JPH11341702A - Uninterruptible power supply device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an uninterruptible power supply device which is quickly and accurately capable of detecting the abnormality of an AC power supply and stably supplying power to a normal load. SOLUTION: A changeover relay 16 switches a power supply route between a a route, through which a power is supplied from a commercial power supply 12 to a load and a route through which a reserve power is supplied from a battery 14 to the load, in accordance with the level of a changeover command signal Sa. A full-wave rectifying circuit 17 generates a power supply voltage signal Sp obtained through the full-waver rectification of the commercial power supply voltage. A reference signal generating circuit 19 generates a reference signal Sr, which has a same period as the period of the commercial power supply voltage and has an absolute-value sinusoidal waveform which rises from the zero-cross point of each period. The amplitude of the reference signal Sr is selected to be smaller than the amplitude of the power supply voltage signal Sp, which is generated while the commercial power supply 12 is in a normal state. A power supply abnormality monitor circuit 18 makes the level of the changeover command signal Sa H, when the power supply voltage signal Sp becomes smaller than the reference signal Sr. As a result of this process, an inverter circuit 15 is driven, and moreover the changeover relay 16 is switched to a contact 16a side.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an uninterruptible power supply of an always-supply type which supplies an output to a load when an AC power supply is normal and supplies an output of a standby power supply to the load when abnormal.

[0002]

The conventional structure of this type of uninterruptible power supply will be described with reference to FIGS. 5 and 6. FIG. In FIG. 5, the uninterruptible power supply 1 includes a power supply abnormality detection circuit 2, a battery 3 as a standby power supply, an inverter circuit 4, and a switching relay 5. The power supply abnormality detection circuit 2 constantly monitors the voltage abnormality and the frequency abnormality of the commercial power supply 6, and when the commercial power supply 6 is normal, switches the switching relay 5 to the commercial power supply 6 side, and directly connects the commercial power supply 6 to the load 7. Control to supply power. When detecting an abnormal voltage or abnormal frequency of the commercial power supply 6, the power supply abnormality detection circuit 2 activates the inverter circuit 4 and simultaneously switches the switching relay 5 to the output side of the inverter circuit 4. Control is performed so as to output the converted energy of the battery 3 to the load 7.

In the power supply abnormality detecting circuit 2, the operation of detecting a voltage abnormality or a frequency abnormality of the commercial power supply 6 is as follows. That is, the power failure detection circuit 2 includes a power failure detection counter (not shown) for constantly counting up internally generated pulse signals of a fixed cycle, and the power failure detection counter is as shown in FIG. The zero cross point of the AC voltage of the commercial power supply 6 is cleared to 0 at each timing when the power supply signal is detected.

Therefore, when the commercial power supply 6 is normal,
The power failure detection counter is always 0 every 1/2 cycle of the commercial power supply 6.
, The count value is always kept below the predetermined power failure detection value. However, when a power failure occurs in the commercial power supply 6, the zero-crossing point, that is, the power signal is no longer detected, so that the power failure detection counter is not cleared to 0, and as a result, the count value exceeds the power failure detection value. With this operation, the power supply abnormality detection circuit 2 can detect a power failure of the commercial power supply 6. The power failure detection circuit 2 monitors a count value immediately before the power failure detection counter is cleared to 0 when a power signal is input, and the count value is set in advance in accordance with the cycle of the commercial power supply 6. If it deviates from the reference count value, it is determined that the frequency is abnormal.

However, the above-described power supply abnormality detection circuit 2
Even if the commercial power supply 6 fails, the power failure cannot be detected until the scheduled time of the next zero crossing that occurs thereafter. As a result, at least a half cycle of the commercial power supply voltage until the power failure is detected. There is a problem that the above detection delay occurs. In addition, an instantaneous power failure of less than 1/2 cycle may not be detected in some cases, and furthermore, there is a problem that a drop in power supply voltage cannot be detected at all. for that reason,
When a power failure occurs, the operation of the switching relay 5 is delayed,
The state in which power is not supplied to the load 7 may last for about 10 [ms] at worst, and in some cases, the operation of the load 7 may be stopped or a malfunction may occur.

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and has as its object to quickly and accurately detect an abnormality in an AC power supply and to stably supply power to a load at all times. An object of the present invention is to provide an uninterruptible power supply having the following features.

[0007]

In order to achieve the above object, an uninterruptible power supply according to the present invention supplies its output to a load when the AC power supply is normal, and supplies a standby power supply output to the load when the AC power supply is abnormal. In the continuous power supply uninterruptible power supply, the AC power supply output is synchronized every predetermined cycle, has a signal length corresponding to a normal cycle of the AC power supply output, and has a normal voltage level of the AC power supply output. A reference signal generating unit that generates a reference signal that is always smaller than an absolute value; a voltage signal generating unit that generates a power supply voltage signal obtained by converting an output voltage of the AC power supply into an absolute value; and the reference signal and the power supply voltage signal. When the power supply voltage signal becomes smaller than the reference signal, it is determined that the voltage of the AC power supply is abnormal, and a power supply signal for outputting a switching command signal from the AC power supply to the standby power supply is output. Characterized in that with an abnormality monitoring unit (claim 1).

With this configuration, when the output voltage and the frequency of the AC power supply are normal, the power supply voltage signal is always higher than the reference signal, so that the power supply abnormality monitoring means switches from the AC power supply to the standby power supply. No command signal is output, and the output of the AC power supply is directly applied to the load. On the other hand, if the voltage of the AC power supply drops or a power failure or instantaneous power failure occurs, the power supply voltage signal immediately becomes smaller than the reference signal irrespective of the occurrence timing, and at that time, the power supply abnormality monitoring means does not have any delay in detecting the AC power supply. A switching command signal from the power supply to the standby power supply can be output, and the standby power supply output is immediately applied to the load instead of the AC power supply output.

Further, the uninterruptible power supply according to the present invention is characterized in that the AC power supply is synchronized every one cycle or a plurality of cycles, has a signal length corresponding to a normal cycle of the AC power supply output, and has the AC power supply. Reference signal generating means for generating a reference signal that is always smaller than the absolute value of the normal voltage level of the output; voltage signal generating means for generating a power supply voltage signal obtained by converting the output voltage of the AC power supply into an absolute value; Comparing the signal and the power supply voltage signal, for each cycle of the AC power supply,
A power supply abnormality monitoring unit that determines that the frequency of the AC power supply is abnormal when the change in which the power supply voltage signal is smaller than the reference signal is repeated a set number of times or more, and outputs a switching command signal from the AC power supply to the standby power supply. It is also possible to adopt a configuration provided with (claim 2).

With this configuration, when the frequency of the AC power supply output increases or decreases from the normal frequency, the AC power supply has a signal length corresponding to a normal cycle within one cycle section or a plurality of cycle sections. Synchronization is lost between the reference signal and the power supply voltage signal having an abnormal cycle, and a period occurs in which the power supply voltage signal is smaller than the reference signal for each cycle of the AC power supply. Therefore, the frequency abnormality of the AC power supply can be reliably determined based on the fact that this change is repeated a set number of times or more.

[0011] In these cases, the reference signal generating means exhibits a value of 0 or a value close to 0 in a period before and after a normal zero crossing point of the AC power supply, and a sine waveform in an absolute value in other periods. Is preferably generated. With such a configuration, since the power supply signal having a sine waveform obtained by converting the output voltage of the AC power supply into an absolute value and the reference signal have a similar waveform, the reference signal approaches the power supply voltage signal in a normal state at each time. The power supply abnormality detection accuracy can be further improved. Further, since the reference signal is set to 0 or a value close to 0 in a period before and after a normal zero crossing point of the AC power supply, erroneous detection in that period can be prevented.

Further, the reference signal generating means may be configured to generate a rectangular waveform reference signal having a value of 0 or a value close to 0 before and after a normal zero crossing point of the AC power supply. With such a configuration, it is possible to easily generate the reference signal without providing sine waveform data in advance or performing calculations or the like. Further, as described above, it is possible to prevent erroneous detection in a period before and after a normal zero-cross point of the AC power supply.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First Embodiment A first embodiment of the present invention will be described below with reference to FIGS. FIG. 1 shows the configuration of the uninterruptible power supply. In FIG. 1, the uninterruptible power supply 11
AC power supply, for example, single-phase 100 [V] voltage value, 50 [H]
z] or a commercial power supply 12 having a frequency of 60 [Hz]
, And is configured to always output a single-phase AC voltage having the above-described voltage value and frequency to the load 13.

The battery 14 is a backup power supply for supplying energy to the load 13 instead of the commercial power supply 12 when a voltage abnormality or a frequency abnormality occurs in the commercial power supply 12. It is designed to be driven. Although not specifically shown, the inverter circuit 15 includes, for example, an inverter control circuit including a microcomputer, and a well-known main circuit in which a switching element such as a transistor and a return diode are connected in a single-phase bridge. In addition, the DC-AC conversion operation is started at the same time when the switching command signal Sa described later is input, and the above-described single-phase AC voltage is generated.

The switching relay 16 is provided for switching a power supply path to the load 13. When a driving signal Sb described later is inputted, the switching relay 16 is energized to an exciting coil (not shown) and is normally opened. Presents a closed state,
When the input of the drive signal Sb is stopped, the excitation coil is turned off and the normally closed contact 16b is closed. In this case, in the steady state in which the contact 16b is closed, the output of the commercial power
3, the output of the inverter circuit 15 is applied to the load 13 when the contact 16a is closed.

The full-wave rectifying circuit 17 functioning as a voltage signal generating means has a well-known circuit configuration in which a rectifying element, for example, a diode is connected in a single-phase bridge, and furthermore, a power-supply abnormality monitoring circuit to be described later. 18 to a control voltage level corresponding to the control power supply voltage, and the power supply voltage signal Sp
(See FIG. 2).

A reference signal generating circuit 19, which functions as a reference signal generating means, detects a zero-cross point every one cycle of the commercial power supply voltage by a zero-cross detecting circuit (not shown). A reference signal Sr (see FIG. 2) having a frequency at the time and having a sinusoidal waveform whose absolute value is zero with a voltage value of 0 during a predetermined period before and after the zero crossing point is generated. The amplitude of the reference signal Sr is set to be always smaller than the amplitude of the power supply voltage signal Sp when the commercial power supply 12 is normal. In addition, the sine waveform data of the reference signal Sr is stored in a storage circuit (not shown) in the reference signal generation circuit 19 in a state where the sine waveform data is converted into an absolute value in advance. The zero-cross point may be detected either when the voltage of the commercial power supply 12 changes from negative to positive or when the voltage changes from positive to negative.

The power supply abnormality monitoring circuit 18 as a power supply abnormality monitoring means includes a comparing means, for example, a comparator 20 and a driving circuit 21.
It is composed of Comparator 20 compares power supply voltage signal Sp input to its non-inverting input terminal with reference signal Sr input to the inverting input terminal, and uses the comparison output as switching command signal Sa as inverter circuit 15 and driving circuit Give to 21. The drive circuit 21 is a circuit for turning off the excitation coil of the switching relay 16 in accordance with the level (H level or L level) of the switching command signal Sa.

Although the connection relationship is not shown in FIG. 1, the full-wave rectifier circuit 17, the power supply abnormality monitoring circuit 18, the reference signal generation circuit 19, and the inverter control circuit receive power supply from the battery 14. To work.

Next, the operation of this embodiment when an instantaneous power failure occurs in the commercial power supply 12 will be described. FIG.
Shows an output timing chart of the reference signal Sr, the power supply voltage signal Sp, and the switching command signal Sa.
This timing chart is drawn on the assumption that the commercial power supply 12 supplies a normal voltage from time t1 to time t4, and after a momentary power failure from time t4 to time t5, the power is restored.

The reference signal generation circuit 19 constantly detects the zero-cross point of the output voltage from the commercial power supply 12 by the zero-cross detection circuit.
Starting from time t6,..., The sine waveform data stored in the storage circuit in an absolute value state in advance is sequentially read out for one cycle and converted into an analog value, and the power supply abnormality monitoring circuit 18 is used as the reference signal Sr. Output to In this case, the sine waveform data in the storage circuit is stored as, for example, amplitude data at each time obtained by dividing one cycle of the commercial power supply voltage into a plurality of equal time widths. Note that the reference signal Sr
Becomes the same waveform (half sine wave waveform) every half cycle, so it is sufficient to actually store data for half a cycle.

The comparator 20 in the power supply abnormality monitoring circuit 18
The read reference signal Sr is compared with a power supply voltage signal Sp obtained by full-wave rectifying the voltage of the commercial power supply 12 in the full-wave rectifier circuit 17 to output a switching command signal Sa. The comparator 20 outputs the H level when the non-inverted input terminal voltage is higher than the inverted input terminal voltage, and outputs the L level when the non-inverted input terminal voltage is lower than the inverted input terminal voltage. Operate. As described above, when the commercial power supply 12 supplies a normal voltage, the amplitude of the reference signal Sr is set to be always smaller than the amplitude of the power supply voltage signal Sp. Therefore, the commercial power supply 12
, The switching command signal Sa is always at the H level except for the zero cross point.

At the zero cross point, the power supply voltage signal S
Since both p and the reference signal Sr become 0, the operation of the comparator 20 becomes unstable, and the switching command signal Sa may momentarily become L level. Even in this case, a mask circuit (not shown) using a zero-cross detection signal is provided at the output of the comparator 20 or a low-pass filter (not shown) is inserted so that the instantaneous L level can be obtained. Can be removed. Further, the power supply voltage signal Sp
Since the reference signal Sr is set to 0 in a period before and after the zero-cross point at which the amplitude of the
Even if noise is mixed in the data, it is possible to prevent the occurrence of a comparison error. It should be noted that the voltage abnormality cannot be detected during the period in which the reference signal Sr is set to 0, but since the voltage of the commercial power supply 12 is low during this period, the effect of the delay in detecting the voltage abnormality is sufficiently small.

When a power failure occurs in the commercial power supply 12 at time t4, the power supply voltage signal Sp becomes 0 as shown in FIG. 2, so that the power supply voltage signal Sp becomes smaller than the reference signal Sr, and the switching command signal Sa Changes from the H level to the L level. This L level state indicates that the commercial power supply 12
Continues until time t5 when power is restored. This change to the L level occurs not only due to a power failure of the commercial power supply 12 but also due to a voltage drop. Therefore, the power supply abnormality monitoring circuit 18 can instantaneously detect that an abnormality such as a power failure or a voltage drop has occurred in the commercial power supply 12 due to the change of the switching command signal Sa from the H level to the L level.

The inverter circuit 15 operates only while the switching command signal Sa is at the L level,
4 to generate an AC voltage. Further, the drive circuit 21 outputs the drive signal Sb only during a period when the switching command signal Sa is at the L level, and energizes the exciting coil of the switching relay 16. As a result, the contact 16 of the switching relay 16
a is closed, the commercial power supply 12 exhibiting an abnormal voltage is disconnected, and the AC output from the inverter circuit 15 can be supplied to the load 13 instead.

As described above, the uninterruptible power supply 11 of the present embodiment using the constant power supply system supplies the normal path for supplying power from the commercial power supply 12 to the load 13 and the reserve power of the battery 14 via the inverter circuit 15. Relay 16 for switching the backup path to supply the load 13 to the load 13. In particular, the power supply voltage signal Sp is obtained by full-wave rectifying the commercial power supply voltage, and the power supply voltage signal Sp has the same cycle as the commercial power supply voltage. It is characterized in that a power supply voltage abnormality is determined and the switching relay 16 is switched based on a comparison result with an absoluteized sine waveform reference signal Sr sequentially read from the zero cross point as a starting point.

As described above, the power supply abnormality monitoring circuit 18 generates the power supply voltage signal S based on the actual voltage waveform of the commercial power supply 12.
Since p and the reference signal Sr based on the normal voltage waveform of the commercial power supply 12 are constantly compared, even in the case where not only the power failure of the commercial power supply 12 but also the voltage drop occurs, it is quickly and almost simultaneously performed. An abnormality can be accurately detected, and power can be stably supplied to the load 13.

Since the reference signal Sr has a sine waveform approximating the power supply voltage signal Sp, the amplitudes of the two signals Sp and Sr when the commercial power supply 12 is normal are set close to each other (where Sp> Sr). As a result, an abnormality can be detected even with a slight decrease in the commercial power supply voltage, and the detection accuracy of the voltage abnormality can be further improved. In this case, the power supply voltage signal Sp and the reference signal Sr are closer to the zero crossing point.
The difference between the reference signal Sr and the reference signal Sr in the present embodiment is set to be 0 in the period before and after the zero-cross point. Occurrence can be prevented.

(Second Embodiment) Next, a second embodiment of the present invention will be described with reference to FIGS. 3 and 4 while referring to differences from the first embodiment. FIG. 3 shows the configuration of the uninterruptible power supply 22. In FIG. 3, a power supply abnormality monitoring circuit 23 as a power supply abnormality monitoring means
Is composed of a comparator 20, a counting circuit 24, and a driving circuit 21. Although not specifically shown, the counting circuit 24 includes a counter for counting the number of occurrences of the edge change from the H level to the L level in the output of the comparator 20, a count value and a preset abnormality determination value. (2, 3, 4,...). The output of the counting circuit 24 is used as the switching command signal Sa as the inverter circuit 15 and the driving circuit 2.
Given to one.

Hereinafter, the operation of the present embodiment when the frequency abnormality occurs in the output voltage of the commercial power supply 12 will be described with reference to FIG. FIG. 4 shows an output timing chart of the output voltage of the commercial power supply 12, the reference signal Sr, and the comparator 20. This timing chart is
The commercial power supply 12 supplies a normal voltage from time t11 to time t13, and is illustrated on the assumption that the frequency decreases after time t13.

As in the first embodiment, the reference signal generation circuit 19 detects the zero-cross detection time t11 of the commercial power supply voltage,
Time t12, time t13, time t17, time t20,.
A sine waveform of one cycle (two half-wave sine waveforms) having a normal frequency is output as a reference signal Sr starting from.

The comparator 20 in the power supply abnormality monitoring circuit 23
The output reference signal Sr is compared with a power supply voltage signal Sp obtained by full-wave rectifying the voltage of the commercial power supply 12. When the commercial power supply 12 is normal, the output of the comparator 20 is always at the H level except for the zero cross point. The instantaneous change to the L level at the zero cross point is removed by the above-described mask circuit or low-pass filter. Therefore, the counter in the counting circuit 24 is kept at 0. Since this count value is smaller than the abnormality determination value, the output of the counting circuit 24, that is, the switching command signal Sa becomes H level, and the commercial power supply 12 is directly supplied to the load 13.

When the frequency of the commercial power supply 12 decreases at time t13, the cycle of the power supply voltage signal Sp becomes longer than the cycle of the reference signal Sr, and from the zero-cross detection point of the commercial power supply voltage to the next zero-cross detection point. During one cycle, the power supply voltage signal Sp and the reference signal Sr are out of synchronization. As a result, for example, at time t15, a case occurs where the reference signal Sr has a non-zero value at the zero cross point of the power supply voltage signal Sp. In this case, the comparator 20 detects the time t1 when the power supply voltage signal Sp and the reference signal Sr match before the zero cross point.
4, after the zero-cross point, the power supply voltage signal S
The L level is output until time t16 when p and the reference signal Sr match. Then, from the time t16 to the next zero-cross detection time t17, the reference signal Sr is again at the H level. Such an output pattern of the comparator 20 is
The same is repeated in each subsequent cycle (for example, from t17 to t20).

Therefore, the power supply abnormality monitoring circuit 23 changes the output of the comparator 20 from H level to L
By counting the number of occurrences of the edge change to the level and comparing the count value with the abnormality determination value, it is possible to recognize the occurrence of the above-mentioned output pattern peculiar to a frequency abnormality, and determine the frequency abnormality of the commercial power supply 12. can do. When the count value becomes equal to or larger than the abnormality determination value, the switching command signal Sa changes from the H level to the L level, and at the same time the inverter circuit 15 is started, the contact 16a of the switching relay 16 is closed, and the energy of the battery 14 is reduced. The power is supplied to the load 13 via the inverter circuit 15.

As described above, when the frequency of the commercial power supply 12 deviates from the specified value, the uninterruptible power supply 22 of the present embodiment synchronizes the power supply voltage signal Sp with the reference signal Sr between the zero-cross detection times. Comparator 2
0 level to L level repeatedly appearing at the output of 0 every cycle
It is characterized in that the change to the level is counted by the counting circuit 24, and when the value becomes equal to or larger than the abnormality determination value, the frequency abnormality is detected.

When noise is mixed in the commercial power supply voltage, a zero-cross signal may be output at a point other than the original zero-cross point. In a conventional configuration in which pulse signals of a fixed cycle are counted between the zero-cross points, Misjudgment has occurred for such noise. On the other hand, the uninterruptible power supply 22 of the present invention can eliminate such erroneous determination based on the erroneous zero-cross signal if the abnormality determination value is set to an appropriate value according to the state of the commercial power supply voltage. Frequency abnormality can be reliably detected, and power can be stably supplied to the load 13.

(Other Embodiments) The present invention is not limited to the embodiment described above and shown in the drawings.
For example, the configuration may be as follows. The sine waveform data of the reference signal Sr may be generated by calculation using a microcomputer.

If the reference signal Sr is not a sine waveform but a rectangular waveform, the abnormality of the commercial power supply 12 can be detected. If such a reference signal Sr is used, the reference signal generation circuit 19 can have a simpler configuration. In this case, similarly to the sine waveform described above, it is desirable to set the voltage value to 0 during a predetermined period before and after the zero-cross point when the commercial power supply 12 is normal. However, if there is no erroneous detection in the reference signal Sr of the sine waveform or the rectangular waveform, it is not necessary to set the voltage value to 0 in a predetermined period before and after the zero cross point.

In the first embodiment, the reference signal generation circuit 1
In FIG. 9, the zero cross is detected every cycle, but the zero cross may be detected every half cycle or every two or more cycles. Further, in the second embodiment, the reference signal generation circuit 19 detects a zero cross every cycle, but may be configured to detect a zero cross every two or more cycles.

[0040]

As described above, the uninterruptible power supply according to claim 1 has a normal cycle of the AC power supply output and is always smaller than the absolute value of the normal voltage level of the AC power supply output. By comparing the signal with a power supply voltage signal obtained by converting the AC power supply voltage into an absolute value, a voltage abnormality of the AC power supply is detected, and the power supply is switched to a standby power supply. This voltage abnormality can be detected at the same time that the power supply voltage signal becomes smaller than the reference signal regardless of the timing of occurrence of the abnormality. Therefore, even if a voltage abnormality such as a power failure occurs in the AC power supply, the power supply can be maintained to the load almost instantaneously without interruption, and a stable and reliable uninterruptible power supply can be supplied.

The uninterruptible power supply according to claim 2 is
The power supply voltage signal is compared with the reference signal synchronized every one cycle or a plurality of cycles of the AC power supply, and when a change in which the power supply voltage signal becomes smaller than the reference signal is repeated for a set number of times or more, the AC power supply is turned off. It is configured to detect a frequency abnormality and switch to a standby power supply. Therefore, a frequency abnormality can be detected in about two cycles from the occurrence thereof, and a stable and reliable uninterruptible power supply can be supplied.

[Brief description of the drawings]

FIG. 1 is an electrical configuration diagram of an uninterruptible power supply according to a first embodiment of the present invention;

FIG. 2 is a timing chart of a reference signal Sr, a power supply voltage signal Sp, and a switching command signal Sa.

FIG. 3 is a view corresponding to FIG. 1, showing a second embodiment of the present invention;

FIG. 4 is a timing chart of a commercial power supply voltage, a reference signal Sr, and a comparator output.

FIG. 5 is a diagram corresponding to FIG. 1 in a conventional configuration.

FIG. 6 is a timing chart of a commercial power supply voltage, a power supply signal, and a power failure detection counter.

[Explanation of symbols]

11 and 22 are uninterruptible power supplies, 12 is a commercial power supply (AC power supply), 13 is a load, 14 is a battery (stand-by power supply), 1
7 is a full-wave rectifier circuit (voltage signal generating means), 18 and 23 are power supply abnormality monitoring circuits (power supply abnormality monitoring means), 19 is a reference signal generation circuit (reference signal generating means), and 20 is a comparator.

Claims (4)

[Claims] 1. An uninterruptible power supply system of an always-on power supply type wherein an output of an AC power supply is supplied to a load when the AC power supply is normal and a standby power supply output is supplied to the load when the AC power supply is abnormal. Reference signal generating means for generating a reference signal having a signal length corresponding to a normal cycle of the AC power supply output and always being smaller than an absolute value of a normal voltage level of the AC power supply output; Voltage signal generating means for generating a power supply voltage signal obtained by converting an output voltage of a power supply into an absolute value; comparing the reference signal and the power supply voltage signal, and when the power supply voltage signal becomes smaller than the reference signal, An uninterruptible power supply device comprising: a power supply abnormality monitoring unit that determines that the power supply voltage is abnormal and outputs a switching command signal from the AC power supply to the standby power supply. 2. An uninterruptible power supply of an always-on power supply type wherein an output of the AC power supply is supplied to a load when the AC power supply is normal and a standby power supply output is supplied to the load when the AC power supply is abnormal. A reference signal generating means for generating a reference signal which is synchronized every time, has a signal length corresponding to a normal cycle of the AC power supply output, and is always smaller than an absolute value of a normal voltage level of the AC power supply output; A voltage signal generating unit that generates a power supply voltage signal obtained by converting an output voltage of the AC power supply into an absolute value; and comparing the reference signal and the power supply voltage signal, and for each cycle of the AC power supply, A power supply that determines that the frequency of the AC power supply is abnormal when a change smaller than the reference signal is repeated a set number of times or more, and outputs a switching command signal from the AC power supply to the standby power supply. An uninterruptible power supply device comprising: an abnormality monitoring unit. 3. The reference signal generating means, wherein the reference signal has a value of 0 or a value close to 0 in a period before and after a normal zero crossing point of the AC power supply, and has a sine waveform of an absolute value in other periods. The uninterruptible power supply according to claim 1 or 2, wherein 4. The reference signal generating method according to claim 1, wherein the reference signal generating means generates a rectangular waveform reference signal having a value of 0 or a value close to 0 before and after a normal zero crossing point of the AC power supply. Uninterruptible power system.