Uninterruptible power supply

An uninterruptible power supply is configured to receive, together with a first load, AC power supplied from an AC power source. The uninterruptible power supply includes: a converter configured to convert AC power from the AC power source into DC power; an inverter configured to convert DC power generated by the converter or DC power in a battery into AC power to supply the converted power to a second load; a controller configured to control reactive power generated at the converter to compensate at least a part of reactive power generated at the first load; and a limiter configured to limit reactive power generated at the converter to upper limit power or lower. The upper limit power is set to a value according to the difference between the rated capacity of the uninterruptible power supply and AC power supplied to the second load.

TECHNICAL FIELD

The present invention relates to an uninterruptible power supply, and more particularly to an uninterruptible power supply that receives, together with a first load, AC power supplied from an AC power source and supplies AC power to a second load.

BACKGROUND ART

Japanese Patent Laying-Open No. 2011-55570 (PTD 1) discloses a technique where parallelly connected load and uninterruptible power supply are connected to an AC power source, where reactive power generated at the load is compensated with reactive power generated at the uninterruptible power supply, and where the power factor of the load and the uninterruptible power supply is controlled to 1.

CITATION LIST

Patent Document

SUMMARY OF INVENTION

Technical Problem

In PTD 1, however, reactive power generated at the load is wholly compensated with reactive power generated at the uninterruptible power supply. Accordingly, if large reactive power is generated at the load, a high-capacity uninterruptible power supply is required, which disadvantageously causes upsizing of the apparatus and increase in cost.

Therefore, a main object of the present invention is to provide an uninterruptible power supply that can compensate reactive power generated at a load within the range of rated capacity.

Solution to Problem

An uninterruptible power supply according to the present invention is an uninterruptible power supply configured to receive, together with a first load, AC power supplied from an AC power source, the uninterruptible power supply including: a converter configured to convert AC power from the AC power source into DC power; an inverter configured to convert DC power generated by the converter or DC power in a power storage device into AC power to supply the converted power to a second load; a controller configured to control reactive power generated at the converter to compensate at least a part of reactive power generated at the first load; and a limiter configured to limit reactive power generated at the converter to upper limit power or lower. The upper limit power is set to a value according to the difference between the rated capacity of the uninterruptible power supply and AC power supplied to the second load.

Advantageous Effects of Invention

An uninterruptible power supply according to the present invention limits reactive power generated at a converter to less than or equal to upper limit power having a value according to the difference between the rated capacity of the uninterruptible power supply and AC power supplied to a second load. Accordingly, reactive power generated at a first load can be compensated within the range of rated capacity of the uninterruptible power supply. Therefore, downsizing of the apparatus and reduction in cost can be achieved compared to a case where reactive power generated at a first load is wholly compensated.

DESCRIPTION OF EMBODIMENTS

FIG. 1is a circuit block diagram showing a configuration of an uninterruptible power supply1according to Embodiment 1 of the present invention. Uninterruptible power supply1receives, together with a load53(first load), three-phase AC power having a commercial frequency supplied from a commercial AC power source51through a transformer52and supplies three-phase AC power having a commercial frequency to a load54(second load).FIG. 1, however, shows a part related to only a single phase for simplicity of the drawing and the explanation.

Input terminal T1receives AC power having a commercial frequency supplied from commercial AC power source51through transformer52. To output terminal T2, load53to be driven with AC power is connected. To output terminal T3, load54to be driven with AC power is connected.

Reactor L1has one terminal connected to input terminal T1and has the other terminal connected to output terminal T2. Reactor L1constitutes a low-pass filter to allow passage of AC power having a commercial frequency supplied from commercial AC power source51through transformer52and to interrupt a signal having a high frequency generated at load53. Current detector CD1detects an instantaneous value of an AC current flowing between the other terminal of reactor L1and output terminal T2, and outputs a signal representing the detection value.

Actually, corresponding to a three-phase AC current, there are three sets of input terminal T1, reactor L1, current detector CD1, and output terminal T2. Each of three current detectors CD1(first current detector) detects an instantaneous value of a three-phase AC current flowing between the other terminal of a corresponding one of three reactors L1(first three-phase reactor) and a corresponding one of three output terminals T2, and outputs a signal representing the detection value.

Switch S1has one terminal connected to input terminal T1and has the other terminal connected to one terminal of reactor L2through fuse F1. Reactor L2has the other terminal connected to an input node of converter2. Switch S1is ON during a normal time in which AC power is supplied from commercial AC power source51, and is OFF during a power failure time in which supply of AC power from commercial AC power source51is stopped. Fuse F1is blown when an overcurrent flows, so as to protect components, such as converter2.

Reactor L2constitutes a low-pass filter to allow passage of AC power having a commercial frequency supplied from commercial AC power source51through transformer52and to interrupt a signal having a switching frequency generated at converter2. Current detector CD2detects an instantaneous value of an AC current flowing between the other terminal of reactor L2and the input node of converter2, and outputs a signal representing the detection value.

Actually, corresponding to a three-phase AC current, there are three sets of input terminal T1, switch S1, fuse F1, reactor L2, and current detector CD2. Each of three current detectors CD2(second current detector) detects an instantaneous value of a three-phase AC current flowing between a corresponding one of three reactors L2(second three-phase reactor) and a corresponding one of three input nodes of converter2, and outputs a signal representing the detection value.

During a normal time in which AC power is supplied from commercial AC power source51, converter2converts AC power from commercial AC power source51into DC power to store the DC power in battery3and to supply the DC power to inverter4through DC bus Bl. During a power failure time in which supply of AC power from commercial AC power source51is stopped, the operation of converter2is stopped. Converter2is also used to compensate reactive power generated at load53. The operation for compensating reactive power will be described in detail later.

Fuse F2and switch S2are connected in series between DC bus B1and battery3. Fuse F2is blown when an overcurrent flows, so as to protect components, such as converter2and battery3. Switch S2is ON during a normal time, and is OFF during, for example, maintenance of battery3. During a normal time, battery3(power storage device) stores DC power generated by converter2. During a power failure time, battery3supplies DC power to inverter4. Capacitor C1is connected to DC bus B1to smooth and stabilize a DC voltage of DC bus B1.

During a normal time in which AC power is supplied from commercial AC power source51, inverter4converts DC power generated by converter2into AC power having a commercial frequency. During a power failure time in which supply of AC power from commercial AC power source51is stopped, inverter4converts DC power in battery3into AC power having a commercial frequency.

Reactor L3has one terminal connected to an output node of inverter4, and has the other terminal connected to one terminal of switch S3. Switch S3has the other terminal connected to output terminal T3. Capacitor C2is connected to the other terminal of reactor L3. Reactor L3and capacitor C2constitute a low-pass filter to allow passage of AC power having a commercial frequency generated by inverter4and to interrupt a signal having a switching frequency generated at inverter4. In other words, reactor L3and capacitor C2shape the waveform of an AC voltage generated by inverter4into a sinusoidal wave. Switch S3is ON during a normal time, and is ON during, for example, maintenance of uninterruptible power supply1.

Actually, there are three sets of reactor L3, capacitor C2, switch S3, and output terminal T3. Three-phase AC power generated by inverter4is supplied to load54through the three sets of these components, such as reactor L3.

Uninterruptible power supply1further includes a phase detector10, coordinate transformers11,14, a reactive current instructing unit12, a limiter13, current controllers15,23, a DC voltage instructing unit20, a voltage detector21, a voltage controller22, a voltage vector calculating unit24, and a PWM (pulse width modulation) controller25.

Phase detector10detects phases θ of a three-phase AC voltage between three switches S1and three fuses F1, and outputs signals representing the detection values θ to coordinate transformers11,14. Based on detection values θ from phase detector10, coordinate transformer11(first coordinate transformer) performs coordinate transformation of a three-phase AC current detected by three current detectors CD1, and generates a d-axis current and a q-axis current Iq1. The d-axis current corresponds to an active current of the three-phase AC current, and the q-axis current Iq1corresponds to a reactive current of the three-phase AC current. The d-axis current is not used here.

Coordinate transformer11performs two-step coordinate transformation to transform a three-phase AC current into d-axis current and q-axis current Iq1. First, coordinate transformer11transforms a UVW three-phase coordinate system into an αβ fixed coordinate system by using the nature of a three-phase AC current having phases shifted by 120° from each other where the sum of the phase currents is zero, so as to transform the three-phase AC current into a biphasic AC current (α-axis current and β-axis current). Coordinate transformer11then transforms the αβ fixed coordinate system into a dq rotating coordinate system by using a biphasic AC current rotating while keeping a phase difference of 90°, so as to transform the biphasic AC current into two DC signals (d-axis current and q-axis current Iq1). The transformation of the three-phase AC current into two DC signals can achieve simplification of the control.

Based on q-axis current Iq1generated by coordinate transformer11, reactive current instructing unit12generates a reactive current instruction value (q-axis current instruction value) for compensating a reactive current generated at load53. Limiter13limits the reactive current instruction value output from reactive current instructing unit12to an upper limit value or less. The upper limit value is a value acquired by converting the current value of the difference between the rated current of uninterruptible power supply1and the rated current of load54, into a DC signal (q-axis current), and is predetermined for limiter13. The reactive current instruction value limited to the upper limit value or less is provided to current controller15.

Based on detection values θ from phase detector10, coordinate transformer14(second coordinate transformer) performs coordinate transformation of a three-phase AC current detected by three current detectors CD2, so that the three-phase AC current is transformed into d-axis current Id2and q-axis current Iq2. The d-axis current Id2corresponds to an active current of the three-phase AC current, and the q-axis current Iq2corresponds to a reactive current of the three-phase AC current. The d-axis current Id2and the q-axis current Iq2generated by coordinate transformer14are provided to current controller23and current controller15, respectively.

Current controller15acquires the deviation of q-axis current Iq2from coordinate transformer14from a reactive current instruction value limited to an upper limit value or less from reactive current instructing unit12, and outputs, to voltage vector calculating unit24, a voltage instruction value for removing the deviation.

DC voltage instructing unit20outputs a DC voltage instruction value for determining a DC voltage of DC bus B1. Voltage detector21detects an instantaneous value of a DC voltage of DC bus B1, and outputs a signal representing the detection value. Voltage controller22acquires the deviation of a detection value from voltage detector21from a DC voltage instruction value from DC voltage instructing unit20, and outputs an active current instruction value (d-axis current instruction value) for removing the deviation.

Current controller23acquires the deviation of d-axis current Id2from coordinate transformer14from an active current instruction value from voltage controller22, and outputs, to voltage vector calculating unit24, a voltage instruction value for removing the deviation.

Voltage vector calculating unit24performs coordinate transformation opposite to that of coordinate transformers11,14, so as to transform two voltage instruction values from current controllers15,23into a three-phase AC voltage instruction value. PWM controller25generates a three-phase PWM signal in accordance with the three-phase AC voltage instruction value from voltage vector calculating unit24, and provides the three-phase PWM signal to converter2.

Converter2is controlled with a three-phase PWM signal from PWM controller25and converts three-phase AC power supplied from commercial AC power source51into DC power. Further, converter2generates, on the input node side, a reactive current for compensating a reactive current generated at load53. Accordingly, a reactive current generated at load53is compensated with a reactive current generated at converter2, thus reducing a reactive current flowing from transformer52to input terminal T1.

FIG. 2shows the relationship between a reactive current153generated at load53and a reactive current I2generated at converter2. InFIG. 2, the horizontal axis is an active current axis, and the vertical axis is a reactive current axis. InFIG. 2, an AC voltage Vs supplied from commercial AC power source51through transformer52is defined as a reference vector. At reactor L2, an AC current Is flows, the AC current Is having a phase retarded relative to AC voltage Vs. AC current Is includes an active current Ip and reactive current I2. The sum of input voltage Vc of converter2and jωL Is is Vs, where L denotes the reactance of reactor L2.

By controlling converter2, each of active current Ip and reactive current I2can be controlled. Reactive current153generated at load53is the same in meaning as reactive current153flowing into load53. Reactive current I2generated at converter2is the same in meaning as reactive current I2flowing into converter2.

If the rated current of uninterruptible power supply1is large enough relative to reactive current153generated at load53, the sum of reactive current153generated at load53and reactive current I2generated at converter2is 0 A. In such a case, reactive current153generated at load53is wholly compensated with reactive current I2generated at converter2, a reactive current flowing from transformer52to input terminal T1is 0 A, and the power factor of the electrical equipment on the input terminal T1side relative to transformer52is 1.

Next, the operation of uninterruptible power supply1is described. During a normal time in which AC power is supplied from commercial AC power source51, all of switches S1to S3are ON. AC power from commercial AC power source51is provided to input terminal T1through transformer52. AC power provided to input terminal T1is supplied to load53through reactor L1and is supplied to converter2through switch S1, fuse F1, and reactor L2to be converted to DC power. DC power generated by converter2is supplied to battery3through fuse F2and switch S2, and is converted into AC power by inverter4. AC power generated by inverter4is supplied to load54through a filter including reactor L3and capacitor C2and through switch S3.

At this time, a three-phase AC current flowing through load53is detected by three current detectors CD1, and the detection values of the three-phase AC current are each transformed into q-axis current Iq1by phase detector10and coordinate transformer11. This q-axis current Iq1corresponds to a reactive current generated at load53. Based on this q-axis current Iq1, a reactive current instruction value for compensating the reactive current generated at load53is generated by reactive current instructing unit12. The reactive current instruction value is limited to an upper limit value or less by limiter13. The upper limit value is set to a value according to the difference between the rated current of uninterruptible power supply1and the rated current of load54.

On the other hand, a three-phase AC current flowing through converter2is detected by three current detectors CD2, and the detection values of the three-phase AC current are each transformed into d-axis current Id2and q-axis current Iq2by phase detector10and coordinate transformer14. The d-axis current Id2corresponds to an active current flowing through converter2, and the q-axis current Iq2corresponds to a reactive current flowing through converter2. Current controller15generates a voltage instruction value so as to remove the deviation of q-axis current Iq2from coordinate transformer14from a reactive current instruction value from reactive current instructing unit12.

Further, voltage detector21detects a DC voltage of DC bus B1, and voltage controller22generates an active current instruction value so as to remove the deviation of the detection value from voltage detector21from a DC voltage instruction value from DC voltage instructing unit20. So as to remove the deviation of d-axis current Id2from coordinate transformer14from the active current instruction value, current controller23generates a voltage instruction value.

Two voltage instruction values generated by current controllers15,23are transformed into a three-phase AC voltage instruction value by voltage vector calculating unit24. Based on the three-phase AC voltage instruction value, PWM controller25generates a three-phase PWM signal with which to control converter2.

Thus, a reactive current lower than or equal to the upper limit value of a reactive current generated at load53(the difference between the rated current of uninterruptible power supply1and the rated current of load54) is compensated. That is, if a reactive current generated at load53is more than the upper limit value, a part of the reactive current generated at load53is compensated, whereas, if a reactive current generated at load53is less than the upper limit value, all of the reactive current generated at load53is compensated.

During a power failure time in which supply of AC power from commercial AC power source51is stopped, switch S1is OFF and the operation of converter2is stopped. DC power in battery3is converted by inverter4into AC power having a commercial frequency to be supplied to load54. Thus, the operation of load54can be continued as long as battery3stores DC power.

In Embodiment 1, limiter13is provided to limit a reactive current generated at converter2to an upper limit value or less, the upper limit value being a value according to the difference between the rated current of uninterruptible power supply1and the rated current of load54. Accordingly, a reactive current generated at load53can be compensated within the range of rated current of uninterruptible power supply1. Therefore, downsizing of the apparatus and reduction in cost can be achieved compared to a case where a reactive current generated at load53is wholly compensated by an uninterruptible power supply.

Note that, in Embodiment 1, although a reactive current generated at converter2is limited to an upper limit value or less, the upper limit value being a value according to the difference between the rated current of uninterruptible power supply1and the rated current of load54, reactive power generated at converter2may be limited to an upper limit value or less, the upper limit value being a value according to the difference between the rated capacity of uninterruptible power supply1and the rated power of load54.

FIG. 3is a block diagram showing a configuration of an uninterruptible power supply system according to Embodiment2of the present invention. InFIG. 3, the uninterruptible power supply system includes an uninterruptible power supply (UPS)31and a plurality of uninterruptible power supplies31A. Each of uninterruptible power supply31and a plurality of uninterruptible power supplies31A receives, together with load53, AC power supplied from commercial AC power source51through transformer52and supplies AC power to load54.

Uninterruptible power supply31and a plurality of uninterruptible power supplies31A are coupled to one another with a communication line34, and each uninterruptible power supply exchanges various types of information with each of other uninterruptible power supplies through communication line34. Uninterruptible power supply31and a plurality of uninterruptible power supplies31A compensate reactive power generated at load53in cooperation with one another.

FIG. 4is a circuit block diagram showing a configuration of uninterruptible power supply31,FIG. 4being contrasted withFIG. 1. With reference toFIG. 4, uninterruptible power supply31is different from uninterruptible power supply1in that the former additionally includes a communication unit32and a sharing current calculation unit33.

Communication unit32exchanges various types of information with each of a plurality of uninterruptible power supplies31A through communication line34. In particular, communication unit32sends a signal representing that uninterruptible power supply31is in operation and sends the value of q-axis current Iq1generated at coordinate transformer11, to each uninterruptible power supply31A, and receives, from each uninterruptible power supply31A, a signal representing whether the uninterruptible power supply31A is in operation.

Sharing current calculation unit33acquires the number n of uninterruptible power supplies in operation among uninterruptible power supply31and a plurality of uninterruptible power supplies31A, based on a signal from each uninterruptible power supply31or31A representing whether the uninterruptible power supply31or31A is in operation. Further, sharing current calculation unit33acquires a sharing current Iq1/n by dividing q-axis current Iq1, which is provided from coordinate transformer11through communication unit32, by the number n of uninterruptible power supplies in operation, and provides the sharing current Iq1/n to reactive current instructing unit12. Note that n is an integer of 1 or more.

Reactive current instructing unit12generates a reactive current instruction value (q-axis current instruction value) for compensating a reactive current generated at load53, based on sharing current Iq1in provided from sharing current calculation unit33. Limiter13limits the reactive current instruction value output from reactive current instructing unit12to an upper limit value or less. The reactive current instruction value limited to the upper limit value or less is provided to current controller15.

FIG. 5is a circuit block diagram showing a configuration of uninterruptible power supply31A,FIG. 5being contrasted withFIG. 4. With reference toFIG. 5, uninterruptible power supply31A is different from uninterruptible power supply31in that the former is not provided with output terminal T2, reactor L1, current detector CD1, and coordinate transformer11.

Communication unit32exchanges various types of information with each of other uninterruptible power supplies31or31A through communication line34. In particular, communication unit32sends a signal representing that this uninterruptible power supply31A is in operation, to each uninterruptible power supply31or31A, receives the value of q-axis current Iq1from uninterruptible power supply31, and receives, from each uninterruptible power supply31or31A, a signal representing whether the uninterruptible power supply31or31A is in operation.

Sharing current calculation unit33acquires the number n of uninterruptible power supplies in operation among uninterruptible power supply31and a plurality of uninterruptible power supplies31A, based on a signal from each uninterruptible power supply31or31A representing whether the uninterruptible power supply31or31A is in operation. Further, sharing current calculation unit33acquires sharing current Iq1/n by dividing q-axis current Iq1, which is provided from uninterruptible power supply31through communication unit32, by the number n of uninterruptible power supplies in operation, and provides the sharing current Iq1/n to reactive current instructing unit12.

Reactive current instructing unit12generates a reactive current instruction value (q-axis current instruction value) for compensating a reactive current generated at load53, based on sharing current Iq1/n provided from sharing current calculation unit33. Limiter13limits the reactive current instruction value output from reactive current instructing unit12to an upper limit value or less. The reactive current instruction value limited to the upper limit value or less is provided to current controller15. The other features and operations of each uninterruptible power supply31,31A are the same as those of uninterruptible power supply1ofFIG. 1, and thus the explanations for them are not repeated.

With Embodiment 2, the same advantageous effects as those of Embodiment 1 can be obtained. In addition, since reactive power generated at load53is compensated by n uninterruptible power supplies in operation, a larger reactive power can be compensated than in Embodiment 1.

Note that, in a case where reactive power generated at load53cannot be compensated by n uninterruptible power supplies in operation, uninterruptible power supply31A that has been stopped and that is ready for operation may be activated so as to increase the number n of operating uninterruptible power supplies.

FIG. 6is a circuit block diagram showing a configuration of an uninterruptible power supply35according to Embodiment 3 of the present invention,FIG. 6being contrasted withFIG. 1. With reference toFIG. 6, uninterruptible power supply35is different from uninterruptible power supply1ofFIG. 1in that the former additionally includes a current detector CD3and includes a limiter36instead of limiter13.

Current detector CD3detects an instantaneous value of an AC current flowing between the other terminal of switch S3and output terminal T3, and outputs a signal representing the detection value to limiter36. Actually, there are three current detectors CD3corresponding to a three-phase AC current. Each of three current detectors CD3detects an instantaneous value of a three-phase AC current and outputs a signal representing the detection value to limiter36. Limiter36limits a reactive current instruction value generated by reactive current instructing unit12to an upper limit value or less. The upper limit value is the difference between the rated current of uninterruptible power supply35and a three-phase AC current (load current) detected by three current detectors CD3. The other features and operations are the same as those of uninterruptible power supply1ofFIG. 1, and thus the explanations for them are not repeated.

With Embodiment 3, the same advantageous effects as those of Embodiment 1 can be obtained. In addition, if load54is changed and a current flowing through load54is changed, a reactive current generated at load53can be compensated within the range of rated current of uninterruptible power supply35.

Note that, in Embodiment 3, although a reactive current generated at converter2is limited to an upper limit value or less, the upper limit value being a value according to the difference between the rated current of uninterruptible power supply35and a current supplied to load54, reactive power generated at converter2may be limited to an upper limit value or less, the upper limit value being a value according to the difference between the rated capacity of uninterruptible power supply35and AC power supplied to load54.

FIG. 7is a block diagram showing a configuration of an uninterruptible power supply system according to Embodiment4of the present invention,FIG. 7being contrasted withFIG. 3. InFIG. 7, the uninterruptible power supply system includes an uninterruptible power supply41and a plurality of uninterruptible power supplies41A. Each of uninterruptible power supply41and a plurality of uninterruptible power supplies41A receives, together with load53, AC power supplied from commercial AC power source51through transformer52and supplies AC power to load54.

Uninterruptible power supply41and a plurality of uninterruptible power supplies41A are coupled to one another with communication line34, and each uninterruptible power supply exchanges various types of information with each of other uninterruptible power supplies through communication line34. Uninterruptible power supply41and a plurality of uninterruptible power supplies41A compensate reactive power generated at load53in cooperation with one another.

FIG. 8is a circuit block diagram showing a configuration of uninterruptible power supply41,FIG. 8being contrasted withFIG. 6. With reference toFIG. 8, uninterruptible power supply41is different from uninterruptible power supply35ofFIG. 6in that the former additionally includes communication unit32and sharing current calculation unit33.

Communication unit32exchanges various types of information with each of a plurality of uninterruptible power supplies41A through communication line34. In particular, communication unit32sends a signal representing that uninterruptible power supply41is in operation and sends the value of q-axis current Iq1generated at coordinate transformer11, to each uninterruptible power supply41A, and receives, from each uninterruptible power supply41A, a signal representing whether the uninterruptible power supply41A is in operation.

Sharing current calculation unit33acquires the number n of uninterruptible power supplies in operation among uninterruptible power supply41and a plurality of uninterruptible power supplies41A, based on a signal from each uninterruptible power supply41or41A representing whether the uninterruptible power supply41or41A is in operation. Further, sharing current calculation unit33acquires sharing current Iq1/n by dividing q-axis current Iq1, which is provided from coordinate transformer11through communication unit32, by the number n of uninterruptible power supplies in operation, and provides the sharing current Iq1/n to reactive current instructing unit12. Note that n is an integer of 1 or more.

Reactive current instructing unit12generates a reactive current instruction value (q-axis current instruction value) for compensating a reactive current generated at load53, based on sharing current Iq1/n provided from sharing current calculation unit33. Limiter36limits the reactive current instruction value output from reactive current instructing unit12to an upper limit value or less. The reactive current instruction value limited to the upper limit value or less is provided to current controller15.

FIG. 9is a circuit block diagram showing a configuration of uninterruptible power supply41A,FIG. 9being contrasted withFIG. 8. With reference toFIG. 9, uninterruptible power supply41A is different from uninterruptible power supply41in that the former is not provided with output terminal T2, reactor L1, current detector CD1, and coordinate transformer11.

Communication unit32exchanges various types of information with each of other uninterruptible power supplies41or41A through communication line34. In particular, communication unit32sends a signal representing that the uninterruptible power supply41A is in operation, to each uninterruptible power supply41or41A, receives the value of q-axis current Iq1from uninterruptible power supply41, and receives, from each uninterruptible power supply41or41A, a signal representing whether the uninterruptible power supply41or41A is in operation.

Sharing current calculation unit33acquires the number n of uninterruptible power supplies in operation among uninterruptible power supply41and a plurality of uninterruptible power supplies41A, based on a signal from each uninterruptible power supply41or41A representing whether the uninterruptible power supply41or41A is in operation. Further, sharing current calculation unit33acquires sharing current Iq1/n by dividing q-axis current Iq1, which is provided from uninterruptible power supply41through communication unit32, by the number n of uninterruptible power supplies in operation, and provides the sharing current Iq1/n to reactive current instructing unit12.

Reactive current instructing unit12generates a reactive current instruction value (q-axis current instruction value) for compensating a reactive current generated at load53, based on sharing current Iq1/n provided from sharing current calculation unit33. Limiter13limits the reactive current instruction value output from reactive current instructing unit12to an upper limit value or less. The reactive current instruction value limited to the upper limit value or less is provided to current controller15. The other features and operations of each uninterruptible power supply41,41A are the same as those of uninterruptible power supply1ofFIG. 1, and thus the explanations for them are not repeated.

With Embodiment 4, the same advantageous effects as those of Embodiment 3 can be obtained. In addition, since reactive power generated at load53can be compensated by n uninterruptible power supplies in operation, a larger reactive power can be compensated than in Embodiment 3.

Note that, in a case where reactive power generated at load53cannot be compensated by n uninterruptible power supplies in operation, uninterruptible power supply41A that has been stopped and that is ready for operation may be activated so as to increase the number n of operating uninterruptible power supplies.

FIG. 10is a circuit block diagram showing a configuration of an uninterruptible power supply45according to Embodiment 5 of the present invention,FIG. 10being contrasted withFIG. 6. With reference toFIG. 10, uninterruptible power supply45is different from uninterruptible power supply35ofFIG. 6in that the former is not provided with reactor L1, locates current detector CD1in a different position, and includes a current controller37instead of current controller15.

Output terminal T2is connected to a node N1between input terminal T1and one terminal of switch S1. Current detector CD1detects an instantaneous value of an AC current flowing between input terminal T1and node N1, and outputs a signal representing the detection value to coordinate transformer11. Actually, three current detectors CD1are provided corresponding to a three-phase AC current.

Each of three current detectors CD1detects an instantaneous value of a three-phase AC current, and outputs a signal representing the detection value to coordinate transformer11. Coordinate transformer11performs three-phase to two-phase transformation of the three-phase AC current detected by three current detectors CD1and generates d-axis current and q-axis current Iq1. The q-axis current Iq1is a DC signal of a value according to a reactive current acquired by adding a reactive current generated at load53to a reactive current generated at converter2.

Reactive current instructing unit12generates a reactive current instruction value (q-axis current instruction value) for compensating a reactive current generated at load53and converter2, based on q-axis current Iq1generated by coordinate transformer11. Limiter36limits the reactive current instruction value output from reactive current instructing unit12to an upper limit value or less. The upper limit value is a value acquired by converting the current value of the difference between the rated current of uninterruptible power supply45and a detection value from current detector CD3, into a DC signal (q-axis current). The reactive current instruction value limited to the upper limit value or less is provided to current controller37.

Current controller37generates a voltage instruction value for reducing the reactive current instruction value, based on the reactive current instruction value from reactive current instructing unit12and q-axis current Iq2from coordinate transformer14. Reduction in reactive current instruction value leads to reduction in reactive current detected by current detector CD1, and thus leads to improvement in power factor of the electrical equipment on the input terminal T1side relative to transformer52. When the reactive current instruction value is 0, the reactive current detected by current detector CD1is 0 A and the power factor of the electrical equipment on the input terminal T1side relative to transformer52is 1.

Next, the operation of uninterruptible power supply45is described. In an initial state, AC power is supplied from commercial AC power source51through transformer52to load53so as to operate load53, and the operation of converter2is stopped.

A three-phase AC current flowing from transformer52to load53is detected by three current detectors CD1, and the detection values are each subjected to coordinate transformation by phase detector10and coordinate transformer11, so that q-axis current Iq1is generated. The q-axis current Iq1is transformed into a reactive current instruction value by reactive current instructing unit12. The reactive current instruction value is limited to an upper limit value or less by limiter36. The upper limit value at this time is a value according to the rated current of uninterruptible power supply45since an AC current has not yet been supplied to load54. Since the operation of converter2is stopped, the detection value from current detector CD2is 0 A and q-axis current Iq2is 0.

Then, the operation of converter2and inverter4is started, and supply of AC power to load54is started. A reactive current generated at load53and converter2is transformed into q-axis current Iq1by current detector CD1, phase detector10, and coordinate transformer11, and is further transformed into a reactive current instruction value by reactive current instructing unit12and limiter36. A reactive current generated at converter2is transformed into q-axis current Iq2by current detector CD2, phase detector10, and coordinate transformer14.

Current controller37generates a voltage instruction value based on the reactive current instruction value and q-axis current Iq2and controls q-axis current Iq2so as to reduce the reactive current instruction value. That is, I1=−I2+I53is true, where T1denotes a reactive current flowing through current detector CD1, −I2denotes a reactive current flowing through current detector CD2(a reactive current generated at converter2), and153denotes a reactive current generated at load53. Current controller37controls I2so as to reduce T1to 0 A. The equation I2=I53is true when I1=0. The other features and operations are the same as those of uninterruptible power supply35ofFIG. 6, and thus the explanations for them are not repeated.

With Embodiment 5, the same advantageous effects as those of Embodiment 3 can be obtained. In addition, reactive power generated at load53and converter2can be compensated within the range of rated capacity of uninterruptible power supply45.

FIG. 11is a block diagram showing a configuration of an uninterruptible power supply system according to Embodiment 6 of the present invention,FIG. 11being contrasted withFIG. 3. InFIG. 11, the uninterruptible power supply system includes an uninterruptible power supply46and a plurality of uninterruptible power supplies46A. Each of uninterruptible power supply46and a plurality of uninterruptible power supplies46A receives, together with load53, AC power supplied from commercial AC power source51through transformer52and supplies AC power to load54.

Uninterruptible power supply46and a plurality of uninterruptible power supplies46A are coupled to one another with communication line34, and each uninterruptible power supply exchanges various types of information with each of other uninterruptible power supplies through communication line34. Uninterruptible power supply46and a plurality of uninterruptible power supplies46A compensate reactive power generated at load53in cooperation with one another.

FIG. 12is a circuit block diagram showing a configuration of uninterruptible power supply46,FIG. 12being contrasted withFIG. 10. With reference toFIG. 12, uninterruptible power supply46is different from uninterruptible power supply45in that the former additionally includes communication unit32and sharing current calculation unit33.

Communication unit32exchanges various types of information with each of a plurality of uninterruptible power supplies46A through communication line34. In particular, communication unit32sends a signal representing that uninterruptible power supply46is in operation and sends the value of q-axis current Iq1generated at coordinate transformer11, to each uninterruptible power supply46A, and receives, from each uninterruptible power supply46A, a signal representing whether the uninterruptible power supply46A is in operation.

Sharing current calculation unit33acquires the number n of uninterruptible power supplies in operation among uninterruptible power supply46and a plurality of uninterruptible power supplies46A, based on a signal from each uninterruptible power supply46or46A representing whether the uninterruptible power supply46or46A is in operation. Further, sharing current calculation unit33acquires sharing current Iq1/n by dividing q-axis current Iq1, which is provided from coordinate transformer11through communication unit32, by the number n of uninterruptible power supplies in operation, and provides the sharing current Iq1/n to reactive current instructing unit12. Note that n is an integer of 1 or more.

Reactive current instructing unit12generates a reactive current instruction value (q-axis current instruction value) for compensating a reactive current generated at load53, based on sharing current Iq1/n provided from sharing current calculation unit33. Limiter36limits the reactive current instruction value output from reactive current instructing unit12to an upper limit value or less. The reactive current instruction value limited to the upper limit value or less is provided to current controller37. Current controller37controls q-axis current Iq2so as to reduce the reactive current instruction value.

FIG. 13is a circuit block diagram showing a configuration of uninterruptible power supply46A,FIG. 13being contrasted withFIG. 12. With reference toFIG. 13, uninterruptible power supply46A is different from uninterruptible power supply46in that the former has input terminal T1connected to node N1of uninterruptible power supply46and is not provided with output terminal T2, current detector CD1, and coordinate transformer11.

Since input terminal T1is connected to node N1of uninterruptible power supply46, current detector CD1of uninterruptible power supply46detects the sum of AC current supplied from commercial AC power source51through transformer52to load53, uninterruptible power supply46, and a plurality of uninterruptible power supplies46A.

Communication unit32exchanges various types of information with each of other uninterruptible power supplies46or46A through communication line34. In particular, communication unit32sends a signal representing that the uninterruptible power supply46A is in operation, to each uninterruptible power supply46or46A, receives the value of q-axis current Iq1from uninterruptible power supply46, and receives, from each uninterruptible power supply46or46A, a signal representing whether the uninterruptible power supply46or46A is in operation.

Sharing current calculation unit33acquires the number n of uninterruptible power supplies in operation among uninterruptible power supply46and a plurality of uninterruptible power supplies46A, based on a signal from each uninterruptible power supply46or46A representing whether the uninterruptible power supply46or46A is in operation. Further, sharing current calculation unit33acquires sharing current Iq1/n by dividing q-axis current Iq1, which is provided from uninterruptible power supply46through communication unit32, by the number n of uninterruptible power supplies in operation, and provides the sharing current Iq1/n to reactive current instructing unit12.

Reactive current instructing unit12generates a reactive current instruction value (q-axis current instruction value) for compensating a reactive current generated at load53, based on sharing current Iq1/n provided from sharing current calculation unit33. Limiter36limits the reactive current instruction value output from reactive current instructing unit12to an upper limit value or less. The reactive current instruction value limited to the upper limit value or less is provided to current controller37. The other features and operations of each uninterruptible power supply46,46A are the same as those of uninterruptible power supply45ofFIG. 10, and thus the explanations for them are not repeated.

With Embodiment 6, the same advantageous effects as those of Embodiment 5 can be obtained. In addition, since reactive power generated at load53is compensated by n uninterruptible power supplies in operation, a larger reactive power can be compensated than in Embodiment 5.

Note that, in a case where reactive power generated at load53cannot be compensated by n uninterruptible power supplies in operation, uninterruptible power supply46A that has been stopped and that is ready for operation may be activated so as to increase the number n of operating uninterruptible power supplies.

Further, it is needless to say that Embodiments 1 to 6 described above can be combined with one another as appropriate.

The embodiments disclosed herein should be considered illustrative in all respects, not limitative. It is intended that the scope of the present invention is defined not by the above description but by the claims, and that the scope of the invention includes all the modifications in the meaning and scope equivalent to the claims.

REFERENCE SIGNS LIST