Electric power conversion systems – Current conversion – Including automatic or integral protection means
Reexamination Certificate
1996-03-21
2001-12-04
Wong, Peter S. (Department: 2111)
Electric power conversion systems
Current conversion
Including automatic or integral protection means
C363S072000, C323S207000, C323S217000, C323S246000, C307S103000, C307S105000, C307S064000
Reexamination Certificate
active
06327162
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention relates generally to electrical distribution switchgear, and, more particularly, to a voltage regulator for buffering loads from momentary voltage dips on utility grids.
BACKGROUND OF THE INVENTION
In the past, electric power consumers ignored service interruptions of less than three seconds. With the increasing use of digital computer equipment electric power consumers are becoming less tolerant of short term voltage dips and sags. These disturbances cause flashing clock displays, resets to VCRs and personal computers, and lost production on automated assembly lines and businesses that rely on computer networks. Voltage sags and voltage dips are two closely related problems. Voltage sag is a partial reduction in the magnitude of voltage and voltage dip describes a situation wherein the magnitude of the voltage phasor drops significantly, often to zero. Voltage sags often persist for extended periods and are usually related to system loading conditions, while voltage dips generally occur for brief durations and are caused by faults on the power system. In either case, it is desirable to support the voltage to prevent service interruption and inconvenience.
One source of complaints regarding power quality are single phase disturbances on the transmission and distribution network. Most utility distribution system disturbances are single phase events. These disturbances may have relatively long durations because ground faults can require several seconds to clear on some distribution feeders. Balanced three phase disturbances are generally less common. Major three phase voltage dips are primarily due to closing into grounded equipment, but generally last only a few cycles. Less severe voltage sags occur during transient conditions following energization of large loads or during recovery from a major network disturbance. The latter events can last for a second or more. Uninterruptible power supply is a good technical solution it is expensive especially when a large load area is to be served. Accordingly it will be appreciated that it would be highly desirable to have a relatively inexpensive voltage regulator that boosts voltage during voltage dips and sags and supports the load during severe unbalanced voltage disturbances.
SUMMARY OF THE INVENTION
The present invention is directed to overcoming one or more of the problems set forth above. Briefly summarized, according to one aspect of the invention, a static series voltage regulator for an electrical distribution system comprises a voltage source inverter having a dc input and an ac output; a source bridge, fed from a power source, supplying the dc input of the inverter; a series transformer connected between the power source and a load coupling the inverter output to appear between the power source and the load; and control means for controlling operation of the inverter so that during normal operation the inverter acts as a short on the series transformer. During a power supply fault that causes a dip in the source voltage, the inverter output is injected in series with the source voltage to provide a boost action to maintain load voltage at a desired magnitude and balance.
A relatively simple 3-phase voltage source inverter is used instead of separate inverters for each phase. A second inverter and series transformer may be connected in series and coordinated to synchronize their sampling and computation cycles so that the effective pulse width modulation (PWM) carrier frequency is doubled for more efficient boost action.
According to another aspect of the invention, a method for regulating voltage applied to a load from a power distribution transformer by providing a boost action from a static series voltage regulator includes measuring a voltage V
T
at an output of the power transformer: measuring a load current I
LMEAS
drawn by the load; constructing what load voltage V
LX
would be, using V
T
and I
LMEAS
, in absence of a boost action (V
LX
compensating for impedance drop of the static series voltage regulator itself); processing the load voltage V
LX
through a minimum dip detector and obtaining a signal V
LXM
that rapidly responds to a voltage dip while remembering a dip happened for a predetermined period of time; and beginning a boost action when V
LXM
falls below a preselected threshold voltage and bringing the load voltage V
LX
to a desired level V
LORDLER
.
The static series voltage regulator senses the incoming voltage from the utility and load current. From these is constructed what the load voltage would be in the absence of boost action. This information is processed through a minimum detector to obtain a signal V
LXM
which rapidly responds to a voltage dip but remembers a dip happened for a few cycles. When V
LXM
falls below some threshold, then boosting begins to bring load voltage to a desired level. Boosting is halted after V
LXM
exceeds the threshold, or when the load voltage exceeds a high voltage threshold.
These and other aspects, objects, features and advantages of the present invention will be more clearly understood and appreciated from a review of the following detailed description of the preferred embodiments and appended claims, and by reference to the accompanying drawings.
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patent: 5323330 (1994-06-01), Asplund et al.
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Clark Kara
Larsen Einar V.
Walling Reigh
General Electric Company
Horton Carl B.
Pham Hanh T.
Riley Shawn
Wong Peter S.
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