TECHNICAL FIELD

The present invention relates to an uninterruptible power supply apparatus and more particularly to a three-phase four-wire uninterruptible power supply apparatus.

BACKGROUND ART

For example, Japanese Patent Laying-Open No. 2013-176296 (PTL 1) discloses a three-phase three-wire uninterruptible power supply apparatus. This uninterruptible power supply apparatus includes a converter that converts AC voltage supplied from an AC power supply to first to third DC voltages and outputs the first to third DC voltages to first to third DC lines in a sound state of the AC power supply and stops operation in a power failure of the AC power supply, a DC voltage converter that converts a fourth DC voltage from a DC power supply source to first to third DC voltages and supplies the first to third DC voltages to the first to third DC lines in a power failure of the AC power supply, and an inverter that converts the first to third DC voltages from the converter and the DC voltage converter to AC voltage and supplies the AC voltage to a load.

This uninterruptible power supply apparatus further includes a first capacitor connected between the first and second DC lines, a second capacitor connected between the second and third DC lines, first and second voltage detectors that detect terminal-to-terminal voltages of the first and second capacitors, respectively, a computing unit that obtains a first voltage that is the sum of the detected values of the first and second voltage detectors and a second voltage that is the difference between the detected values of the first and second voltage detectors, a first control unit that controls the converter such that the first voltage becomes a reference voltage and the second voltage is eliminated in a sound state of the AC power supply, and a second control unit that controls the DC voltage converter such that the first voltage becomes a reference voltage and the second voltage is eliminated in a power failure of the AC power supply.

In this uninterruptible power supply apparatus, therefore, the sum of terminal-to-terminal voltages of the first and second capacitors can be kept to a reference voltage, and the difference between terminal-to-terminal voltages of the first and second capacitors can be kept to 0 V.

CITATION LIST

Patent Literature

PTL 1: Japanese Patent Laying-Open No. 2013-176296

SUMMARY OF INVENTION

Technical Problem

When the load includes a three-phase transformer and a load body, three-phase excitation current flows through the three-phase transformer, and each of three-phase output currents of the uninterruptible power supply apparatus has a positive-negative asymmetric waveform. When all of the AC power supply, the uninterruptible power supply apparatus, the three-phase transformer, and the load body are three-phase three-wire systems, the sum of three-phase AC currents flowing through the three-phase transformer is zero and therefore it is less likely that the terminal-to-terminal voltages of the first and second converters are imbalanced.

When the AC power supply, the uninterruptible power supply apparatus, the three-phase transformer, and the load body are three-phase four-wire systems, however, current flows through a neutral point line. Therefore, the sum of three-phase AC currents flowing through the three-phase transformer is not zero and has a positive-negative asymmetric waveform, and the terminal-to-terminal voltages of the first and second capacitors may be imbalanced. In particular, when load current is small, output current of the DC voltage converter is small in a power failure of the AC power supply, so that the terminal-to-terminal voltages of the first and second capacitors may be imbalanced.

A main object of the present invention is therefore to provide a three-phase four-wire uninterruptible power supply apparatus that can eliminate the imbalance between the terminal-to-terminal voltages of the first and second capacitors in a power failure even when a load includes a three-phase transformer and a load body, and load current is small.

Solution to Problem

An uninterruptible power supply apparatus according to the present invention includes: first to third DC lines; a first capacitor connected between the first and second DC lines; a second capacitor connected between the second and third DC lines; a neutral point line connected to a neutral point terminal of a three-phase four-wire AC power supply, the second DC line, and a neutral point terminal of a three-phase four-wire load; a switch provided corresponding to each phase of the AC power supply, having one terminal receiving AC voltage of a corresponding phase supplied from the AC power supply, turned on in a sound state of the AC power supply, and turned off in a power failure of the AC power supply; an AC input filter including a third capacitor connected between the other terminal of the switch and the neutral point line, and a reactor having one terminal connected to the other terminal of the switch; a converter connected between the other terminal of the reactor and the first to third DC lines and including a first multi-level circuit that converts AC voltage and first to third DC voltages to each other, the converter converting AC power from the AC power supply to DC power and supplying the DC power to the first to third DC lines in a sound state of the AC power supply; a DC voltage converter connected between a DC power supply source and the first to third DC lines and including a second multi-level circuit that converts a fourth DC voltage supplied from the DC power supply source and the first to third DC voltages to each other, the DC voltage converter supplying DC power from the DC power supply source to the first to third DC lines in a power failure of the AC power supply; an inverter provided between the first to third DC lines and the load and including a third multi-level circuit that converts the first to third DC voltages and AC voltage to each other, the inverter converting DC power supplied from the converter and the DC voltage converter to AC power and supplying the AC power to the load; first and second voltage detectors that detect terminal-to-terminal voltages of the first and second capacitors, respectively; a computing unit that obtains a first voltage that is a sum of terminal-to-terminal voltages of the first and second capacitors and a second voltage that is a difference between terminal-to-terminal voltages of the first and second capacitors, based on detection results of the first and second voltage detectors; a first control unit, wherein in a sound state of the AC power supply, the first control unit controls the converter such that the first voltage becomes a first reference voltage and the second voltage is eliminated, and in a power failure of the AC power supply, the first control unit stops the converter when an absolute value of the second voltage is smaller than a predetermined threshold voltage, and controls the converter to reduce the second voltage when an absolute value of the second voltage is larger than the predetermined threshold voltage; and a second control unit that controls the DC voltage converter such that the first voltage becomes the first reference voltage and the second voltage is eliminated, in a power failure of the AC power supply.

Advantageous Effects of Invention

In this uninterruptible power supply apparatus, in a power failure of the AC power supply, the switch is turned off to electrically cut off the AC power supply from the AC input filter, and the DC voltage converter is controlled such that the second voltage that is the difference between the terminal-to-terminal voltages of the first and second capacitors is eliminated, and when the absolute value of the second voltage exceeds a predetermined threshold voltage, the converter is controlled so that the second voltage is reduced. Therefore, the imbalance between the terminal-to-terminal voltages of the first and second capacitors in a power failure can be eliminated even when a load includes a three-phase four-wire transformer and a load body, and load current is small.

