JP2774051B2 - Low-voltage uninterruptible switching device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to two transformers having the same high-voltage system, in which the load of a transformer to be constructed is switched to the other transformer without interruption via this device, and
The present invention relates to a low-voltage uninterruptible switching device used for construction work such as changing a transformer.

[0002]

2. Description of the Related Art In recent years, with the diversification of lifestyles and the rapid progress of a highly information-oriented society, the power system, which is one of the pillars supporting them, has become larger and more complex. High stable supply is required. From these demands, a construction method of a low-voltage instantaneous switching method in electric power facility construction is being promoted. For example, JP-A-3-232006, JP-A-3-32006
As shown in US Pat. No. 232067 and JP-A-3-238400, the present applicant has already proposed a low-voltage uninterruptible switching device (hereinafter referred to as a switching device).

[0003] When any of these switching devices are constructed,
It is installed between the A bank on the healthy side and the B bank on the construction side, and switches the power supply to the load on the B bank between the A and B banks using the same switching device. The switching / switchback is performed by.

[0006] The switching device has a schematic configuration as shown in FIG. 6. That is, a rectifier (not shown) is connected to the input terminal of the switching device 21 that can be connected to the A bank, and the alternating current at the input terminal is rectified into direct current. An inverter circuit 23 is connected to the rectifier via a smoothing capacitor 22, and the direct current is supplied to the inverter circuit 23 (only one phase is shown).
Is converted to AC. A waveform correction unit 24 is connected to the inverter circuit 23. The output of the inverter circuit 23 is subjected to waveform correction by the filter coil 24a of the waveform correction unit, and then output from the output terminal b of the switching device 21 to the low voltage side of the B bank. Is done.

[0005] In order to perform switching / returning by the switching device 21 configured as described above, when the output voltage of the switching device 21 and the voltage on the low voltage side of the B bank are different in the inverter circuit 23, they match. Is controlled.

The inverter circuit 23 uses a plurality of (two or three) single-phase inverters to control the voltage for each of the three phases. May be controlled. But,
The switching device 21 is demanded to be downsized for practical use such as being moved to a construction site on a vehicle. Therefore, since the inverter circuit 23 has a smaller and simpler circuit configuration using a three-phase inverter than using a plurality of single-phase inverters, a three-phase inverter circuit using a three-phase inverter is mainly used. With this configuration, the size of the switching device 21 can be reduced, and further, the cost can be reduced. The output voltage of the three-phase inverter circuit that controls the voltage collectively for each phase is a three-phase balanced voltage.

[0007]

However, generally, different loads exist on the B-bank side load, which is the output side of the switching device 21, and the low-voltage side of each phase on the B-bank side is different. The low-voltage side of the construction-side transformer of bank B may become unbalanced. For this reason, a voltage difference may occur between this voltage and the output voltage of the three-phase inverter circuit that balances and outputs each phase, that is, the output voltage of the switching device 21. In this state, when the switching device 21 and the B-bank-side transformer are operated in parallel, if a voltage difference occurs between the two phases, the following problem occurs.

When (output voltage of the switching device)> (low voltage on the B bank side), a loop current exceeding the rating flows out of the switching device 21 to the transformer on the B bank side due to the voltage difference between the two. However, there is a problem that a failure occurs in the switching device 21 and the transformer on the bank B side.

When (output voltage of the switching device) <(low voltage on the B bank side), an overvoltage occurs in the smoothing capacitor portion on the inverter input side. The cause of this overvoltage will be described with reference to FIGS. For convenience of explanation, a voltage difference due to unbalance is referred to as a power supply W. And
A plurality of transistors Tr1 to Tr4 and diodes C1 to C4 forming a three-phase inverter circuit 25 (only one phase is shown in FIG. 7 for the sake of description) in FIG.
In the case where the transistor Tr1 is turned on and the other transistors Tr2 to Tr4 are turned off, the equivalent circuit 26 shown in FIG. 8 is formed. This equivalent circuit 26 has the same principle as that of the DC-DC converter circuit for boosting which is a known technique.

In FIG. 8, the power supply W, which is an unbalanced voltage between the output voltage of the switching device 21 and the low voltage on the B bank side, becomes the power supply W1 of the equivalent circuit 26 (DC-DC converter circuit). When this circuit is formed, when the transistor Tr1 is turned off, a back electromotive force is generated in the coil 24a of the waveform correction unit 24. Then, according to the principle of the DC-DC converter, the smoothing capacitor 22 is charged by the generated back electromotive force, and the DC power supply of the switching device 21 is boosted. Therefore, if the withstand voltage of the smoothing capacitor 22 is exceeded, the smoothing capacitor 22 is damaged, and as a result, there is a problem that the three-phase inverter circuit 25 is damaged.

The present invention has been made in view of such problems existing in the prior art, and an object of the present invention is to use a three-phase inverter circuit that outputs each phase in balance. Another object of the present invention is to provide a low-voltage uninterruptible switching device capable of performing parallel operation.

[0012]

SUMMARY OF THE INVENTION In order to solve the above-mentioned problems, the present invention provides a method in which a bank A and a bank B are connected to each other.
A low-voltage uninterruptible switching device that switches power supply to a bank-side load between the A and B banks and performs switching and switching back without interruption when switching back; A smoothing capacitor for smoothing the direct current rectified by the rectifier circuit, a three-phase inverter circuit for converting the direct current smoothed by the smoothing capacitor to an alternating current; Control means for controlling the conversion so that the voltage is the same as the voltage of the lowest phase among the phases of the bank; and determining whether or not the voltage between both terminals of the smoothing capacitor has exceeded a predetermined voltage value. The gist of the invention is to provide a determination means for outputting an overvoltage generation signal when the voltage exceeds the voltage, and a discharge means for discharging the smoothing capacitor based on the overvoltage generation signal from the determination means.

[0013]

According to the present invention constructed as described above, the input side AC is converted into DC by the rectifier circuit, further smoothed by the smoothing capacitor, and converted into AC by the three-phase inverter circuit. Then, when the power supply to the B bank side load is switched from the B bank to the A bank,
When switching back to the bank, the control means controls the output of the three-phase inverter circuit so that the output voltage of the switching device and the voltage of the lowest phase among the low-voltages on the bank B, which is the output side, are the same. . As a result, the output voltage of each phase of the switching device is
Since the voltage is the same as the lowest voltage on the low voltage side of bank B,
Outflow of loop current to the bank side is prevented.

In the above state, when an imbalance occurs between the output voltage and the voltage on the bank B side, the determining means determines whether or not the voltage between both terminals of the smoothing capacitor has exceeded a predetermined voltage value. When the voltage exceeds a predetermined voltage value, an overvoltage generation signal is output. The discharge circuit discharges the smoothing capacitor based on the overvoltage generation signal from the determination means. Therefore, without damaging the smoothing capacitor, the smoothing capacitor can be prevented from being damaged due to overvoltage, and the three-phase inverter circuit can be prevented from being damaged.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention in which the high voltage system is the same and the input side and the output side correspond to a three-phase four-wire system will be described with reference to the drawings.

As shown in FIGS. 1 and 2, in the transformers Tra and Trb provided in both the banks A and B, the primary cutout PCs are respectively connected to the respective phases U, V and W of the distribution line L on the high voltage side. Have been. A low-voltage uninterruptible switching device F (hereinafter, referred to as a switching device) is detachably connected between the low-voltage sides of the A and B banks via a bypass cable K.

The switching device F includes a conversion circuit 1, a bypass circuit 2, and a discharging circuit 3 as discharging means. The conversion circuit 1 includes phase rotation switching units M1 and M2, a rectifier unit 4, a waveform correction unit 5, a smoothing capacitor 13, a three-phase inverter circuit 6, an overcurrent limiting circuit 7, a waveform correction unit 8, a control circuit 9 as control means, and A changeover switch M5 is provided. Conversion circuit 1
The input terminal of can be connected to PC1 (power transformer side) and C1-NC2 terminal (common transformer side) of bank A, and the output terminal is PC1 (bank of power transformer) of bank B. Side) and C1-NC2 terminal (shared transformer side).

As shown in FIG. 1, the bypass circuit 2 is provided so as to be opened and closed by switches M3 and M4. The input terminal of the bypass circuit 2 is shared with the input terminal of the conversion circuit 1 and can be connected to the PC1 -NC2 terminal of the A bank via the low voltage line of the A bank. The output terminal of the bypass circuit 2 is also used as the output terminal of the conversion circuit 1, and the P-C1 -N of the B bank is connected via the low voltage line of the B bank.
Connectable to -C2 terminal.

Next, the conversion circuit 1 will be described in detail. The rectifier unit 4 is configured by a three-phase bridge circuit, and performs full-wave rectification on the input voltage from the A bank. The waveform corrector 5 is connected to the rectifier 4 and corrects the waveform after rectification of the input voltage from the A bank. Both ends of the smoothing capacitor 13 are connected to the three-phase inverter circuit 6 to further smooth the DC waveform from the waveform corrector 5. The three-phase inverter circuit 6 is composed of a three-phase bridge unit including transistors and diodes and an input DC voltage adjusting unit (both not shown), and converts the phase and voltage value of the voltage in each phase U, V, W. At the same time, three phases are collectively output in a balanced manner. Further, the waveform correction unit 8 includes a filter coil (not shown), and further corrects the voltage waveform output from the three-phase inverter circuit 6.

The phase rotation switching units M1 and M2 are provided with a switching device F
The operation can be instantaneously switched via the control circuit 9 by operating various switches provided on the operation panel. By switching the two-phase rotation switching units M1 and M2, the input phase rotation of the switching device F is switched.

The switches M3 and M4 and the changeover switch M5 can be instantaneously switched via the control circuit 9 by operating various switches provided on the operation panel of the changeover device F. That is, the changeover switch M5 and the switch section M3,
By the instantaneous switching of M4, the bypass circuit 2 and the conversion circuit 1 are instantaneously switched.

As shown in FIG. 3, the control circuit 9 includes a central processing unit (CPU), a phase detection circuit, and a three-phase inverter control unit. The unit 8 is connected to an input terminal and an output terminal of the system. The control circuit 9 is connected to a readable / writable memory (hereinafter referred to as RAM) 11 and a read-only memory (hereinafter referred to as ROM) 12 storing a control program and the like.

When a voltage is applied to the input terminal and output terminal of the system, the phase detection circuit of the control circuit 9 can detect each voltage value, phase and phase rotation applied to both terminals. It is. Further, the control circuit 9 controls the three-phase inverter circuit 6 based on the detection signal from the overcurrent limiting circuit 7 to decrease the inverter output voltage.

The control circuit 9 includes a conversion switch for adjusting the input voltage waveform from the A bank of the operation panel provided on the switching device F to the B bank voltage waveform and the inverter output voltage waveform of the switching device F to the input voltage of the A bank. The three-phase inverter circuit 6 is controlled by operating a reverse conversion switch (not shown) for adjusting to a waveform. With this control, the phases of the input voltages on the common transformer side and the power transformer side can be changed in a range of -180 ° to + 180 ° in three phases.

The no-fuse breaker NFB is a conversion circuit 1
Are provided on the input terminal side and the output terminal side of the switching device F, and P-C1 -C2
The circuit of the switching device F can be opened and closed, and is opened when the switching device F is short-circuited internally and when the bank B is short-circuited or overloaded.

An output voltage setting volume 18 made of a variable resistor is connected to the control circuit 9, and a DC power supply 19 is connected to the output voltage setting volume 18.
With the output voltage setting volume 18, the output voltage of the switching device F can be set to the same voltage as the lowest voltage value among the U, V, and W phases on the low voltage side of the B bank.

Next, the discharge circuit 3, which is a main part of the present embodiment, will be described. The discharge circuit 3 includes a discharge resistor 14 and an IGBT (registered trademark of JEDEC: Insulated)
A gate bipolar transistor) 15 is provided, and is configured by a series circuit in which the discharge resistor 14 and the collector terminal of the IGBT 15 are connected. The positive terminal of the discharge resistor 14 of the discharge circuit 3 and the emitter terminal of the IGBT 15 are connected to both ends of the smoothing capacitor 13.

As described in the prior art, when power is supplied to the B-bank-side load in parallel operation of the switching device F and the B-bank-side transformer, the output voltage of the switching device F is changed to the low voltage of the B-bank. When the voltage is lower than the side voltage, a counter electromotive force of the coil of the waveform correction unit 8 is generated by the principle of the DC-DC converter, and the counter electromotive force charges the smoothing capacitor 13.

A discriminating circuit 16 as discriminating means is connected to both ends of the smoothing capacitor 13. The discriminating circuit 16 is a driving circuit 17 for controlling ON / OFF of the IGBT 15
Is connected to the base terminal of the IGBT 15 via the.
Then, the determination circuit 16 determines whether the voltage between both terminals of the smoothing capacitor 13 has exceeded a predetermined value, and outputs an overvoltage generation signal to the drive circuit 17 when the voltage exceeds the predetermined value. The drive circuit 17 applies a predetermined voltage to the base terminal of the IGBT 15 based on the overvoltage generation signal from the determination circuit 16 to turn on the IGBT 15. This IGBT
By the ON operation of 15, a closed loop circuit of the smoothing capacitor 13, the discharging resistor 14 and the IGBT 15 is formed, and the smoothing capacitor 13 is discharged via the discharging resistor 14.

Now, a case where the load LB of the bank B is switched to the bank A and switched back to the bank A in a state where the switching device F is connected via the bypass cable K between the banks A and B as described above will be described with reference to FIGS. This will be described with reference to the flowchart shown in FIG. In this flowchart, steps (hereinafter abbreviated as S) 1 to S14 indicate a switching time, and S15 to S25 indicate a switching back.

As shown in FIG. 4, when the changeover switch M5 of the conversion circuit 1 in the changeover device F is open, the control circuit 9 operates in S1 by an external operation from the operation panel to adjust the adaptive route (three-phase in this embodiment). After three-phase output from the input), the no-fuse breaker NFB is turned on in S2. S
At 3, the phase rotation of the bank A low voltage side voltage and the bank B low voltage side voltage are compared by the phase detection circuit of the control circuit 9. If the phase rotation is reversed in S4, the control circuit 9 sets the phase rotation switching units M1 and M
2 to switch the connection between the PC1 and C2 terminals of bank A and each phase. If the phase rotation is normal, the process proceeds to S5.

In S5, the control circuit 9 compares the phase of the output voltage of the switching device F with the phase of the B bank low voltage side, and if different, in S6, the phase of the output voltage of the switching device F and the B bank low voltage side voltage. Are gradually shifted so as to be in phase. When the two phases are in phase, the control circuit 9 stores the phase difference between the input and output of the switching device F in the RAM 1.
1 and display the completion of conversion on a display unit (not shown) of the operation panel, and then proceed to S7.

In S7, the control circuit 9 sets the output voltage of each phase of the switching device F to the same value as the voltage value of the lowest phase of the bank B low voltage. In S8, the output voltage of the switching device F and B
When the voltage value of the lowest phase among the bank low-voltage voltages becomes the same voltage, the control circuit 9 displays the completion of the conversion on the display unit of the operation panel. Thereafter, in S9, the control circuit 9 turns on the changeover switch M5 by operating the operation panel of the changeover device F.

In this state, the switching device F and the transformer Trb in the B bank start operating in parallel, and the load LB on the B bank side is connected to the transformer Tr in the A bank via the switching device F.
Power is supplied from both a and the transformer Trb of bank B, which is the construction side bank. At this time, since the output voltage of each phase of the switching device F is set to the same voltage as the lowest voltage on the low voltage side of the B bank, it is always lower than the voltage value on the low voltage side of the B bank. Therefore, even if the voltages of the two are unbalanced and a voltage difference occurs, the loop current exceeding the rating does not flow from the switching device F to the bank B side.

Also, due to the voltage difference due to the unbalance between the two voltages, the smoothing capacitor 13 is generated by the back electromotive force generated in the coil of the waveform correction unit 8 according to the principle of the DC-DC converter described in the related art. It is charged, and the voltage across the smoothing capacitor 13 rises. When the determination circuit 16 determines that the voltage charged in the smoothing capacitor 13 has exceeded a predetermined value, it inputs an overvoltage generation signal to the drive circuit 17. Then, the drive circuit 17 turns on the IGBT 15 based on the overvoltage generation signal.

At this time, a closed loop circuit is formed in the smoothing capacitor 13, the discharging resistor 14 and the IGBT 15,
The smoothing capacitor 13 is discharged via the discharging resistor 14. Therefore, since the voltage higher than the withstand voltage of the smoothing capacitor 13 is discharged before being charged, the smoothing capacitor 13 is not damaged and the three-phase inverter circuit 6 is not damaged.

Next, the operator confirms with the ammeter of the operation panel that the output current from the switching device F is flowing to the low voltage side of the B bank, and disconnects the low voltage side of the B bank in S10.
Primary cutout P of bank B by manual operation
Release C.

Thereafter, in S11, according to the adaptation route,
The control circuit 9 determines whether or not to convert the output of the switching device F to the bypass circuit 2, and displays the result of the determination on the display unit of the operation panel. As a result, when it is determined that the conversion is performed from the conversion circuit 1 to the bypass circuit 2, S is operated by the operation panel.
At 12, the control circuit 9 changes the output voltage of the switching device F
And the same voltage and the same phase as the input side voltage, and the completion of the voltage and phase adjustment is displayed on the display unit of the operation panel. The operator checks the display and operates the operation panel to
In step S13, the control circuit 9 turns on the switch section M3 to form the bypass circuit 2, and opens the changeover switch M5 to stop the output of the conversion circuit 1.

Accordingly, thereafter, power is supplied from the bypass circuit 2 of the switching device F to the load LB of the B bank. In this state, the switching is completed, and the bank B may be constructed in S14. If it is determined in S11 that there is no need to convert to a bypass circuit, the process may proceed to S14 and construction of bank B may be performed.

Next, the switching back operation will be described. As shown in FIG. 5, after the construction of the bank B is completed in S15, the control circuit 9 determines whether or not the bypass circuit 2 of the switching device F is formed according to the adaptation route in S16 by an external operation of the operation panel. Is determined, and the result of the determination is displayed on the display unit of the operation panel. When it is determined that the bypass circuit 2 is formed, the control circuit 9 controls the three-phase inverter circuit 6 in S17 by an operation from the operation panel, and outputs the output of the switching device F to the input voltage and the same voltage as the input phase. The same phase is displayed on the display of the operation panel. Thereafter, the operator checks the display unit and operates the operation panel, in S18, the control circuit 9 turns on the changeover switch M5, opens the switch unit M3, and disconnects the bypass circuit 2.

In S19, the control circuit 9 gradually shifts the phase of the output voltage of the three-phase inverter circuit 6 to the phase stored in the RAM 11 in S6. After the completion of the phase shift, the control circuit 9 displays the completion of the phase shift on the display unit of the operation panel. After confirming the display section, the operator inputs the primary cutout PC of the B bank by manual operation in S20. At the same time, B
The phase is compared between the low voltage side of the bank and the output of the switching device F to perform phase detection. If it is determined in S16 that the bypass circuit 2 has not been formed, the process proceeds to S20.

If the phase of the output of the switching device F is different from the low pressure side of the bank B as a result of the phase detection of S20 in S21, S
At 25, the operator changes the connection of the transformer Trb of the B bank to the distribution line L, and matches the phases of the low voltage side of the B bank and the output of the switching device F. If the phases are the same, the process proceeds to S22.

In step S22, the operator manually operates the output voltage setting volume 18 to set the output voltage value of the switching device F to the same voltage value as the voltage value of the lowest phase among the low-voltage phases of the bank B. Thereafter, in S23, the operator connects the low pressure side of the bank B and the output side of the switching device F. In this state, there is a parallel operation state in which power is supplied from both the switching device F and the B bank to the load LB of the B bank. At this time, as in the case of switching, the output voltage of each phase of the switching device F is set to the same voltage as the lowest voltage on the low voltage side of the B bank, so
It is lower than the voltage value on the bank low voltage side. Therefore, even if the voltages of the two are unbalanced and a voltage difference occurs, the loop current exceeding the rating does not flow from the switching device F to the bank B side.

The smoothing capacitor 13 is charged by the back electromotive force generated in the coil of the waveform corrector 8 due to the voltage difference due to the imbalance between the two voltages, and the voltage across the smoothing capacitor 13 rises. Also, since the discharge circuit 3 discharges the voltage before the voltage higher than the withstand voltage of the smoothing capacitor 13 is charged, the smoothing capacitor 13 is not damaged and the three-phase inverter circuit 6 is not damaged.

Thereafter, in S24, the control circuit 9 opens the changeover switch M5, opens the no-fuse breaker NFB by external operation of the operation panel, and removes the switching device F and the bypass cable K from both banks A and B.

Therefore, in the switching device F of the present embodiment, when switching or switching back, the control circuit 9
The conversion control is performed so that the output voltage of each phase of the switching device F becomes the same voltage as the lowest voltage among the phases on the low voltage side of the B bank. For this reason, even when a voltage difference occurs between the output side of the switching device F and the low voltage side of the bank B at the time of the loop connection between the banks A and B, the output voltage of each phase of the switching device F is the highest on the low voltage side of the bank B. Since the same voltage as the low voltage is set, it is possible to prevent the loop current from flowing from the switching device F to the bank B side.

The discharge circuit 3 was connected to both ends of the smoothing capacitor 13. Then, the discrimination circuit 16 monitors the voltage between both terminals of the smoothing capacitor 13 which is a DC portion during the equilibrium operation, and drives the drive circuit 17 when the voltage between both terminals of the smoothing capacitor 13 exceeds a predetermined value. And the smoothing capacitor 13, the discharge resistor 14, and the IGBT 1
5, a smoothing capacitor 13 was discharged by forming a closed loop circuit. Therefore, the smoothing capacitor 13
According to the principle of the C-DC converter, it is possible to prevent the voltage from being boosted and damaged, and to prevent a failure in the three-phase inverter circuit 6. As a result, each phase U,
Even if the three-phase inverter circuit 6 that outputs V and W collectively and balanced is used, the parallel operation of supplying power from both the switching device F and the B bank to the load LB of the B bank can be performed.

Further, at the time of switching back, the output voltage value of the switching device F was set to the same voltage value as the voltage value of the lowest phase among the low-voltage side phases of the bank B by the output voltage setting volume 18. According to this method, even if the voltage of the low voltage side is changed before and after the construction of the bank B is the construction for switching the tap of the transformer, the phase of the switching device F is changed by the output voltage setting volume 18. Is set to the same voltage as the lowest voltage value on the low voltage side of the B bank, it is possible to prevent the loop current from flowing out from the switching device F to the B bank side.

The present invention is not limited to the above-described embodiment, and may be appropriately changed as follows without departing from the spirit of the present invention. (1) In the wiring path L of the above embodiment, both the input side and the output side are applied to a three-phase four-wire type line, but may be modified as shown below. For example, the input side is a three-phase four-wire line or a single-phase single-wire line, the output side is a three-phase four-wire line or a single-phase single-wire line, and how the input-side line and the output-side line are combined. You may.

(2) In the above embodiment, the three-phase inverter circuit 6 using a transistor as a switching element is used. However, a three-phase inverter circuit using a switching element thyristor may be used.

[0051]

As described above in detail, according to the present invention, power is supplied from both the switching device and the B-bank to the load of the B-bank even when a three-phase inverter circuit that outputs each phase in balance is used. It has an excellent effect that parallel operation can be performed.

[Brief description of the drawings]

FIG. 1 is a schematic circuit diagram showing an embodiment embodying the present invention, in which a switching device is connected to the low voltage side of both banks.

FIG. 2 is an electric circuit diagram of the switching device.

FIG. 3 is a block diagram illustrating an electrical configuration of a switching device.

FIG. 4 is a flowchart at the time of a switching operation.

FIG. 5 is a flowchart at the time of a switchback operation.

FIG. 6 is a schematic configuration diagram of a switching device in the related art.

FIG. 7 is a circuit diagram in a case where a three-phase inverter circuit is used in a conventional switching device.

FIG. 8 is an equivalent circuit diagram when a three-phase inverter circuit is used in a conventional switching device.

[Explanation of symbols]

3 ... discharge circuit (discharge means), 4 ... rectifier circuit, 6 ... three-phase inverter circuit, 9 ... control circuit (control means), 13 ... smoothing capacitor, 16 ... discrimination circuit (discrimination means), Tra ... A
Transformer on bank side, Trb ... Transformer on bank B side

Claims (1)

(57) [Claims] 1. A bank between an A bank and a B bank.
A low-voltage uninterruptible switching device that switches power supply to a bank-side load between the A and B banks and performs switching and switching back with complete uninterruptible power when switching back; A smoothing capacitor for smoothing the direct current rectified by the rectifier circuit, a three-phase inverter circuit for converting the direct current smoothed by the smoothing capacitor to an alternating current, and an output side for outputting the output voltage of each phase in the three-phase inverter circuit. Control means for performing conversion control so as to be the same as the voltage of the lowest phase among the phases of the bank; and determining whether or not the voltage between both terminals of the smoothing capacitor has exceeded a predetermined voltage value, and determining the predetermined voltage value. A low-voltage uninterruptible power supply comprising: a determination unit that outputs an overvoltage generation signal when the voltage exceeds the voltage; and a discharge unit that discharges the smoothing capacitor based on the overvoltage generation signal from the determination unit. Switching device.