Electric power conversion systems – Current conversion – Including an a.c.-d.c.-a.c. converter
Reexamination Certificate
2002-06-26
2003-04-29
Vu, Bao Q. (Department: 2838)
Electric power conversion systems
Current conversion
Including an a.c.-d.c.-a.c. converter
C363S098000, C363S056020, C363S058000, 38, 38
Reexamination Certificate
active
06556459
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an uninterruptible power supply which connects a load to utility power supply through an AC switch.
AC switches used in conventional uninterruptible power supplies, as disclosed in Japanese Patent Laid-Open No. 5-292686, employ switching element with a self turn-off function or thyristors that are turned off by a forced turn-off circuit.
Another type of AC switch is also used which does not use a turn-off circuit and is naturally turned off as a current flowing in the thyristor falls below a holding current. The use of the AC switch incorporating only such thyristors can reduce the cost of the uninterruptible power supply. However, when this kind of AC switch is used, a period occurs without fail during which two AC powers are parallelly connected to and fed to a load, delaying the turn-off of the AC switch, as indicated in Japanese Patent Laid-Open Nos. 5-30683, 5-176461 and 5-292686.
As shown in
FIG. 8A
, a conventional thyristor-based AC switch
1
for each phase has antiparallelly connected thyristors SCR
1
and SCR
2
disposed between a utility or commercial power supply
3
and a power converter
5
for each phase. The power converter
5
has a function of converting an AC power into a DC power and vice versa, and is connected to a DC power supply
7
on a DC side thereof. The electric power from the utility power supply
3
is fed through the AC switch
1
to a load
9
, and the output from the power converter
5
is supplied directly to the load
9
.
To initiate a switchover between the two power supplies
3
,
7
, from the power converter
5
side (second feeding path R
2
) to the utility power supply
3
side (first feeding path R
1
), an ON signal is applied to each of the thyristors SCR
1
and SCR
2
making up the AC switch
1
at a predetermined cycle and an OFF signal is given to the power converter
5
. At this time the current flowing through the power converter
5
is immediately interrupted and the AC switch immediately turns on, causing the feeding path to be changed over instantaneously.
To switch the feeding path from the utility power supply side (first feeding path R
1
) to the power converter side (second feeding path R
2
), an ON signal is applied to the power converter
5
and an OFF signal is supplied to the AC switch
1
. However, since the AC switch
1
uses thyristors without the self turn-off function, the current flowing through the AC switch
1
does not fall immediately to zero and thus there is a period when the utility power supply
3
and the power converter
5
are connected parallelly to the load
9
. During this period, depending on the voltage relation between the utility power supply
3
and the power converter
5
, an uncontrollable cross current flows between the power converter
5
and the utility power supply
3
, rendering the voltage applied to the load
9
abnormal. The inexpensive AC switch
1
comprising only the thyristors SCR
1
and SCR
2
that do not use a forced turn-off circuit, therefore, has a problem that it cannot make an instantaneous switchover of the feeding path.
An object of the present invention is to provide an uninterruptible power supply capable of turning off an AC switch made up of thyristors as quickly as possible.
Another object of the present invention is to provide an uninterruptible power supply capable of turning off an AC switch made up of thyristors as quickly as possible by using a power converter.
Still another object of the present invention is to provide an uninterruptible power supply capable of turning off an AC switch made up of thyristors without providing a separate, forced turn-off circuit.
Yet another object of the present invention is to provide an AC switch turn-off method for an uninterruptible power supply capable of turning off an AC switch made up of thyristors without providing a separate, forced turn-off circuit.
SUMMARY OF THE INVENTION
The present invention improves an uninterruptible power supply which comprises: a first feeding path to feed an AC power to a load from a utility power supply through an AC switch made up of a pair of antiparallelly connected thyristors; a second feeding path including a voltage control type power converter, the power converter using a DC power supply as its source and performing an inverter operation of converting a DC power into an AC power and feeding the converted AC power to the load; a reference sinusoidal voltage generation circuit to produce a reference sinusoidal voltage; a control command generation means to, based on the reference sinusoidal voltage, output to the power converter a voltage control command to cause the power converter to perform the inverter operation, wherein, while the voltage control command is being output from the control command generation means, the AC switch is turned off and the feeding of the AC power to the load is switched over from the first feeding path to the second feeding path. The AC switch is arranged for each phase. Thus, for a three-phase AC power, three single-phase AC switches are provided, one for each phase.
In the uninterruptible power supply of the present invention, the reference sinusoidal voltage generation circuit includes: a sinusoidal voltage generation means to generate a synchronous sinusoidal voltage Vref
1
, a high synchronous sinusoidal voltage Vref
2
and a low synchronous sinusoidal voltage Vref
3
, wherein the synchronous sinusoidal voltage Vref
1
is synchronized with the utility power supply and, when used as the reference sinusoidal voltage, causes an output voltage of the power converter to become almost equal to a voltage of the utility power supply, the high synchronous sinusoidal voltage Vref
2
is in phase with the synchronous sinusoidal voltage Vref
1
and higher in amplitude than the synchronous sinusoidal voltage Vref
1
, and the low synchronous sinusoidal voltage Vref
3
is in phase with the synchronous sinusoidal voltage Vref
1
and lower in amplitude than the synchronous sinusoidal voltage Vref
1
; a current detection means to detect a current flowing through the AC switch; and a sinusoidal voltage selection means to select the synchronous sinusoidal voltage Vref
1
, the high synchronous sinusoidal voltage Vref
2
or the low synchronous sinusoidal voltage Vref
3
and output the selected voltage as the reference sinusoidal voltage to the control command generation means, wherein when the voltage control command is not output, the sinusoidal voltage selection means selects the synchronous sinusoidal voltage Vref
1
and wherein when the voltage control command is output, the sinusoidal voltage selection means selects the high synchronous sinusoidal voltage Vref
2
if an output of the current detection means has a positive polarity, selects the low synchronous sinusoidal voltage Vref
3
if the output of the current detection means has a negative polarity, and selects the synchronous sinusoidal voltage Vref
1
if the output of the current detection means is zero.
In the uninterruptible power supply of the present invention, the reference sinusoidal voltage generation circuit includes: a sinusoidal voltage generation means to generate a synchronous sinusoidal voltage Vref
1
, a high synchronous sinusoidal voltage Vref
2
and a low synchronous sinusoidal voltage Vref
3
, wherein the synchronous sinusoidal voltage Vref
1
is synchronized with the utility power supply and, when used as the reference sinusoidal voltage, causes an output voltage of the power converter to become almost equal to a voltage of the utility power supply, the high synchronous sinusoidal voltage Vref
2
is in phase with the synchronous sinusoidal voltage Vref
1
and higher in amplitude than the synchronous sinusoidal voltage Vref
1
, and the low synchronous sinusoidal voltage Vref
3
is in phase with the synchronous sinusoidal voltage Vref
1
and lower in amplitude than the synchronous sinusoidal voltage Vref
1
; a current detection means to detect a current flowing through the AC switch; and a sinusoidal voltage selection means to select t
Okui Yoshiaki
Yanagisawa Minoru
Rankin, Hill Porter & Clark LLP
Sanyo Denki Co. Ltd.
Vu Bao Q.
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