3a. When the AC power supply 1 fails, the charge / discharge circuit 45 supplies DC power from the battery cell 3a to the capacitor 40 via the DC line PL. The current detector CD3 detects the DC current flowing between the charge / discharge circuit 45 and the battery cell 3a and provides a signal indicating the detected value to the control circuit 54.

[0059] Switch S5 is connected between the second terminal of the reactor 44 and the DC terminal 30c and is controlled by the control circuit 54. Switch S5 is turned ON when the chopper cell 30 is used. Switch S5 is turned OFF when the uninterruptible power supply 10 is started up and when the battery cell 3a is being maintained.

[0060] A fuse 50 is provided between the switch S5 and the DC terminal 30c. If a large current flows between the charge / discharge circuit 45 and the battery cell 3a, the fuse 50 will blow.

[0061] The resistor 46 and switch S6 are connected in series between the first and second terminals of switch S5. Switch S6 is controlled by the control circuit 54 and is turned on when the uninterruptible power supply 10 is started up, and is turned off during normal operation and when the battery cell 3a is being maintained. The resistor 46 limits the current flowing from the battery cell 3a to the capacitor 40 when no DC power is stored in the capacitor 40 when the uninterruptible power supply 10 is started up.

[0062] The voltage detector 48 detects the instantaneous value of the terminal voltage of the corresponding battery cell 3a and provides a signal indicating the detected value to the control circuit 54.

[0063] The first input terminal of the power supply 52 is connected to the DC terminal 30c, and the second input terminal of the power supply 52 is connected to the DC terminal 30d. The power supply 52 steps down the DC voltage supplied from the battery cell 3a via the DC terminals 30c and 30d to generate the power supply voltage for the control circuit 54. As a result, the control circuit 54 can operate even when the uninterruptible power supply 10 is stopped.

[0064] The control circuit 54 is driven by the power supply voltage supplied from the power supply 52 and exchanges signals with the control device 22. The control circuit 54 controls the chopper cell 30 based on signals from the control device 22, the output signals from the voltage detectors 42 and 48, and the output signal from the current detector CD3.

[0065] The control circuit 54 can typically be configured by a microcomputer with a predetermined program pre-stored in it. For example, the control circuit 54 includes a CPU, memory, and input / output circuits. A program is pre-stored in a portion of the memory, and the CPU can execute this program to realize various functions described later.

[0066] Figure 6 is a flowchart showing the control of the chopper cells 30 by the control circuit 54. As shown in Figure 6, in step S01, the control circuit 54 determines whether or not the uninterruptible power supply 10 has been instructed to be started. When the user of the uninterruptible power supply 10 instructs the start of the uninterruptible power supply 10, the control device 22 sends a signal to the control circuit 54 of each chopper cell 30 to command the start of the uninterruptible power supply 10. In S01, the control circuit 54 determines that the start of the uninterruptible power supply 10 has been instructed when it receives this signal from the control device 22.

[0067] If the uninterruptible power supply 10 is instructed to start (when the YES determination is made in S01), the control circuit 54 starts the initial charging of the capacitor 40 by turning on the switch S6 in step S02 (step S03).

[0068] Figure 7 is a circuit block diagram illustrating the initial charging of capacitor 40. During initial charging, bypass switch S4 and switch S5 are turned off.

[0069] If the terminal voltage of capacitor 40 is lower than the terminal voltage of battery cell 3a, when switch S5 is turned on in step S02 in Figure 6, current flows from battery cell 3a through switch S6, resistor 46, reactor 44, switching element SW5, and DC line PL to capacitor 40, as shown by the arrow in Figure 7. This current charges capacitor 40, and the terminal voltage of capacitor 40 rises. In step S03, the control circuit 54 can control the charge / discharge circuit 45 so that the terminal voltage of capacitor 40 becomes a predetermined target voltage.

[0070] Returning to Figure 6, once the initial charging of capacitor 40 is complete, the control circuit 54 turns off switch S6 and turns on switch S5 in step S04.

[0071] During operation of the uninterruptible power supply 10, the control circuit 54 determines in step S05 whether or not a power outage has occurred in the AC power supply 1. The control device 22 determines whether or not a power outage has occurred in the AC power supply 1 based on the AC input voltage VI supplied from the AC power supply 1, and transmits a power outage detection signal indicating the determination result to the control circuit 54 of each chopper cell 30. In S05, the control circuit 54 determines whether or not a power outage has occurred in the AC power supply 1 based on the power outage detection signal from the control device 22.

[0072] If no power outage has occurred in AC power supply 1 and it is determined that AC power supply 1 is healthy (NO determination in S05), the control circuit 54 controls the charge / discharge circuit 45 in step S06 to supply DC power stored in capacitor 40 to battery cell 3a. In S06, the control circuit 54 controls the charge / discharge circuit 45 so that the terminal voltage of battery cell 3a detected by voltage detector 48 becomes the reference DC voltage VBR.

[0073] On the other hand, if it is determined that a power outage has occurred in the AC power supply 1 (when the YES determination is made in S05), the control circuit 54 controls the charge / discharge circuit 45 in step S11 to supply DC power from the battery cell 3a to the capacitor 40. In S11, the control circuit 54 controls the charge / discharge circuit 45 so that the terminal voltage VDc of the capacitor 40 detected by the voltage detector 42 becomes the reference DC voltage VDcR.

[0074] The control circuit 54 obtains the number of normal chopper cells 30 from the signals exchanged with the control device 22. Then, the control circuit 54 calculates the reference DC voltage VDDc by dividing the reference DC voltage VDR by the number of normal chopper cells 30 included in the bidirectional chopper 18.

[0075] In step S07, the control circuit 54 detects a fault in the battery cell 3a based on the output signals of the voltage detector 48 and the current detector CD3. In S07, a fault in the battery cell 3a is detected if at least one of the terminal voltage of the battery cell 3a detected by the voltage detector 48 and the current of the battery cell 3a detected by the current detector CD3 is outside the acceptable range.

[0076] If no failure is detected in battery cell 3a (when NO is determined in S07), the control circuit 54 executes the processes in steps S05 to S06 and S11. On the other hand, if a failure is detected in battery cell 3a (when YES is determined in S07), the control circuit 54 proceeds to step S08 and disconnects battery cell 3a from chopper cell 30 by turning off switch S5. The control circuit 54 also short-circuits chopper cell 30 by turning on bypass switch S4.

[0077] In step S09, the control circuit 54 stops the operation of the charge / discharge circuit 45 by turning off the switching elements SW5 and SW6. In step S10, the control circuit 54 transmits a fault detection signal indicating a failure in the chopper cell 30 to the control device 22. When the control device 22 receives a fault detection signal from the control circuit 54 of one of the chopper cells 30, it identifies the location of the chopper cell 30 based on the received fault detection signal and notifies the user of the uninterruptible power supply 10 and the control circuits 54 of the other chopper cells 30 of the information indicating the identified location.

[0078] This allows the user of the uninterruptible power supply 10 to perform maintenance on the faulty battery cells 3a of the chopper cells 30 based on the acquired information. In addition, the control circuit 54 of each chopper cell 30 can determine the number of normal chopper cells 30 from the acquired information.

[0079] As described above, each of the multiple chopper cells 30 connected in series between the DC buses 14p and 14n includes a smoothing capacitor 40 and a charge / discharge circuit 45 for transferring DC power between the capacitor 40 and the corresponding battery cell 3a. In this configuration, the voltage applied to each chopper cell 30 can be lowered according to the number of chopper cells 30 included in the bidirectional chopper 18. Therefore, electrical isolation between the capacitor 40 and the battery cell 3a in the charge / discharge circuit 45 becomes unnecessary. As a result, the bidirectional chopper 18 can be miniaturized, and consequently, the uninterruptible power supply 10 can be miniaturized.

[0080] Furthermore, since multiple battery cells 3a are provided corresponding to each of the multiple chopper cells 30, even if one of the multiple battery cells 3a fails, the bypass switch S4 of the chopper cell 30 corresponding to the failed battery cell 3a is turned on to short-circuit that chopper cell 30. This allows the operation of the uninterruptible power supply 10 to continue using the other chopper cells 30 while maintenance of the failed battery cell 3a can be performed.

[0081] Next, other configuration examples of the chopper cell 30 will be described. Figure 8 is a circuit diagram showing a second configuration example of the chopper cell 30. As shown in Figure 8, the chopper cell 30 according to the second configuration example differs from the chopper cell 30 according to the first configuration example shown in Figure 5 in that it incorporates a battery cell 3a.

[0082] Since each chopper cell 30 incorporates a battery cell 3a, it becomes unnecessary to install a battery panel containing multiple battery cells 3a outside the uninterruptible power supply 10. Therefore, the size of the power supply system, including the uninterruptible power supply 10, can be reduced.

[0083] Figure 9 is a circuit diagram showing a third configuration example of the chopper cell 30. As shown in Figure 9, the chopper cell 30 according to the third configuration example differs from the chopper cell 30 according to the first configuration example shown in Figure 5 in that it incorporates a battery cell 3a and includes an equalization circuit 56.

[0084] The equalization circuit 56 is provided between the input / output terminals 30a, 30b and the capacitor 40. The equalization circuit 56 is a circuit for equalizing the terminal voltage VDc of the capacitor 40 of the multiple chopper cells 30.

[0085] If there is variation in the terminal voltage VDc of the capacitors 40 of multiple chopper cells 30, then there will also be variation in the operation of the charge / discharge circuit 45 in each chopper cell 30. For example, consider a case where the terminal voltage VDc of the capacitor 40 in some chopper cells 30 exceeds the reference DC voltage VDcR. In this case, the charge / discharge circuit 45 in that chopper cell 30 is controlled to supply DC power from the capacitor 40 to the battery cell 3a, that is, to charge the battery cell 3a, in order to lower the terminal voltage VDc of the capacitor 40.

[0086] Conversely, if the terminal voltage VDc of the capacitor 40 in some chopper cells 30 is below the reference DC voltage VDcR, the charge / discharge circuit 45 in those chopper cells 30 is controlled to supply DC power from the battery cell 3a to the capacitor 40, that is, to discharge the battery cell 3a, in order to raise the terminal voltage VDc of the capacitor 40.

[0087] In this way, the charging and discharging of the battery cells 3a are individually controlled in each chopper cell 30 according to the terminal voltage VDc of the capacitor 40. Therefore, if there is variation in the terminal voltage VDc of the capacitors 40 of multiple chopper cells 30, a difference will occur in the number of times the battery cells 3a are charged and discharged among the multiple chopper cells 30. Battery cells 3a normally deteriorate gradually with repeated charge and discharge cycles. As a result, among the multiple battery cells 3a, the battery cell 3a with a high number of charge and discharge cycles will deteriorate more rapidly than the battery cell 3a with a low number of charge and discharge cycles. This variation in the number of charge and discharge cycles leads to variations in the lifespan of the multiple battery cells 3a. The equalization circuit 56 reduces the variation in the terminal voltage VDc of the capacitors 40 of multiple chopper cells 30, thereby suppressing variations in the lifespan of the multiple battery cells 3a and achieving a longer lifespan.

[0088] The equalization circuit 56 includes switching elements SW7 and SW8. Each of the switching elements SW7 and SW8 is configured by connecting a freewheeling diode in antiparallel to a self-extinguishing semiconductor switching element such as an IGBT or MOSFET. The switching elements SW7 and SW8 are connected in series between the DC line PL and the DC line NL. The connection point between the switching elements SW7 and SW8 is connected to the input / output terminal 30a. The control circuit 54 controls the on and off states of the switching elements SW7 and SW8 based on the signal from the control device 22 and the output signal from the voltage detector 42, as will be described later.

[0089] Figure 10 is a flowchart showing the control of the chopper cell 30 according to the third configuration example by the control circuit 54. The flowchart in Figure 10 differs from the flowchart shown in Figure 6 in that it includes step S04A and includes step S09A instead of step S09.

[0090] As shown in Figure 10, during operation of the uninterruptible power supply 10, the control circuit 54 of each chopper cell 30 controls the equalization circuit 56 in step S04A so that the terminal voltages VDc of the capacitors 40 of the multiple chopper cells 30 become equal.

[0091] In S04A, the control circuit 54 acquires the output signal of the voltage detector 42 from the other chopper cells 30 via the control device 22. Based on the output signal of the voltage detector 42 of its own chopper cell 30 and the acquired output signals of the voltage detector 42 of the other chopper cells 30, the control circuit 54 calculates the average value VDc of the terminal voltages VDc of the capacitors 40 of the multiple chopper cells 30, VDcave. The control circuit 54 calculates the deviation ΔVDc between the calculated average value VDc_ave and the reference DC voltage VDcR (ΔVDc = VDcR - VDc_ave). The control circuit 54 controls the on and off states of the switching elements SW7 and SW8 so that the deviation ΔVDc becomes zero.

[0092] If a failure of battery cell 3a is detected in step S07, the same as in Figure 6 (when YES is determined in S07), the control circuit 54 proceeds to step S08, where it turns off switch S5 to disconnect battery cell 3a from chopper cell 30 and turns on bypass switch S4 to short-circuit chopper cell 30. Furthermore, in step S09A, the control circuit 54 stops the operation of the charge / discharge circuit 45 by turning off switching elements SW5 and SW6, and stops the operation of the equalization circuit 56 by turning off switching elements SW7 and SW8.

[0093] As described above, in this embodiment, each of the converter 12 and inverter 20 is composed of a modular multilevel converter (MMC). The MMC can output a stepped multilevel voltage waveform by shifting the operating timing of each converter cell 26, and the output AC voltage can be made approximately sinusoidal, thus eliminating the need for an AC filter.

[0094] By applying MMC suitable for high voltage and large capacity to both the converter 12 and the inverter 20, the uninterruptible power supply 10 can supply medium-voltage AC power to the AC transmission line 4. Therefore, compared to conventional uninterruptible power supplies that supply low-voltage (e.g., 400V) AC power to the AC transmission line, the current flowing through the AC transmission line can be reduced. This reduces power loss and voltage drop in the AC transmission line. In addition, the number of AC transmission lines can be reduced, which enables miniaturization of the power supply system and reduction of equipment costs.

[0095] In this embodiment, the bidirectional chopper 18 is composed of a plurality of chopper cells 30 connected in series. Each of the plurality of chopper cells 30 includes a smoothing capacitor 40 and a charge / discharge circuit 45 for exchanging DC power between the capacitor 40 and the corresponding battery cell 3a. As the number of chopper cells 30 included in the bidirectional chopper 18 can be adjusted, the voltage applied to each chopper cell 30 can be reduced, eliminating the need for electrical isolation between the capacitor 40 and the battery cell 3a in the charge / discharge circuit 45. As a result, the bidirectional chopper 18 can be miniaturized, and consequently, the uninterruptible power supply 10 can be miniaturized.

[0096] Furthermore, since multiple battery cells 3a are provided corresponding to each of the multiple chopper cells 30, even if one of the multiple battery cells 3a fails, the bypass switch S4 of the chopper cell 30 corresponding to the failed battery cell 3a is turned on to short-circuit that chopper cell 30. This allows the operation of the uninterruptible power supply 10 to continue using the other chopper cells 30 while maintenance of the failed battery cell 3a can be performed.

[0097] Furthermore, as explained in Figure 9, by providing an equalization circuit 56 in each chopper cell 30 to equalize the terminal voltage VDDc of the capacitors 40 of the multiple chopper cells 30, variations in the number of charge and discharge cycles of the multiple battery cells 3a can be suppressed, thereby achieving a longer lifespan for the multiple battery cells 3a.

[0098] Next, an example of the application of the uninterruptible power supply 10 according to this embodiment will be described. Figure 11 is a diagram showing an example of the configuration of a power supply system using the uninterruptible power supply 10 according to this embodiment. The power supply system can be applied, for example, to a large-scale data center.

[0099] As shown in Figure 11, the power supply system includes multiple uninterruptible power supplies 10 connected in parallel between the AC power sources 1 and 1A and the AC transmission line 4. The basic configuration of each uninterruptible power supply 10 is the same as that of the uninterruptible power supply 10 described in Figures 1 to 10. The uninterruptible power supply 10 shown in Figure 11 differs in that it includes a switch S7 instead of a switch S1, and has a transformer 2.

[0100] Switch S7 is controlled by a control device (not shown) and is configured to connect one of the two AC power sources 1 and 1A to the primary side of transformer 2. AC power source 1 is, for example, a commercial AC power source, and AC power source 1A is, for example, a generator. In other words, the uninterruptible power supply 10 is configured to be switchable between a commercial AC power source and a generator as the AC power source.

[0101] As shown in Figure 11, the converter 12 receives 13.8 kV (medium voltage) AC power from either AC power source 1 or 1A via switch S5 and transformer 2. The AC power is 50 MVA.

[0102] Figure 12 shows the main circuit configuration of the converter 12, inverter 20, and bidirectional chopper 18 shown in Figure 11. As explained in Figures 2 to 4, the converter 12 is composed of a modular multilevel converter (MMC) and converts 13.8 kV AC power (50 MVA) to 22.5 kV (medium voltage) DC power and outputs it to the DC bus 14.

[0103] The bidirectional chopper 18 is configured to include a plurality of chopper cells 30 connected in series, as described in Figures 2 and 5 to 10.

[0104] The inverter 20 is composed of a modular multilevel circuit (MMC) and converts 22.5 kV (medium voltage) DC power supplied from the converter 12 or bidirectional chopper 18 via the DC bus 14 into 13.8 kV (medium voltage) AC power which is then supplied to the AC transmission line 4. The AC transmission line 4 is approximately 1,000 feet long. The 13.8 kV (medium voltage) AC power transmitted through the AC transmission line 4 is converted to 415 V (low voltage) AC power by the transformer 5 and supplied to the load.

[0105] The embodiments disclosed herein should be considered in all respects to be illustrative and not restrictive. The scope of this disclosure is indicated by the claims and not by the foregoing description, and all modifications within the meaning and scope of the claims are intended to be included.

[0106] 1, 1A AC power supply, 2, 5 transformer, 3 battery, 3a battery cell, 4 AC transmission line, 6 load, 10 uninterruptible power supply, 12 converter, 12r, 12s, 12t, 20u, 20v, 20w reg circuit, 14 DC bus, 14p DC positive bus, 14n DC negative bus, 18 bidirectional chopper, 20 inverter, 22 control device, 24 upper arm, 25 lower arm, 26 converter cell, 26a, 26b, 30a, 30b input / output terminals, 28, 38, 40 capacitor, 30 chopper cell, 30c, 30d, T2 DC terminal, 42, 48 voltage detector, 44, L1, L2 reactor, 45 charge / discharge circuit, 46 resistor element, 50 fuse, 52 power supply, 54 control circuit, 56 Equalization circuit, CD1-CD3 current detectors, NL, PL DC lines, S1-S3, S5-S7 switches, S4 bypass switch, SW1-SW8 switching elements, T1 input terminal, T3 output terminal.

Claims

1. An uninterruptible power supply connected between an AC power source and an AC transmission line, comprising: a converter that converts AC power supplied from the AC power source into DC power and supplies it between a DC positive busbar and a DC negative busbar; a bidirectional chopper that converts the DC voltage between the DC positive busbar and the DC negative busbar and the DC voltage of a power storage device to each other; and an inverter that converts the DC power received from the DC positive busbar and the DC negative busbar into AC power and supplies it to the AC transmission line, wherein each of the converter and the inverter includes a leg circuit having a plurality of cascaded converter cells; the bidirectional chopper includes a plurality of chopper cells connected in series between the DC positive busbar and the DC negative busbar; the power storage device includes a plurality of power storage cells provided corresponding to each of the plurality of chopper cells and connected in parallel to each other; each of the plurality of chopper cells includes a first input / output terminal and a second input / output terminal, and a capacitor connected between the first input / output terminal and the second input / output terminal. An uninterruptible power supply including the capacitor and a charge / discharge circuit for transferring DC power between the capacitor and a corresponding energy storage cell.

2. The uninterruptible power supply according to claim 1, further comprising a control device for controlling the converter, the inverter and the bidirectional chopper, wherein each of the plurality of chopper cells is communicated with the control device and further includes a control circuit for controlling the charge / discharge circuit and a power supply for generating a power supply voltage for the control circuit from the DC voltage of the corresponding energy storage cell.

3. In each of the plurality of chopper cells, the control circuit controls the charge / discharge circuit to supply DC power from the capacitor to the corresponding energy storage cell when the AC power supply is healthy, and controls the charge / discharge circuit to supply DC power from the corresponding energy storage cell to the capacitor when the AC power supply is interrupted, as described in claim 2.

4. The uninterruptible power supply according to claim 2, wherein each of the plurality of chopper cells further includes a resistor and a switch connected in series between the charge / discharge circuit and the corresponding energy storage cell, and when the uninterruptible power supply is started, the control circuit turns on the switch and supplies DC power from the corresponding energy storage cell to the capacitor via the resistor and the charge / discharge circuit.

5. The uninterruptible power supply according to claim 2, wherein each of the plurality of chopper cells further includes a bypass switch connecting a first input / output terminal and a second input / output terminal, and the control circuit stops the operation of the charge / discharge circuit and turns on the bypass switch when the corresponding energy storage cell fails.

6. The uninterruptible power supply according to claim 2, wherein each of the plurality of chopper cells further includes an equalization circuit comprising a plurality of semiconductor switching elements, and the control circuit controls the on and off of the plurality of semiconductor switching elements so that the terminal voltages of the capacitors of the plurality of chopper cells are equal.

7. The uninterruptible power supply according to claim 1, wherein each of the plurality of chopper cells incorporates the corresponding energy storage cell.

8. The uninterruptible power supply according to any one of claims 1 to 7, wherein the AC power supply is a power system or generator that supplies medium-voltage AC power, and when the AC power supply is healthy, the converter converts the medium-voltage AC power supplied from the AC power supply into medium-voltage DC power and supplies it between the DC positive busbar and the DC negative busbar, and the inverter converts the medium-voltage DC power received from the DC positive busbar and the DC negative busbar into medium-voltage AC power and supplies it to the AC transmission line.

9. The uninterruptible power supply according to claim 7, wherein the energy storage device is configured to store medium-voltage DC power, and in the event of a power outage of the AC power supply, the converter stops operating, and the bidirectional chopper supplies the medium-voltage DC power from the energy storage device to the DC positive busbar and the DC negative busbar.

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