U.S. patent application number 09/956185 was filed with the patent office on 2002-03-28 for uniterruptible power system.
This patent application is currently assigned to Sanyo Denki Co., Ltd.. Invention is credited to Okui, Yoshiaki.
Application Number | 20020036911 09/956185 |
Document ID | / |
Family ID | 18772061 |
Filed Date | 2002-03-28 |
United States Patent
Application |
20020036911 |
Kind Code |
A1 |
Okui, Yoshiaki |
March 28, 2002 |
Uniterruptible power system
Abstract
An uninterruptible power system capable of reducing strain in an
output voltage thereof as compared with the prior art. A DC voltage
control circuit of a current control system obtains an input
current command value on the basis of a multiplied value between a
differential voltage between a DC voltage command and a DC-side
voltage of a power converter and an output of a reference voltage
forming circuit. A current control circuit outputs, as a current
control command, a value obtained by subtracting a value of a load
current flowing to a load and a value of a current flowing from an
AC side into the power converter from the input current command
value. An AC current flowing through an AC switch is used as the
input current command value during a period of time for which an
abnormal voltage detection circuit detects momentary
abnormality.
Inventors: |
Okui, Yoshiaki; (Tokyo,
JP) |
Correspondence
Address: |
RANKIN, HILL, PORTER & CLARK, LLP
700 HUNTINGTON BUILDING
925 EUCLID AVENUE
CLEVELAND
OH
44115-1405
US
|
Assignee: |
Sanyo Denki Co., Ltd.
15-1, 1-chome, Kitaotsuka
Tokyo
JP
|
Family ID: |
18772061 |
Appl. No.: |
09/956185 |
Filed: |
September 19, 2001 |
Current U.S.
Class: |
363/95 |
Current CPC
Class: |
H02J 9/062 20130101 |
Class at
Publication: |
363/95 |
International
Class: |
H02M 003/24 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 22, 2000 |
JP |
2000-288480 |
Claims
What is claimed is:
1. An uninterruptible power system comprising: an abnormal voltage
detection circuit for detecting voltage reduction abnormality of a
commercial power supply; an AC switch turned on to permit AC power
to be fed from said commercial power supply to a load when said
commercial power supply is in a normal condition and turned off
when said commercial power supply falls into an abnormal condition;
a storage battery; a power converter of the voltage control type
arranged between said AC switch and said storage battery; said
power converter carrying out power rectifying operation of
converting AC power of said commercial power supply into DC power
to charge said storage battery by said DC power and active filter
operation of flowing a current for compensating a harmonic current
and/or a reactive current flowing to said load when said commercial
power supply is in the normal condition; said power converter
carrying out power inverting operation of inverting DC power into
AC power to feed the AC power to said load mainly using said
storage battery as a power supply therefor when said commercial
power supply is in a condition other than said silent condition; a
control command generation means for outputting a control command
to said power converter for feeding AC power to said load without a
power failure; said control command generation means including a
voltage control system for outputting a voltage control command
which permits said power converter to carry out said power
inverting operation and a current control system for outputting a
current control command which permits said power converter to carry
out said power rectifying operation and active filter operation;
and a reference voltage generating circuit for generating a
reference sinusoidal voltage synchronized with said commercial
power supply; said current control system being constructed so as
to provide an input current command value on the basis of a
multiplied value obtained by multiplication between a differential
voltage between a DC command voltage commanded by a DC voltage
command and a DC-side voltage of said power converter and an output
of said reference voltage generating circuit, and to thereby output
said current control command on the basis of a value obtained by
subtracting a load current value from said input current command
value; said current control system being constructed so as to use
an AC current flowing through said AC switch or a value
corresponding to said AC current as said input current command
value when said abnormal voltage detection circuit carries out
detection of said voltage reduction abnormality.
2. An uninterruptible power system comprising: an abnormal voltage
detection circuit for detecting voltage reduction abnormality of a
commercial power supply; an AC switch turned on to permit AC power
to be fed from said commercial power supply to a load when said
commercial power supply is in a normal condition and turned off
when said commercial power supply falls into an abnormal condition;
a storage battery; a power converter of the voltage control type
arranged between said AC switch and said storage battery; said
power converter carrying out power rectifying operation of
converting AC power of said commercial power supply into DC power
to charge said storage battery by said DC power and active filter
operation of flowing a current for compensating a harmonic current
and/or a reactive current flowing to said load when said commercial
power supply is in the silent condition; said power converter
carrying out power inverting operation of inverting DC power into
AC power to feed the AC power to said load mainly using said
storage battery as a power supply therefor when said commercial
power supply is in a condition other than said silent condition; a
control command generation means for outputting a control command
for feeding AC power to said load without a power failure to said
power converter; said control command generation means including a
voltage control system for outputting a voltage control command
which permits said power converter to carry out said power
inverting operation and a current control system for outputting a
current control command which permits said power converter to carry
out said power rectifying operation and active filter operation;
and a reference voltage generating circuit for generating a
reference sinusoidal voltage synchronized with said commercial
power supply; said control command generation means being
constructed so as to add said current control command and voltage
control command to each other to output said current control
command and voltage control command thus added to said power
converter; said current control system being configured so as to
provide a momentary load-side effective power value corresponding
to momentary effective power fed to said load on the basis of an
output of said reference voltage forming circuit and a load current
value and provide a momentary input-side effective power command
value which permits an input current to have a sinusoidal waveform
by removing an AC component from said momentary load-side effective
power command value, to thereby output said current control command
on the basis of a value obtained by subtracting said momentary
load-side effective power value from said momentary input-side
effective power command value; said current control system being
constructed so as to use a momentary effective power value obtained
by an output of said reference voltage generating circuit and a
value of an AC current flowing through said AC switch as said
momentary input-side effective power command value.
3. An uninterruptible power system as defined in claim 2, further
comprising: a power failure detection circuit for detecting a power
failure of said commercial power supply; and a DC/DC converter
arranged between said power converter and said storage battery;
said DC/DC converter being configured so as to feed DC power from
said storage battery to said power converter during a period of
time for which said power failure detection circuit detects a power
failure of said commercial power supply.
4. An uninterruptible power system as defined in claim 3, wherein
said power converter includes a capacitor connected in parallel to
said storage battery through said DC/DC converter; and said power
converter feeds said AC current to said load using said capacitor
as a power supply therefor during a period of time of from
detection of a voltage reduction of said commercial power supply by
said abnormal voltage detection circuit to detection of a power
failure of said commercial power supply by said power failure
detection circuit.
5. An uninterruptible power system as defined in claim 2, wherein
said current control system is configured so as to output an added
value obtained by addition between a value obtained by increasing a
difference between an AC current flowing through said AC switch and
said multiplied value by constant times and the AC current flowing
through said AC switch as said input current command value, when
said commercial power supply is in said normal condition.
6. An uninterruptible power system as defined in claim 4, wherein
said power failure detection circuit is configured so as to detect
a power failure of said commercial power supply when a voltage
across said capacitor is reduced to a level equal to or below a
predetermined voltage level.
7. An uninterruptible power system comprising: an abnormal voltage
detection circuit for detecting voltage reduction abnormality of a
commercial power supply; an AC switch turned on to permit AC power
to be fed from said commercial power supply to a load when said
commercial power supply is in a normal condition and turned off
when said commercial power supply falls into an abnormal condition;
a storage battery; a power converter of the voltage control type
arranged between said AC switch and said storage battery; said
power converter carrying out power rectifying operation of
converting AC power of said commercial power supply into DC power
to charge said storage battery by said DC power and active filter
operation of flowing a current for compensating a harmonic current
and/or a reactive current flowing to said load when said commercial
power supply is in the normal condition; said power converter
carrying out power inverting operation of inverting DC power into
AC power to feed the AC power to said load mainly using said
storage battery as a power supply therefor when said commercial
power supply is in a condition other than said normal condition; a
control command generation means for outputting a control command
for feeding AC power to said load without a power failure to said
power converter; said control command generation means including a
voltage control system for outputting a voltage control command
which permits said power converter to carry out said power
inverting operation and a current control system for outputting a
current control command which permits said power converter to carry
out said power rectifying operation and active filter operation;
and a reference voltage generating circuit for generating a
reference sinusoidal voltage synchronized with said commercial
power supply; a power failure detection circuit for detecting a
power failure of said commercial power supply; and a DC/DC
converter arranged between said power converter and said storage
battery; said DC/DC converter being configured so as to feed DC
power from said storage battery to said power converter during a
period of time for which said power failure detection circuit
detects a power failure of said commercial power supply.
8. An uninterruptible power system as defined in claim 7, wherein
said power converter includes a capacitor connected in parallel to
said storage battery through said DC/DC converter; and said power
converter feeds said AC current to said load using said capacitor
as a power supply therefor during a period of time of from
detection of a voltage reduction of said commercial power supply by
said abnormal voltage detection circuit to detection of a power
failure of said commercial power supply by said power failure
detection circuit.
Description
BACKGROUND OF THE INVENTION
[0001] This invention relates to an uninterruptible power system,
and more particularly to an uninterruptible power system of the
normally commercial power feed type adapted to carry out electric
supply to a load through an AC switch when a commercial power
supply is in a normal or silent condition and turn off the AC
switch when power stoppage or a power failure occurs in the
commercial power supply, to thereby convert DC power previously
stored in a storage battery into AC power by means of a power
converter, leading to feeding of the AC power to the load,
resulting in ensuring power electric supply without
interruption.
[0002] A conventional control circuit for an uninterruptible power
system is disclosed in Japanese Patent Application Laid-Open
Publication No. 51735/1996, Japanese Patent Application Laid-Open
Publication No. 56087/1997 and Japanese Patent Application
Laid-Open Publication No. 14251/1998. The conventional control
circuit disclosed is so constructed that a command having a current
control command and a voltage control command added to each other
is inputted to a PWM comparator of a power converter with an active
filter function to prepare a gate signal for the power converter.
The conventional control circuit is adapted to output a current
control command to the uninterruptible power system supposing that
a commercial power supply is normal or silent unless a power
failure detection circuit detects a power failure of the commercial
power supply, even when the commercial power supply is actually
reduced in voltage below a predetermined level.
[0003] Thus, the conventional uninterruptible power system is fed
with a current control command supposing that no abnormality occurs
unless power failure of the commercial power supply is detected,
even when any abnormality in voltage such as a voltage reduction
actually occurs in the commercial power supply. This causes strain
at a large magnitude to occur in an output voltage thereof fed to a
load due to a current which intends to flow from the power
converter toward the commercial power supply.
[0004] Also, the conventional uninterruptible power system often
includes a DC/DC converter arranged between the power converter and
a storage battery. The DC/DC converter acts to frequently operate
when any abnormality in voltage such as a voltage reduction occurs
in the commercial power supply, to thereby feed DC power from the
storage battery to the power converter. Unfortunately, this causes
the storage battery to be deteriorated in durability or operating
life.
SUMMARY OF THE INVENTION
[0005] The present invention has been made in view of the foregoing
disadvantage of the prior art.
[0006] Accordingly, it is an object of the present invention to
provide an uninterruptible power system which is capable of
significantly reducing strain in an output voltage thereof.
[0007] It is another object of the present invention to provide an
uninterruptible power system which is capable of enhancing
durability of a storage battery or an operating life thereof.
[0008] In accordance with the present invention, an uninterruptible
power system is provided. The uninterruptible power system includes
an abnormal voltage detection circuit for detecting voltage
reduction abnormality of a commercial power supply, an AC switch
turned on to permit AC power to be fed from the commercial power
supply to a load when the commercial power supply is in a normal
condition and turned off when the commercial power supply falls
into an abnormal condition, a storage battery, and a power
converter of the voltage control type arranged between the AC
switch and the storage battery. The power converter carries out
power rectifying operation of converting AC power of the commercial
power supply into DC power to charge the storage battery by the DC
power and active filter operation of flowing a current for
compensating a harmonic current and/or a reactive current flowing
to the load when the commercial power supply is in a normal
condition. The power converter carries out power inverting
operation of inverting DC power into AC power to feed the AC power
to the load mainly using the storage battery as a power supply
therefor when the commercial power supply is in a condition other
than the silent condition. The uninterruptible power system further
includes a control command generation means for outputting a
control command for feeding AC power to the load without a power
failure to the power converter. The control command generation
means includes a voltage control system for outputting a voltage
control command which permits the power converter to carry out the
power inverting operation and a current control system for
outputting a current control command which permits the power
converter to carry out the power rectifying operation and active
filter operation. The uninterruptible power system further includes
a reference voltage generating circuit for generating a reference
sinusoidal voltage synchronized with the commercial power
supply.
[0009] In the present invention generally constructed as described
above, the control command generation means is configured so as to
output the current control command and voltage control command to
the power converter while adding them to each other. The current
control system is constructed so as to provide an input current
command value on the basis of a multiplied value obtained by
multiplication between a differential voltage between a DC command
voltage commanded by a DC voltage command and a DC-side voltage of
the power converter and an output of the reference voltage
generating forming circuit, and to thereby output the current
control command on the basis of a value obtained by subtracting a
load current value from the input current command value. The
current control system is constructed so as to use an AC current
flowing through the AC switch or a value corresponding to the AC
current as the input current command value when the abnormal
voltage detection circuit carries out detection of voltage
reduction abnormality in the commercial power supply. The value
obtained by subtracting the load current flowing to the load from
the input current command value acts as a command value indicating
a current flowing through the power converter. A value obtained by
subtracting a value of a current flowing to an AC-side of the power
converter from the command value acts as a corrected value of a
current command of the power converter or a current control
command. The term "value corresponding to AC current" as used
herein is intended to mean a value proportional to the AC current
in predetermined relationship. When voltage reduction abnormality
occurs in the commercial power supply, the AC current flowing
through the AC switch is gradually decreased toward zero with the
voltage reduction. Thus, use of the AC current or the value
corresponding to the AC current as the input current command value
permits a reduction in current control command with the voltage
reduction, to thereby reduce strain in the output voltage as
compared with the prior art.
[0010] The current control system may be configured so as to output
an added value obtained by addition between a value obtained by
increasing a difference between an AC current flowing through the
AC switch and the multiplied value by constant times and the AC
current flowing through the AC switch as the input current command
value. This permits the output to further approach a sinusoidal
waveform.
[0011] When PQ operation is used in the current control system, the
current control system may be configured so as to provide a
momentary load-side effective power value corresponding to
momentary effective power fed to the load on the basis of an output
of the reference voltage forming circuit and the load current value
and provide a momentary input-side effective power command value
which permits an input current to have a sinusoidal waveform by
removing an AC component from the momentary load-side effective
power command value, to thereby output the current control command
on the basis of a value obtained by subtracting the momentary
load-side effective power value from the momentary input-side
effective power command value. Also, the current control system,
when the abnormal voltage detection circuit detects voltage
reduction abnormality in the commercial power supply, may be
configured so as to use a momentary effective power value obtained
from an output of the reference voltage generating circuit and an
AC current flowing through the AC switch as the momentary
input-side effective power command value. In the PQ operation as
well, use of the momentary effective power value obtained from the
output of the reference voltage forming circuit and the AC current
flowing through the AC switch as the momentary input-side effective
power command value likewise leads to a reduction in current
control command with the voltage reduction, to thereby reduce
strain in the output voltage as compared with the prior art.
[0012] The uninterruptible power system of the present invention
may further include a power failure detection circuit for detecting
a power failure of the commercial power supply and a DC/DC
converter arranged between the power converter and the storage
battery. The DC/DC converter may be configured so as to feed DC
power from the storage battery to the power converter during a
period of time for which the power failure detection circuit
detects a power failure of the commercial power supply. Such
configuration eliminates waste discharge of the storage battery, to
thereby enhance durability thereof. The DC/DC converter may include
a capacitor connected in parallel to the storage battery. In this
instance, the power converter may feed an AC current to the load
using the capacitor as a power supply therefor during a period of
time extending from detection of a voltage reduction of the
commercial power supply by the abnormal voltage detection circuit
to detection of a power failure of the commercial power supply by
the power failure detection circuit. This permits discharge of the
capacitor rather than the storage battery to feed AC power to the
load. In general, repeating of charging and discharge of a
capacitor does not cause a rapid deterioration in life thereof as
compared with a deterioration in life of a storage battery due to
frequent discharge thereof. The power failure detection circuit may
be constructed in any desired manner. For example, a power failure
of the commercial power supply may be judged to occur when a period
of time for which an average of an output of the commercial power
supply is equal to or lower than a predetermined level is equal to
or exceeds a predetermined period of time. In this instance, the
predetermined period of time may be set to be shorter than a period
of time required for a voltage of the capacitor to be reduced to a
predetermined level or below. Alternatively, the power failure
detection circuit may be configured so as to detect a power failure
of the commercial power supply when a voltage across the capacitor
is reduced to a level equal to or below a predetermined voltage
level. This permits smooth changing-over from discharge of the
capacitor to that of the storage battery without causing
over-discharge of the capacitor. Such construction or technical
idea may be solely employed.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] These and other objects and many of the attendant advantage
of the present invention will be readily appreciated as the same
becomes better understood by reference to the following detailed
description when considered in connection with the accompanying
drawings; wherein:
[0014] FIG. 1 is a block circuit diagram generally showing an
embodiment of an uninterruptible power system according to the
present invention;
[0015] FIG. 2 is a block diagram showing a current control system
by way of example;
[0016] FIG. 3(A) is a circuit diagram showing operation of
turning-off an AC switch when a power failure occurs in a
commercial power supply;
[0017] FIG. 3(B) is a waveform diagram showing operation of
turning-off an AC switch when a power failure occurs in a
commercial power supply;
[0018] FIG. 4(A) is a graphical representation showing an input
voltage and an input current in the uninterruptible power system of
FIG. 1 when an instantaneous or momentary voltage reduction of a
half cycle or less occurs in a U phase of an input of a commercial
power supply AC;
[0019] FIG. 4(B) is a graphical representation showing an output
voltage and an output current to a load in the uninterruptible
power system of FIG. 1 when a momentary voltage reduction of a half
cycle or less occurs in a U phase of an input of a commercial power
supply AC;
[0020] FIG. 4(C) is a graphical representation showing an DC
voltage of a power converter in the uninterruptible power system of
FIG. 1 when a momentary voltage reduction of a half cycle or less
occurs in a U phase of an input of a commercial power supply
AC;
[0021] FIG. 5(A) is a graphical representation showing an input
voltage and an input current in the uninterruptible power system of
FIG. 1 when a short-circuit occurs in a U phase of an input of a
commercial power supply AC;
[0022] FIG. 5(B) is a graphical representation showing an output
voltage and an output current to a load in the uninterruptible
power system of FIG. 1 when a short-circuit occurs in a U phase of
an input of a commercial power supply AC;
[0023] FIG. 5(C) is a graphical representation showing a DC voltage
of a power converter in the uninterruptible power system of FIG. 1
when a short-circuit occurs in a U phase of an input of a
commercial power supply AC;
[0024] FIG. 6 is a block diagram showing a variation of the current
control system shown in FIG. 2; and
[0025] FIG. 7 is a block diagram showing another embodiment of an
uninterruptible power system according to the present
invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0026] Now, an uninterruptible power system according to the
present invention will be described with reference to the
accompanying drawings.
[0027] Referring first to FIG. 1, an embodiment of an
uninterruptible power system according to the present invention is
illustrated. In the illustrated embodiment, a commercial power
supply designated at reference character AC is a three-phase AC
power supply. However, in FIG. 1, only a connection line for a
single phase or one phase and a control system are merely shown for
the sake of brevity. In FIG. 1, an AC switch SW, a power converter
1 of the voltage control type, a DC/DC converter 2 and a storage
battery 3 may be essentially constructed in a conventional manner.
The AC switch SW arranged between the commercial power supply AC
and the power converter 1 corresponds to one phase and includes two
thyristors Th1 and Th2 connected in reverse-parallel to each other.
A circuit for the AC switch SW has a capacitor C0 arranged between
an input portion thereof and a neutral point thereof. When the
commercial power supply AC is in a normal or silent condition, AC
power is fed from the commercial power supply AC through the switch
SW to a load L. Thus, when the commercial power supply AC is in a
normal condition, the AC switch SW is turned on to permit AC power
to be fed from the commercial power supply AC to the load L
therethrough. When the commercial power supply AC falls into a full
power failure condition, the AC switch SW is turned off to
interrupt feeding of AC power from the commercial power supply AC
to the load L. In the illustrated embodiment, any specific forced
turn-off circuit is not provided for the thyristors Th1 and Th2 of
the AC switch SW. In the illustrated embodiment, when any
abnormality occurs in the commercial power supply AC, a current fed
thereto from the power converter 1 permits turn-off of the
thyristors Th1 and Th2 to be attained, as described below.
[0028] Also, the power converter 1 of the voltage control type,
when the commercial power supply AC is in a normal or silent
condition, functions to convert AC power of the commercial power
supply AC into DC power, to thereby carry out power rectifying
operation of charging the storage battery 3 through the DC/DC
converter 2 and active filter operation of flowing a current for
compensating a harmonic current and/or a reactive current flowing
to the load L. Also, the power converter 1 carries out power
inverting operation of converting DC power into AC power to feed
the load L with the AC power using the storage battery 3 as a power
supply therefor mainly during a period of time for which a power
failure detection circuit 15 detects a power failure of the
commercial power supply AC. The power converter 1 includes a
capacitor C1 arranged between an AC terminal thereof and a neutral
point thereof, a reactor L arranged in series between the AC
terminal thereof and an AC terminal of a converter for power
conversion designated at reference character PCV, and a capacitor
C2 arranged between a DC terminal of the power conversion converter
PCV and the neutral point. It may be considered that the capacitor
C2 is connected in series to the storage battery 3 through the
DC/DC converter 2. Configuration of such a power converter of the
voltage control type and operation thereof are known in the art, as
described in detail in Japanese Patent Application Laid-Open
Publication No. 56087/1997 or the like.
[0029] The AC switch SW is provided on an AC line thereof
positioned on an input side thereof with an instrument transformer
T1 for detecting an AC voltage fed thereto from the commercial
power supply AC. Also, the power converter 1 is provided on an AC
line thereof positioned on a side of the AC terminal thereof with
an instrument transformer T2 for detecting an AC voltage across the
AC line. Further, the power converter 1 is provided on a DC line
thereof on a side of a DC terminal thereof with an instrument
transformer T3 for detecting a DC voltage across the DC line. In
addition, the AC line for feeding AC power to the load L is
provided thereon with a current transformer CT1 for measuring a
load current. Furthermore, the AC switch SW is provided on an AC
line thereof on a load side thereof with a current transformer CT2
for measuring an AC current. Moreover, a current transformer CT3 is
arranged between a connection between the capacitor C1 arranged in
the power converter 1 and the reactor L and the reactor L so as to
measure an AC current flowing through the power converter 1.
[0030] Reference numeral 10 designates a control command generation
means 10, which includes a current control system 11, a voltage
control system 12, a reference voltage generating circuit 13 for
outputting a reference sinusoidal wave in synchronism with an AC
voltage outputted from the commercial power supply AC, an abnormal
voltage detection circuit 14 for detecting voltage reduction
abnormality of the commercial power supply AC, and a power failure
detection circuit 15 for detecting a power failure of the
commercial power supply AC. The abnormal voltage detection circuit
14 is constructed so as to prepare two kinds of reference AC
voltage signals different in offset voltage from each other to form
a predetermined band width, resulting in outputting a voltage
abnormality detection signal indicating that voltage reduction
abnormality occurs in an output voltage of the commercial power
supply AC, only during a period of time for which an AC voltage Vin
generated from the commercial power supply AC is above or out of
the band width between two reference AC voltage signals. The
abnormal voltage detection circuit 14 exhibiting such a function
may be constructed in any suitable conventional manner. The power
failure detection circuit 15 is constructed so as to rectify an
output of the commercial power supply AC to obtain an average
voltage level, and to judge that power failure occurs in the
commercial power supply AC, resulting in outputting a power failure
detection signal, when a period of time during which the average
voltage level is kept equal to or lower than a predetermined power
failure judgement reference voltage level is equal to or longer
than a predetermined period of time. In FIG. 1, reference numeral
16 designates a PWM circuit for outputting a PWM control signal for
subjecting the power converter 1 of the voltage control type to PWM
control.
[0031] The current control system 11 is constituted by a DC voltage
control circuit 11A and a current control circuit 11B. The current
control system 11 may be constructed in such a manner as shown in
FIG. 2 by way of example. More particularly, the DC voltage control
circuit 11A acts to obtain a differential voltage between a DC
command voltage Vdc commanded by a DC voltage command optionally
externally fed thereto and a DC-side voltage Vdc' of the power
converter 1 detected by the meter transformer T3, to thereby
increase the differential voltage by constant times (k.sub.dc). The
current control system shown in FIG. 2 includes a switch circuit
SW2 kept turned off during a period of time for which the power
failure detection circuit 15 detects a power failure of the
commercial power supply AC. In the illustrated embodiment,
arrangement of the switch circuit SW2 is not necessarily required.
When the switch circuit SW2 is not arranged, it is not required to
input an output of the power failure detection circuit 15 to the DC
voltage control circuit 11A.
[0032] The current control circuit 11B functions to obtain an input
current command value Io corresponding to an input current of the
power converter 1 on the basis of a multiplied value between an
output of the DC voltage control circuit 11A (the above-described
differential voltage value increased by the constant times) and an
output of the reference voltage generating circuit 13. Also, the
current control circuit 11B, when the commercial power supply AC is
in a normal or silent condition, functions so that a value
(Io-Iout-Icnv) obtained by subtracting both a load current Iout
flowing to the load L which is detected by the current transformer
CT1 and a current Icnv flowing to the power converter 1 from the
AC-side which is detected by a current transformer CT3 from the
input current command value Io may be outputted in the form of a
current control command. In order to control the power converter 1
of the voltage control type, the PWM circuit 16 is fed with a
control command obtained by adding the current control command to a
voltage control command outputted from a voltage control circuit 12
described below. When a voltage of the commercial power supply AC
is at a normal or silent level, the voltage control signal is
determined to be smaller than the current control signal. Thus, the
power converter 1 carries out both power rectifying operation and
active filter operation.
[0033] When the abnormal voltage detection circuit 14 detects that
voltage reduction abnormality occurs in a voltage of the commercial
power supply AC, to thereby generate a voltage abnormality
detection signal therefrom, the switch circuit SW1 is kept changed
over during a period of time for which the detection signal is
outputted, so that an AC current flowing through the AC switch SW
or an input current Iin detected by the current transformer CT2 is
substituted for the input current command value Io and acts as a
substituted input current command value. Thus, when voltage
reduction abnormality occurs in the commercial power supply AC, the
value Iin-Iout-Icnv may be outputted as the current control
command. This permits the AC current Iin flowing through the AC
switch SW to be fed as the input current command value even when
the voltage reduction abnormality occurs in the commercial power
supply, so that the current control command may be gradually
reduced with the voltage reduction. More particularly, the current
control command at this time corresponds to a current value fed
from the power converter 1 to the load L. This results in an output
of the power converter 1 being varied depending on a reduction in
AC current Iin with the voltage reduction so as to compensate it.
Thus, the illustrated embodiment reduces strain in the voltage
occurring when the voltage reduction abnormality occurs in the
commercial power supply AC, as compared with the prior art. Then,
when the AC current Iin is zero, the thyristors are turned off.
Thereafter, the PWM circuit 16 of the power converter 1 is fed with
only the voltage control command, so that the power converter 1
starts power inverting operation. At this time, the commercial
power supply AC does not yet fall into a power failure condition,
so that the capacitor C2 arranged in the power converter 1 acts as
a power supply therefor to carry out discharge, to thereby feed AC
power to the load. This causes the current Icnv flowing through the
power converter 1 to flow in an opposite direction, however, the
load current Iout is kept unvaried.
[0034] When the commercial power supply AC falls into a power
failure condition beyond the voltage reduction, an AC current fed
through the AC switch SW to the load L is rendered zero, resulting
in the current control command being naturally zero, during which
the power converter 1 acts as a power inverter using the capacitor
C2 as a power supply therefor. When the power failure detection
circuit 15 detects such a power failure of the commercial power
supply AC, it feeds a switch circuit SW2 with its output, so that
the switch circuit SW2 is turned off. Thus, even when the abnormal
voltage detection circuit 14 carries out malfunction or false
detection, the current control command is rendered zero.
[0035] When the power failure detection circuit 15 detects that the
commercial power supply falls into a power failure condition, the
DC/DC converter 2 starts conversion operation for discharging the
storage battery 3. The DC/DC converter 2 carries out boosting
operation so as to feed the power converter 1 with a constant DC
voltage based on a DC voltage accumulated in the storage battery 3
during a period of time for which it has a power failure detection
signal inputted thereto from the power failure detection circuit
15. The DC/DC converter 2 carries out charging operation of
charging the storage battery 3 to a predetermined charging voltage
level but does not carry out discharge operation of discharging the
storage battery, unless it is fed with the power failure detection
signal from the power failure detection circuit 15. This results in
the power converter 1 subsequently carrying out inverting operation
using the storage battery as a power supply therefor, so that
electric supply to the load L may be attained without a power
failure. The power failure detection circuit 15 is constructed so
as to detect a power failure of the commercial power supply AC
before falling of the capacitor C2 into an over-discharge condition
and more particularly within a period as short as a lapse of a half
cycle.
[0036] The voltage control system 12 is fed with a reference
sinusoidal voltage signal outputted from the reference voltage
generating circuit 13 and an AC voltage detected by the meter
transformer T2 for detecting an AC voltage on the AC terminal side
of the power converter 1, to thereby output a voltage control
command which permits the power converter 1 to carry out power
inverting operation of inverting DC power to AC power. The voltage
control command outputted from the voltage control system 12 is
rendered smaller than the above-described current control command
when the commercial power supply AC is in a silent or normal
condition, so that the power converter 1 may be kept from carrying
out power inverting operation by the voltage control command. In
the course of falling of the commercial power supply AC into
voltage reduction abnormality, which is detected by the abnormal
voltage detection circuit 14, the power converter 1 positively
starts the power inverting operation when the voltage control
command is larger than the current control command.
[0037] The reference voltage generating circuit 13 functions to
generate or prepare a reference sinusoidal voltage signal of which
a phase coincides with an AC voltage outputted from the commercial
power supply AC on the basis of phase information on an AC voltage
of the commercial power supply taken in when the commercial power
supply AC is in a silent condition. Then, the reference voltage
generating circuit 13 outputs the reference sinusoidal voltage
signal to the current control system 11 and voltage control system
12.
[0038] In the illustrated embodiment, when the power failure
detection circuit 15 detects a power failure (including one open
phase) of the commercial power supply AC, the power failure
detection circuit 15 outputs a power failure detection signal to
the current control system 11 and DC/DC converter 2 during a period
of time for which it detects the power failure.
[0039] In the illustrated embodiment, the control command
generation means 10 is constructed so as to add the voltage control
command outputted from the voltage control system 12 and the
current control command outputted from the current control system
11 to each other at an addition point SP1 and output the thus-added
commands to the PWM circuit 16. Such construction of the control
command generation means 10 permits the control command generation
means 10 to output the voltage control command and current control
command to the power converter 1 while carrying out instantaneous
changing-over between both control commands, without using any
changing-over switch.
[0040] Now, operation of turning off the AC switch SW when a power
failure occurs in the commercial power supply AC will be described
with reference to FIGS. 3(A) and 3(B).
[0041] First, the power converter 1 may be regarded as a voltage
source because of being the voltage control type. Thus, it may be
considered that the commercial power supply AC and power converter
1 carry out parallel operation or running through the reactor L
(FIG. 1) and the thyristors Th1 and Th2 of the AC switch SW, when
they are seen from a side of the load. The power converter 1, when
the commercial power supply AC is in a normal or silent condition,
acts as an active filter, so that an input current i of a
sinusoidal waveform may be fed to the power converter 1 as shown in
FIGS. 3(A) and 3(B). Then, when a voltage reduction such as an
instantaneous voltage reduction or the like occurs in the
commercial power supply AC to cause a voltage difference to be
generated between an output voltage Vs of the commercial power
supply AC and an output voltage Vc thereof obtained when the power
converter 1 is regarded as a voltage source therefor as shown in
FIG. 3(B), it intends to flow an AC current i.sub.cc from the power
converter 1 through the AC switch SW toward the commercial power
supply AC. In the illustrated embodiment, the voltage difference
which intends to flow the AC current i.sub.cc to the power supply
is applied in the form of a reverse bias to the thyristors Th1 and
Th2, so that the thyristors Th1 and Th2 may be rapidly turned off.
In the illustrated embodiment, the AC current i.sub.cc does not
intend to flow to the commercial power supply AC unless a power
failure or an instantaneous voltage reduction occurs in the
commercial power supply AC. In other words, the thyristors Th1 and
Th2 are subjected to turning-off depending on a reduction in
voltage in the AC power supply AC, resulting in malfunction thereof
due to noise being effectively prevented.
[0042] When an instantaneous or momentary voltage reduction of a
half cycle or less such as a partial lack or deficiency of a
voltage waveform occurs in the commercial power supply AC, the
thyristors Th1 and Th2 are subjected to turning-off, resulting in a
DC voltage on the DC terminal side of the power converter 1 being
reduced. However, when the voltage waveform is returned to the
original state after the next half cycle, the AC switch SW is
turned on, so that the commercial power supply may be returned to a
normal running state.
[0043] The illustrated embodiment will be further described with
reference to FIGS. 4(A) to 4(C), wherein FIG. 4(A) is a graphical
representation showing an input voltage and an input current in the
uninterruptible power system of the illustrated embodiment when an
instantaneous voltage reduction of a half cycle or less occurs in a
U phase of an input of the commercial power supply AC, FIG. 4(B)
shows an output voltage and an output current to the load in the
uninterruptible power system when the instantaneous or momentary
voltage reduction occurs in the U phase, and FIG. 4(C) shows an AC
voltage of the power converter in the uninterruptible power system
when the instantaneous voltage reduction occurs in the U phase.
FIGS. 4(A) to 4(C) indicate that the illustrated embodiment attains
smooth changing-over between active filer operation and so-called
inverter operation or changing-over between control by the current
control command and that by the voltage control command, even when
a voltage reduction occurs for a relatively short period of time,
resulting in the changing-over being accomplished without any
instantaneous interruption. In the illustrated embodiment, the
voltage reduction is a half cycle or less and at a low level, so
that feeding of AC power to the load L may be carried out by only
discharge of the capacitor C2 without discharge of the storage
battery 3 while being free from any power failure.
[0044] Also, the illustrated embodiment will be described with
reference to FIGS. 5(A) to 5(C), wherein FIG. 5(A) shows an input
voltage and an input current in the uninterruptible power system of
the illustrated embodiment when a short-circuit occurs in a U phase
of an input of the commercial power supply AC, FIG. 5(B) shows an
output voltage and an output current to the load in the
uninterruptible power system when a short-circuit occurs in the U
phase, and FIG. 5(C) shows a DC voltage Vdc of the power converter
1 when a short-circuit occurs in the U phase. When the
short-circuit occurs in the U-phase, an input current of the U
phase is abruptly rendered zero, so that the thyristors of the AC
switch are turned off. However, changing-over from the current
control command to the voltage control command is smoothly carried
out in the control command generation means 10. This permits the
changing-over to be carried out without any instantaneous or
momentary interruption, to thereby prevent the load from being
adversely affected, although an output voltage of the U phase is
somewhat reduced. The thyristors are not rendered turned off in the
remaining phase, so that the current continues to flow during the
maximum of a half-wave period. However, a transient state is
considered in the current and voltage control systems, to thereby
prevent strain from occurring in the output voltage. Likewise, the
input current is gradually increased during recovery of the power
failure, however, the load is prevented from being adversely
affected. The DC voltage is gradually reduced after generation of a
power failure in the commercial power supply and then gradually
increased. The power failure detection circuit 15 judges the power
failure at a changing-over point at which changing-over from the
reduction to the increase is carried out, so that the DC/DC
converter 2 starts feeding of DC power from the storage battery 3
to the power converter 1. This permits the power converter 1 to act
as an inverter using the storage battery 3 as a power supply
therefor, to thereby feed the load with AC power at a U phase.
[0045] Thus, it will be noted that the uninterruptible power system
of the regularly commercial feed type of the illustrated embodiment
permits changing-over of the output voltage from a commercial feed
mode to a feed mode from the storage battery at the time of power
failure of the commercial power supply to be accomplished without
any instantaneous or momentary interruption, resulting in
accommodating to any load.
[0046] Referring now to FIG. 6, a modification of the control
command generation means 10 is illustrated. In the modification, a
current control system 11 includes a current control circuit 11B,
which is constructed so as to output an added value as the input
current command value when the commercial power supply is in a
silent condition. The added value is obtained by adding a value
obtained by increasing a difference between an AC current I.sub.in
flowing through an AC switch SW and a multiplied value Io by
constant times (k2) and the AC current I.sub.in to each other. In
the modification, the switch SW1 is turned off or rendered open
when the abnormal voltage detection circuit 14 detects voltage
reduction abnormality of the commercial power supply. Thus, when a
power voltage falls into a voltage reduction abnormality condition,
the AC current I.sub.in flowing through the AC switch SW is used as
a current command value corresponding to an input current as in the
current control system 11 shown in FIG. 2. Such construction
permits an output to a load to further approach a sinusoidal
wave.
[0047] In the illustrated embodiment, the power failure detection
circuit 15 is configured so as to detect an average voltage,
leading to detection of a power failure of the commercial power
supply. Alternatively, it may be constructed in any other suitable
manner. For example, the power failure detection circuit may be
configured so as to measure a voltage across the capacitor C2 of
the power converter 1, to thereby judge that a power failure of the
commercial power supply occurs when the voltage measured is below a
predetermined level.
[0048] Referring now to FIG. 7, another embodiment of an
uninterruptible power system according to the present invention is
illustrated, wherein operation in a current control system is
carried out by PQ operation. In the embodiment shown in FIG. 7, a
current control circuit 11B includes first to third 3 phase-2 phase
converters 11C to 11E, first and second pq arithmetic units 11F and
11G, a low-pass filter 11H, a switch circuit 11I, an inverse pq
arithmetic unit 11J, and a 2 phase-3 phase converter 11K. A DC
voltage control circuit 11A is constructed in substantially the
same manner as in the embodiment described above with reference to
FIG. 2.
[0049] In the illustrated embodiment thus constructed, a reference
sinusoidal voltage signal outputted from a reference voltage
forming circuit 13 is converted into a two-phase signal. Also, a
three-phase output of a current transformer CT1 is converted into a
two-phase signal by means of the 3 phase-2 phase converter 11E.
Then, an output of the 3 phase-2 phase converter 11E is inputted to
the first and second pq arithmetic units 11F and 11G. This permits
the first pq arithmetic unit 11F to operate momentary effective
power (momentary load-side effective power) and momentary false
power on a side of a load and the second pq arithmetic unit 11G to
operate momentary effective power and momentary false power on a
side of a commercial power supply AC. The first pq arithmetic unit
11F and second pq arithmetic unit 11G include output terminals p1
and p2, respectively, through each of which momentary effective
power is outputted from each of the arithmetic units 11F and 11G.
Momentary load-side effective power outputted from the output
terminal p1 of the first pq arithmetic unit 11F is inputted to the
low-pass filter 11H, in which a DC component is removed from the
power. The momentary load-side effective power is then fed to one
connection terminal (or a "positive" terminal in FIG. 7) of the
switch circuit 11I in the form of a momentary input-side effective
power command value which permits an input current to have a
sinusoidal wave. Momentary effective power on a power supply side
outputted through the output terminal p2 of the second pq
arithmetic unit 11G is fed to the other connection terminal (or a
"different" terminal in FIG. 7) of the switch circuit 11I. The
switch circuit 11I, when an abnormal voltage detection circuit 14
detects voltage reduction abnormality of the commercial power
supply AC to output an abnormal voltage detection signal, changes
over a switch contact from a "positive" terminal position to a
"different" terminal position.
[0050] When a voltage of the commercial power supply AC is normal,
the switch contact is kept at the "positive" terminal position. At
this time, the low-pass filter 11H removes a DC component from the
momentary load-side effective power, so that the momentary
load-side effective power from which the DC component is removed is
inputted as a momentary input-side effective power command value to
a subtraction point SP4, at which the momentary load-side effective
power is subtracted from the momentary input-side effective power
command value, so that a current control command may be formed or
prepared on the basis of the momentary input-side effective power
command value thus subtracted.
[0051] When voltage reduction abnormality occurs in the commercial
power supply AC, the switch contact of the switch circuit 11I is
changed over from the "positive" terminal position to the
"different" terminal position, so that momentary effective power on
a power supply side outputted through an output terminal p2 of the
second arithmetic unit 11G or a momentary effective power value
obtained by an output of the reference voltage forming circuit 13
and an AC current flowing through the AC switch SW is inputted as
an input current command value to the subtraction point SP4 through
the switch circuit 11I. Thus, when voltage reduction abnormality
occurs in the commercial power supply AC, use of the momentary
effective power on the power supply side reduced with the voltage
reduction as the momentary input-side effective power permits a
reduction in current control command with the voltage reduction, to
thereby minimize strain in the output voltage as in the embodiment
described above with reference to FIG. 1.
[0052] An output at the subtraction point SP4 is inputted to an
addition point SP5, to which an output of the switch circuit SW2 as
well is inputted. The switch circuit SW2 is fed with a signal
obtained by increasing a difference between a DC command voltage
Vref obtained at a subtraction point SP7 and a DC-side voltage Vdc'
by k.sub.dc times. The thus-obtained signal, when a power failure
detection circuit 15 detects a power failure of the commercial
power supply to turn off the switch circuit SW2, fails to be fed to
the subtraction point SP5. This results in malfunction of the
device being prevented.
[0053] The switch circuit SW2 is kept turned on unless a power
failure occurs in the commercial power supply, so that a sum of a
difference of the momentary effective power and power
k.sub.dc.multidot.(Vdc-Vdc') required on the DC side added to each
other at the addition point SP5 is inputted to an input terminal px
of the inverse pq arithmetic unit 11J. This permits charging
operation of the storage battery 3. Then, when occurrence of a
power failure in the commercial power supply is detected, the
switch circuit SW2 is turned off, so that only a difference between
the load-side momentary effective power value and the momentary
input-side effective power command value may be inputted to the
input terminal px of the inverse pq arithmetic unit 11J. This
results in charging operation of the storage battery 3 being
interrupted.
[0054] Also, an added part of momentary false power outputted
through an addition point SP6 passes through a transfer element of
-1, resulting in being inputted as a value obtained by subtracting
the momentary false power from zero (0) to an input terminal qx of
the inverse pq arithmetic unit 11J. The inverse pq arithmetic unit
11J has both momentary effective power to be compensated and
momentary false power to be compensated inputted thereto, to
thereby carry out inverse pq operation, resulting in outputting
results of the operation to the 2 phase-3 phase converter 11K.
Then, the 2 phase-3 phase converter 11K outputs a command value of
a harmonic current to be compensated in a power converter 1. A
subtraction point SP8 has the command value of the harmonic current
and a value of a current actually flowing to an AC terminal of a
power converter 1 inputted thereto. Then, a difference between the
command value of the harmonic current and the current value is
increased by k.sub.c times and then inputted to the addition point
SP1, at which addition between the difference increased by k.sub.c
times and a voltage control command is carried out. During a period
of time for which the power failure detection circuit 15 detects a
power failure of the commercial power supply, a combination or sum
of the voltage control command and a current control command
generated depending on the voltage control command is outputted
from the addition point SP1. During a period of time for which the
power failure detection circuit 15 does not detect the power
failure, the current control command is predominantly outputted
through the addition point SP1 to a PWM control circuit 16 of the
power converter 1. At this time, the voltage control command is
highly reduced as compared with the current control command.
[0055] As can be seen form the foregoing, the uninterruptible power
system of the present invention is so constructed that the current
control system uses an AC current flowing through the AC switch as
the input current command value during a period of time for which
the abnormal voltage detection circuit detects voltage reduction
abnormality of the commercial power supply. This permits the
current control command to be reduced with the voltage reduction
when voltage reduction abnormality occurs in the commercial power
supply, leading to a reduction in strain in the output voltage at
the time of the voltage reduction, as compared with the prior
art.
[0056] Also, in the present invention, the DC/DC converter
functions to feed DC power from the storage battery to the power
converter, to thereby minimize the number of times of discharge of
the storage battery, resulting in preventing a deterioration in
life or durability of the storage battery.
[0057] While preferred embodiments of the invention have been
described with a certain degree of particularity with reference to
the drawings, obvious modifications and variations are possible in
light of the above teachings. It is therefore to be understood that
within the scope of the appended claims, the invention may be
practiced otherwise than as specifically described.
* * * * *