Uninterruptible power supply device

Abstract

An uninterruptible power supply device (10) comprises: a converter (12) that converts AC power supplied from an AC power supply (1) into DC power and supplies the DC power to a DC bus (14); an inverter (18) that converts DC power received from the DC bus (14) into AC power and supplies the AC power to an AC transmission line (4); and a power storage device (3). The converter (12) includes a plurality of cascaded first converter cells. The inverter (18) includes a plurality of cascaded second converter cells. The power storage device (3) includes a plurality of power storage cells (3a) provided respectively corresponding to the plurality of first converter cells. Each of the plurality of first converter cells includes: a first input / output terminal and a second input / output terminal; a first bridge circuit including a plurality of semiconductor switching elements; a first capacitor connected to the first and second input / output terminals via the first bridge circuit; and a charge / discharge circuit. The charge / discharge circuit exchanges DC power between the first capacitor and the 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. Also, 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 one aspect of the present disclosure is connected between an AC power source and an AC transmission line. The UPS comprises a converter that converts AC power supplied from the AC power source into DC power and supplies it to a DC bus, an inverter that converts DC power received from the DC bus into AC power and supplies it to an AC transmission line, and a power storage device. The converter includes a plurality of first converter cells cascaded together for each phase of the AC power source. The inverter includes a plurality of second converter cells cascaded together for each phase of the AC power source. The power storage device includes a plurality of power storage cells provided corresponding to each of the plurality of first converter cells. Each of the plurality of first converter cells includes a first input / output terminal and a second input / output terminal, a first bridge circuit including a plurality of semiconductor switching elements, a first capacitor connected to the first input / output terminal and the second input / output terminal via the first bridge circuit, and a charge / discharge circuit. The charge / discharge circuit is configured to exchange DC power between the first capacitor and the corresponding power 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 and inverter. This is a circuit diagram showing a first configuration example of a converter cell included in the inverter. This is a circuit diagram showing a second configuration example of a converter cell included in the inverter. This is a circuit diagram showing a first configuration example of a converter cell included in the converter. This is a flowchart showing the control of the converter 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 converter cell included in the converter. This is a circuit diagram showing a third configuration example of a converter cell included in the converter. 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 and inverter shown in Figure 10.

[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 capacitor 16, an inverter 18, and a control device 20.

[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 20. 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 the control device 20 with a signal Iif indicating the detected value.

[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 S7 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 S7 connected to each load 6 is turned on or off.

[0019] When switch S7 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. When the battery 3 is fully charged, the terminal voltage VB of the battery 3 is, for example, several kV. The instantaneous value of the terminal voltage VB of the battery 3 is detected by the control device 20.

[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 converter cells included in the converter 12. The battery cells 3a constitute an "energy storage cell".

[0022] Switch S1 is connected between the input terminal T1 and the AC node 12a of the converter 12 and is controlled by the control device 20. 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 has an AC node 12a and DC nodes 12b and 12c. The AC node 12a is connected to input terminal T1 via switch S1, the DC node 12b is connected to DC terminal T2, and the DC node 12c is connected to DC bus 14.

[0024] The converter 12 is controlled by the control device 20 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 is also configured to exchange DC power between the DC bus 14 and the battery 3 by performing bidirectional DC voltage conversion between the DC bus 14 and the battery 3. The converter 12 will be described in more detail later.

[0025] Capacitor 16 is connected to the DC bus 14 and smooths and stabilizes the DC voltage VD of the DC bus 14. The instantaneous value of the DC voltage VD of the DC bus 14 is detected by the control device 20. When the AC power supply 1 is healthy, the control device 20 controls the converter 12 so that the DC voltage VD becomes the reference DC voltage VDR and the terminal voltage VB of the battery 3 becomes the reference DC voltage VBR. When the AC power supply 1 fails, the control device 20 controls the converter 12 so that the DC voltage VD becomes the reference DC voltage VDR.

[0026] The DC bus 14 is connected to the DC node 18a of the inverter 18, and the AC node 18b of the inverter 18 is connected to the output terminal T3 via switch S2. Switch S2 is controlled by the control device 20. When the uninterruptible power supply 10 is in use, switch S2 is turned on. When the inverter 18 is being maintained, switch S2 is turned off.

[0027] The current detector CD2 detects the AC output current Io of the inverter 18 and provides the control device 20 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 20.

[0028] The inverter 18 is controlled by the control device 20 and converts the DC power supplied from the converter 12 via the DC bus 14 into AC power of a predetermined frequency, which is then supplied to the AC transmission line 4. The inverter 18 will be described in more detail later.

[0029] The control device 20 controls switches S1, S2, converter 12, and inverter 18 based on the AC input voltage VI, AC output voltage VO, DC voltage VD, terminal voltage VB of battery 3, AC input current Ii, and AC output current Io.

[0030] The control device 20 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.

[0031] Figure 2 is a circuit diagram showing the main circuit configuration of the converter 12 and inverter 18. As shown in Figure 2, each of the converter 12 and inverter 18 is composed of a modular multilevel converter (MMC) in which multiple converter cells are cascaded together.

[0032] 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".

[0033] 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. The AC input terminals Nr, Ns, and Nt correspond to the AC node 12a shown in Figure 1. In the following description, when referring to the AC input terminals Nr, Ns, and Nt collectively, they will also be referred to as "AC input terminal Nx".

[0034] The leg circuit 12r includes an upper arm circuit 24 from the DC positive busbar 14p to the AC input terminal Nr, and a lower arm circuit 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.

[0035] The upper arm circuit 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 circuit 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. The transducer cells 26 correspond to one embodiment of the "first transducer cell".

[0036] Reactors L1 and L2 are buffer reactors for suppressing short-circuit current. Reactor L1 may be inserted at any position in the upper arm circuit 24, and reactor L2 may be inserted at any position in the lower arm circuit 25. Multiple reactors L1 and L2 may be provided. Alternatively, only reactor L1 in the upper arm circuit 24 or only reactor L2 in the lower arm circuit 25 may be provided.

[0037] The inverter 18 includes multiple leg circuits 18u, 18v, and 18w. The multiple leg circuits 18u, 18v, and 18w 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 18u, 18v, and 18w collectively, they will also be referred to as "leg circuit 18x".

[0038] The leg circuits 18x 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 18u, 18v, and 18w 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 18u, 18v, and 18w are connected to the output terminal T3 via switch S2. The AC output terminals Nu, Nv, and Nw correspond to the AC node 18b shown in Figure 1. In the following description, when referring to the AC output terminals Nu, Nv, and Nw collectively, they will also be referred to as "AC output terminal Nx".

[0039] The leg circuit 18u includes an upper arm circuit 34 from the DC positive busbar 14p to the AC output terminal Nu, and a lower arm circuit 35 from the DC negative busbar 14n to the AC output terminal Nu. Leg circuits 18v and 18w have a similar configuration, so the configuration of leg circuit 18u will be described representatively below.

[0040] The upper arm circuit 34 includes a plurality of cascaded transducer cells 36 and a reactor L1. The plurality of transducer cells 36 and reactor L1 are connected in series. Similarly, the lower arm circuit 35 includes a plurality of cascaded transducer cells 36 and a reactor L2. The plurality of transducer cells 36 and reactor L2 are connected in series. The transducer cells 36 correspond to one embodiment of the "second transducer cell".

[0041] Reactors L1 and L2 are buffer reactors for suppressing short-circuit current. The position where reactor L1 is inserted may be any position in the upper arm circuit 34, and the position where reactor L2 is inserted may be any position in the lower arm circuit 35. A plurality of reactors L1 and L2 may be provided respectively. Or, only reactor L1 in the upper arm circuit 34 or only reactor L2 in the lower arm circuit 35 may be provided.

[0042] FIG. 3 is a circuit diagram showing a first configuration example of a converter cell 36 included in the inverter 18. As shown in FIG. 3, the converter cell 36 according to the first configuration example includes a series body formed by connecting two switching elements SW1 and SW2 in series, a capacitor 38, and input / output terminals 36a and 36b.

[0043] In the upper arm circuit 34, the input / output terminal 36a of the uppermost converter cell 36 is connected to the DC positive bus 14p. The input / output terminal 36b of each converter cell 36 other than the lowermost one is connected to the input / output terminal 36a of the adjacent converter cell 36. The input / output terminal 36b of the lowermost converter cell 36 is connected to the AC output terminal Nx.

[0044] In the lower arm circuit 35, the input / output terminal 36a of the uppermost converter cell 36 is connected to the AC output terminal Nx. The input / output terminal 36b of each converter cell 36 other than the lowermost one is connected to the input / output terminal 36a of the adjacent converter cell 36. The input / output terminal 36b of the lowermost converter cell 36 is connected to the DC negative bus 14n.

[0045] The series body of the switching elements SW1 and SW2 and the capacitor 38 are connected in parallel. The series body of the switching elements SW1 and SW2 constitutes a half-bridge circuit. The switching elements SW1 and SW2 correspond to an embodiment of the "second semiconductor switching element", and the half-bridge circuit corresponds to an embodiment of the "second bridge circuit". The capacitor 38 constitutes a "power storage element" for storing DC power. The capacitor 38 corresponds to an embodiment of the "second capacitor".

[0046] Both terminals of the switching element SW2 are connected to the input / output terminals 36a and 36b, respectively. The converter cell 36 outputs the voltage Vc between the terminals of the capacitor 38 or zero voltage between the input / output terminals 36a and 36b by the switching operations 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 38 is output from the converter cell 36. When the switching element SW1 is off and the switching element SW2 is on, the converter cell 36 outputs zero voltage.

[0047] FIG. 4 is a circuit diagram showing a second configuration example of the converter cell 36 included in the inverter 18. As shown in FIG. 4, the converter cell 36 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 38, and input / output terminals 36a and 36b. 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. The full-bridge circuit corresponds to an embodiment of the "second bridge circuit".

[0048] The connection point of the switching element SW1 and the switching element SW2 is connected to the input / output terminal 36a. Similarly, the connection point of the switching element SW3 and the switching element SW4 is connected to the input / output terminal 36b. The converter cell 36 outputs the voltage Vc, -Vc between the terminals of the capacitor 38 or zero voltage between the input / output terminals 36a and 36b by the switching operations of the switching elements SW1 to SW4.

[0049] The switching elements SW1 to SW4 are configured by connecting a freewheeling diode (FWD) in anti-parallel to a self-extinguishing semiconductor switching element such as an IGBT (Insulated Gate Bipolar Transistor) or a MOSFET (Metal Oxide Semiconductor Field Effect Transistor).

[0050] Figure 5 is a circuit diagram showing a first configuration example of a converter cell 26 included in the converter 12. As shown in Figure 5, the converter cell 26 according to the first configuration example includes input / output terminals 26a, 26b, DC terminals 26c, 26d, switching elements SW1, SW2, a capacitor 40, a bypass switch S3, a charge / discharge circuit 44, switches S4, S5, a resistor R1, voltage detectors 42, 48, a current detector CD3, a fuse 50, a power supply 52, and a control circuit 54.

[0051] In the upper arm circuit 24, the input / output terminal 26a of the uppermost converter cell 26 is connected to the DC positive busbar 14p. The input / output terminal 26b of each converter cell 26, except for the bottommost one, is connected to the input / output terminal 26a of the adjacent converter cell 26. The input / output terminal 26b of the bottommost converter cell 26 is connected to the AC input terminal Nx.

[0052] In the lower arm circuit 25, the input / output terminal 26a of the uppermost converter cell 26 is connected to the AC input terminal Nx. The input / output terminal 26b of each converter cell 26 other than the bottommost one is connected to the input / output terminal 26a of the adjacent converter cell 26. The input / output terminal 26b of the bottommost converter cell 26 is connected to the DC negative busbar 14n.

[0053] DC terminal 26c is connected to the positive terminal of the corresponding battery cell 3a, and DC terminal 26d is connected to the negative terminal of the corresponding battery cell 3a.

[0054] Switching elements SW1 and SW2 are connected in series between DC line PL and DC line NL. The connection point between switching element SW1 and switching element SW2 is connected to input / output terminal 26a. The connection point between switching element SW2 and DC line NL is connected to input / output terminal 26b and DC terminal 26d.

[0055] The series-connected switching elements SW1 and SW2 form a half-bridge circuit. The half-bridge circuit corresponds to one embodiment of the "first bridge circuit". The switching elements SW1 and SW2 correspond to one embodiment of the "first semiconductor switching element". The capacitor 40 is connected between the DC line PL and the DC line NL. That is, the capacitor 40 is connected in parallel with the series-connected switching elements SW1 and SW2, forming an "energy storage element" that stores DC power. The capacitor 40 corresponds to one embodiment of the "first capacitor". The voltage detector 42 detects the instantaneous value of the terminal voltage of the capacitor 40 and provides a signal indicating the detected value to the control circuit 54.

[0056] The converter cell 26 outputs the terminal voltage of the capacitor 40 or a zero voltage between the input / output terminals 26a and 26b by the switching operation of switching elements SW1 and SW2. The on and off states of switching elements SW1 and SW2 are controlled by the control circuit 54. Switching elements SW1 and SW2 are switched on and off complementaryly. When switching element SW1 is on and switching element SW2 is off, the converter cell 26 outputs the terminal voltage of the capacitor 40. When switching element SW1 is off and switching element SW2 is on, the converter cell 26 outputs a zero voltage.

[0057] The bypass switch S3 is connected between input / output terminals 26a and 26b. The bypass switch S3 is configured to short-circuit input / output terminals 26a and 26b by being turned on in response to a command from the control circuit 54. In some cases, the bypass switch S3 can be used to short-circuit the converter cell 26 when the corresponding battery cell 3a fails. In this case, even if the battery cell 3a fails in some of the converter cells 26 among the multiple converter cells 26, the operation of the uninterruptible power supply 10 can be continued by using the other converter cells 26.

[0058] The charge / discharge circuit 44 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 44 is composed of switching elements SW5 and SW6 and a reactor 46. 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.

[0059] 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 46. The second terminal of reactor 46 is connected to DC terminal 26c via switch S4.

[0060] Although Figure 3 illustrates an example in which the charge / discharge circuit 44 includes a bidirectional chopper, it is not limited to this. The charge / discharge circuit 44 can include a well-known non-isolated DC / DC converter.

[0061] The charge / discharge circuit 44 is controlled by the control circuit 54. When the AC power supply 1 is functioning properly, the charge / discharge circuit 44 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 44 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 44 and the battery cell 3a and provides a signal indicating the detected value to the control circuit 54.

[0062] Switch S4 is connected between the second terminal of the reactor 46 and the DC terminal 26c and is controlled by the control circuit 54. Switch S4 is turned ON when the converter cell 26 is in use. Switch S4 is turned OFF when the uninterruptible power supply 10 is started up and when the battery cell 3a is being maintained.

[0063] A fuse 50 is provided between the switch S4 and the DC terminal 26c. If a large current flows between the charge / discharge circuit 44 and the battery cell 3a, the fuse 50 will blow.

[0064] The resistor R1 and switch S5 are connected in series between the first and second terminals of switch S4. Switch S5 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 R1 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.

[0065] 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.

[0066] The first input terminal of power supply 52 is connected to DC terminal 26c, and the second input terminal of power supply 52 is connected to DC terminal 26d. Power supply 52 steps down the DC voltage supplied from battery cell 3a via DC terminals 26c and 26d to generate the power supply voltage for control circuit 54. As a result, control circuit 54 can operate even when the uninterruptible power supply 10 is stopped.

[0067] The control circuit 54 is driven by the power supply voltage supplied from the power supply 52 and exchanges signals with the control device 20. The control circuit 54 controls the converter cell 36 based on signals from the control device 20, the output signals from the voltage detectors 42 and 48, and the output signal from the current detector CD3.

[0068] 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.

[0069] Figure 6 is a flowchart showing the control of the converter cells 26 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 start. When the user of the uninterruptible power supply 10 instructs the start of the uninterruptible power supply 10, the control device 20 transmits a signal to the control circuit 54 of each converter cell 26 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 20.

[0070] 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 S5 in step S02 (step S03).

[0071] Figure 7 is a circuit block diagram illustrating the initial charging of capacitor 40. During initial charging, bypass switches S3 and S4 are turned off, and switching elements SW1 and SW2 are turned off.

[0072] 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 S5, resistor R1, reactor 46, 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 44 so that the terminal voltage of capacitor 40 becomes a predetermined target voltage.

[0073] Returning to Figure 6, once the initial charging of the capacitor 40 is complete, the control circuit 54 turns off switch S5 and turns on switch S4 in step S04.

[0074] 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 20 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 converter cell 26. 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 20.

[0075] 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 half-bridge circuit in step S06 to convert the AC power input between input / output terminals 26a and 26b from AC power supply 1 into DC power. Furthermore, in step S07, the control circuit 54 controls the charge / discharge circuit 44 to supply the DC power stored in capacitor 40 to battery cell 3a.

[0076] 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 44 in step S12 to supply DC power from the battery cell 3a to the capacitor 40. Furthermore, in step S13, the control circuit 54 controls the half-bridge circuit to output the DC power stored in the capacitor 40 between the input / output terminals 26a and 26b.

[0077] In step S08, 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 S08, 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.

[0078] If no failure is detected in battery cell 3a (when S08 is determined to be NO), the control circuit 54 executes the processes in steps S05 to S07, S12, and S13. On the other hand, if a failure is detected in battery cell 3a (when S08 is determined to be YES), the control circuit 54 proceeds to step S09 and disconnects battery cell 3a from converter cell 26 by turning off switch S4. The control circuit 54 also short-circuits converter cell 26 by turning on bypass switch S3.

[0079] In step S10, the control circuit 54 stops the operation of the half-bridge circuit by turning off the switching elements SW1 and SW2. The control circuit 54 also stops the operation of the charge / discharge circuit 44 by turning off the switching elements SW5 and SW6.

[0080] In step S11, the control circuit 54 transmits a fault detection signal indicating a failure in the converter cell 26 to the control device 20. When the control device 20 receives a fault detection signal from the control circuit 54 for one of the converter cells 26, it identifies the location of that converter cell 26 based on the received fault detection signal and notifies the user of the uninterruptible power supply 10 of the information indicating the identified location. This allows the user of the uninterruptible power supply 10 to perform maintenance on the battery cell 3a of the faulty converter cell 26 based on the acquired information.

[0081] The control device 20 may further short-circuit one converter cell 26 in each leg circuit other than the leg circuit containing the faulty converter cell 26. For example, if one converter cell 26 fails in the upper arm circuit 24 of leg circuit 12r, the control circuit 54 short-circuits that converter cell 26 and also selects one of the multiple converter cells 26 included in the upper arm circuit 24 of leg circuit 12s and one of the multiple converter cells 26 included in the upper arm circuit 24 of leg circuit 12t, and short-circuits the two selected converter cells 26. In this way, the number of operating converter cells 26 can be matched among the leg circuits 12r, 12s, and 12t.

[0082] As explained above, each of the multiple converter cells 26 included in the converter 12 is configured to include a capacitor 40, which is an energy storage element, and a charge / discharge circuit 44 that exchanges DC power between it and the corresponding battery cell 3a. Therefore, in the uninterruptible power supply 10 shown in Figure 1, it is not necessary to install a bidirectional DC / DC converter for exchanging DC power between the DC bus 14 and the battery 3.

[0083] Furthermore, since multiple converter cells 26 are cascaded for each phase of the AC power supply 1, the peak voltage applied to each converter cell 26 can be reduced according to the number of converter cells 26 included in each phase. As a result, electrical isolation between the capacitor 40 and the battery cell 3a is unnecessary in the charge / discharge circuit 44. For these reasons, it is possible to miniaturize the uninterruptible power supply 10.

[0084] Furthermore, since multiple battery cells 3a are provided corresponding to each of the multiple converter cells 26, even if one of the multiple battery cells 3a fails, the bypass switch S3 of the converter cell 26 corresponding to the failed battery cell 3a is turned on to short-circuit the converter cell 26, allowing the uninterruptible power supply 10 to continue operating using the other converter cells 26 while maintenance of the failed battery cell 3a can be performed.

[0085] Next, other configuration examples of the converter cell 26 included in the converter 12 will be described. Figure 8 is a circuit diagram showing a second configuration example of the converter cell 26 included in the converter 12. The converter cell 26 according to the second configuration example differs from the converter cell 26 according to the first configuration example shown in Figure 5 in that it includes switching elements SW3 and SW4.

[0086] As shown in Figure 8, in the converter cell 26 according to the second configuration example, switching elements SW1 and SW2 are connected in series between DC line PL and DC line NL. The connection point between switching element SW1 and switching element SW2 is connected to input / output terminal 26a. The connection point between switching element SW2 and DC line NL is connected to input / output terminal 26b and DC terminal 26d. Switching elements SW3 and SW4 are connected in series between DC line PL and DC line NL. The connection point between switching element SW3 and switching element SW4 is connected to input / output terminal 26b.

[0087] The first series configuration of switching elements SW1 and SW2, and the second series configuration of switching elements SW3 and SW4, constitute a full-bridge circuit. The full-bridge circuit corresponds to one embodiment of the "first bridge circuit".

[0088] In the second configuration example, the full-bridge circuit is controlled by the control circuit 54, and the switching operation of the switching elements SW1 to SW4 outputs the terminal voltage of the capacitor 40, a voltage with the polarity of the terminal voltage of the capacitor 40 reversed, or a zero voltage between the input and output terminals 26a and 26b. The other configurations of the second configuration example are the same as those of the first configuration example.

[0089] Figure 9 is a circuit diagram showing a third configuration example of the converter cell 26 included in the converter 12. The converter cell 26 according to the third configuration example differs from the converter cell 26 according to the first configuration example shown in Figure 5 in that it incorporates a battery cell 3a. By incorporating a battery cell 3a into each converter cell 26, 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. Although not shown in the diagram, it is also possible to incorporate a battery cell 3a into the converter cell 26 according to the second configuration example shown in Figure 8.

[0090] In the embodiment described above, a configuration was described in which each of the multiple converter cells 26 included in the converter 12 has a charge / discharge circuit 44. However, the same effects as in the embodiment described above can be obtained by replacing the multiple converter cells 26 with a configuration in which each of the multiple converter cells 36 included in the inverter 18 has a charge / discharge circuit 44. In this case, each of the multiple converter cells 36 will have the circuit configuration shown in Figure 5, Figure 8, or Figure 9, and each of the multiple converter cells 26 will have the circuit configuration shown in Figure 3 or Figure 4.

[0091] Next, an example of the application of the uninterruptible power supply 10 according to this embodiment will be described. Figure 10 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.

[0092] As shown in Figure 10, 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 9. The uninterruptible power supply 10 shown in Figure 10 differs in that it includes a switch S6 instead of a switch S1, and has a transformer 2.

[0093] Switch S6 is controlled by a control device 20 (not shown) and is configured to connect one of the two AC power sources 1 and 1A to the primary side of the 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.

[0094] As shown in Figure 10, 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.

[0095] Figure 11 shows the main circuit configuration of the converter 12 and inverter 18 shown in Figure 10. As explained in Figure 2, 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, and also exchanges DC power between the DC bus 14 and the battery 3. The battery 3 stores 1.5 kV (medium voltage) DC power. The battery 3 has a plurality of battery cells 3a connected in parallel.

[0096] Each of the plurality of converter cells 26 constituting the converter 12 is configured to include a bridge circuit, a capacitor 40, and a charge / discharge circuit 44 provided between the capacitor 40 and the corresponding battery cell 3a, as described in Figures 5 to 9.

[0097] The inverter 18 is composed of a modular multilevel converter (MMC) and converts 22.5 kV (medium voltage) DC power supplied from the converter 12 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.

[0098] In this embodiment, each of the converter 12 and inverter 18 is composed of a modular multilevel converter (MMC). The MMC can output a stepped multilevel voltage waveform by staggering the operating timing of each converter cell, and since the output AC voltage can be made approximately sinusoidal, an AC filter is not required.

[0099] In this embodiment, by applying MMC suitable for high voltage and large capacity to each of the converter 12 and inverter 18, 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 4 can be reduced. This reduces power loss and voltage drop in the AC transmission line 4. Furthermore, since the number of AC transmission lines can be reduced, it becomes possible to miniaturize the power supply system and reduce equipment costs.

[0100] Furthermore, in this embodiment, as described above, each of the plurality of converter cells 26 included in the converter 12 is configured to include a charge / discharge circuit 44 that exchanges DC power between the capacitor 40 and the corresponding battery cell 3a. Therefore, it is not necessary to install a bidirectional DC / DC converter for exchanging DC power between the DC bus 14 and the battery 3, and the power supply system can be made smaller.

[0101] Furthermore, since multiple battery cells 3a are provided corresponding to each of the multiple converter cells 26, even if one of the multiple battery cells 3a fails, the bypass switch S3 of the converter cell 26 corresponding to the failed battery cell 3a is turned on to short-circuit the converter cell 26, allowing the uninterruptible power supply 10 to continue operating using the other converter cells 26 while maintenance of the failed battery cell 3a can be performed.

[0102] 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.

[0103] 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, 18u, 18v, 18w reg circuit, 14 DC bus, 14p DC positive bus, 14n DC negative bus, 16, 38, 40 capacitor, 18 inverter, 20 control device, 24, 34 upper arm circuit, 25, 35 lower arm circuit, 26a, 26b, 36a, 26b input / output terminals, 26, 36 converter cell, 26c, 26d, T2 DC terminal, 42, 48 voltage detector, 44 charge / discharge circuit, 46, L1, L2 reactor, 50 fuse, 52 power supply, 54 control circuit, CD1-CD3 Current detector, PL, NL DC lines, SW1 to SW6 switching elements, R1 resistance element, S1 to S2, S4 to S7 switches, S3 bypass switch, 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 to a DC bus; an inverter that converts DC power received from the DC bus into AC power and supplies it to the AC transmission line; and a power storage device, wherein the converter includes a plurality of cascaded first converter cells; the inverter includes a plurality of cascaded second converter cells; the power storage device includes a plurality of power storage cells provided corresponding to each of the plurality of first converter cells; each of the plurality of first converter cells includes a first input / output terminal and a second input / output terminal; a first bridge circuit including a plurality of first semiconductor switching elements; a first capacitor connected to the first input / output terminal and the second input / output terminal via the first bridge circuit; and a charge / discharge circuit for exchanging DC power between the first capacitor and the corresponding power storage cell.

2. The uninterruptible power supply according to claim 1, further comprising a control device for controlling the converter and the inverter, wherein each of the plurality of first converter cells is communicated with the control device and further includes a control circuit for controlling the first bridge circuit and 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 first converter cells, the control circuit controls the first bridge circuit to convert the AC power input from the AC power source between the first input / output terminal and the second input / output terminal into DC power when the AC power source is healthy, and controls the charge / discharge circuit to supply DC power from the first capacitor to the corresponding energy storage cell, and controls the charge / discharge circuit to supply DC power from the corresponding energy storage cell to the first capacitor when the AC power source is out of service, and controls the first bridge circuit to output the DC power from the first capacitor between the first input / output terminal and the second input / output terminal, as described in claim 2.

4. The uninterruptible power supply according to claim 2, wherein each of the plurality of first converter 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 first capacitor via the resistor and the charge / discharge circuit.

5. The uninterruptible power supply according to claim 2, wherein each of the plurality of first converter 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 the first bridge circuit and turns on the bypass switch when the corresponding energy storage cell fails.

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

7. The uninterruptible power supply according to claim 1, wherein each of the plurality of second converter cells includes a third input / output terminal and a fourth input / output terminal, a second bridge circuit including a plurality of second semiconductor switching elements, and a second capacitor connected to the third input / output terminal and the fourth input / output terminal via the second bridge circuit.

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, the converter converts the medium-voltage AC power supplied from the AC power supply into medium-voltage DC power and supplies it to the DC bus when the AC power supply is functioning properly, the converter supplies the medium-voltage DC power stored in the energy storage device to the DC bus when the AC power supply fails, and the inverter converts the medium-voltage DC power received from the DC bus into medium-voltage AC power and supplies it to the AC transmission line.

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