Electrical transmission or interconnection systems – Switching systems – Condition responsive
Reexamination Certificate
2002-03-06
2003-05-06
Riley, Shawn (Department: 2838)
Electrical transmission or interconnection systems
Switching systems
Condition responsive
C361S091700, C323S255000
Reexamination Certificate
active
06559562
ABSTRACT:
TECHNICAL FIELD
The present invention relates generally to the field of electric distribution power systems and, more particularly, relates to a voltage sag and over-voltage compensation device for an AC electric power distribution system employing a pulse width modulated autotransformer.
BACKGROUND OF THE INVENTION
Voltage sags and over-voltage conditions occasionally occur on AC power distribution systems for a variety of reasons, such as high resistance faults in the distribution system, fault clearing, switching large loads characterized by arcing during connection or disconnection of the load, other types of transient circuit overloading (e.g., dynamic disturbances), high load inductance during unusually heaving load periods, and line capacitance during unusually light load periods. Although these voltage conditions may be short lived, such as a few cycles in a, 50 or 60 Hertz electric power system for transient disturbances and fault clearing events, they can nonetheless cause sensitive loads, such as computer systems and manufacturing operations, to experience equipment damage and, in some cases, to drop off line. Therefore, devices that compensate for these voltage sags and over-voltage conditions, so that the loads receive an uninterrupted supply of the intended line voltage, serve an important function for these types of sensitive loads.
Certain conventional approaches to AC voltage compensation use traditional inverter technology, which rectifies the AC line power into DC power and stores the DC energy, typically in capacitors, batteries, or a flywheel during normal system operations. Then, during a voltage sag, the sag supporter device inverts the stored DC energy into AC power and delivers this power through a series-connected transformer to supply the missing voltage. This conventional inverter approach is complex and requires a large number of power switching elements to create the replacement voltage profile. The switching elements are relatively expensive and render the sag supporter financially infeasible for many applications. In addition, the duration of the available voltage support is limited by the amount of energy that can be stored prior to the voltage sag, and can therefore require large storage devices. Large storage devices can significantly increase the size of the device, often making pole-mounted configurations impractical. Alternatively, the circuitry required to repeatedly discharge and charge capacitors during the voltage sag condition presents complex control and timing challenges associated with continually recharging the capacitors to the proper level, and further increases the cost and sophistication of the device. In addition, the presence of a series-connected transformer in the power line during normal circuit operation causes significant power losses, even when voltage support is not required.
In another conventional approach, a tap switching series-connected transfonner, often called a voltage regulator, may be used to compensate for voltage sag conditions. However, the large number of windings and switching elements required to provide a range of voltage sag compensation increases the cost of the voltage regulator and, in any event, limits the device to providing a discrete number of voltage steps in the output power supply. In addition, due the implementation time required for tap-changing voltage correction, these systems are ill suited to following fast changing voltage sag or over-voltage events, which typically occur when the cause of the voltage sag or over-voltage event involves a fault or switching event characterized by arcing. Arcing, by it's very nature, is erratic in behavior and changes quickly during the event as attachment points move around. Again with this type of device, the presence of a series-connected transformer in the power line during normal operations causes significant power losses.
Transient over-voltage conditions caused by the tap switching series-connected transformer presents another significant disadvantage of the tap-switching voltage regulator approach. This typically occurs when a breaker or fuse clears a fault causing the voltage sag, which abruptly returns the system to normal voltage. This typically occurs at a zero-current condition, which is followed buy the series-connected transformer boosting the voltage on it's output well above it's normal level for approximately 8 milliseconds until the transformer can be returned to its normal setting, which usually occurs at the next zero-current condition. This “current-zero switching” limitation occurs with these devices because they typically employ thyristor switching elements, which can only switch during zero-current conditions. Thus, notwithstanding multiple winding ratios and multiple switching elements, these systems still impose a significant over-voltage on the load at the conclusion of many voltage sag events.
Therefore, there is a need in the art for a compact, cost effective voltage sag and over-voltage compensation device that does not routinely impose over-voltage conditions on the loads they are designed to protect. There is also a need for a voltage sag and over-voltage compensation device that does not require a large number of switching devices, large power storage devices, or a series-connected transformer in the power line during normal operation of the circuit.
SUMMARY OF THE INVENTION
The present invention meets the needs described above in a voltage sag and over-voltage compensation device employing a pulse width modulated autotransformer. The device preferably operates at AC electric power distribution system voltages, but may be designed to operate at other voltage levels. This device significantly improves over conventional inverter technology in that no energy storage devices are required. The present invention also significantly improves over conventional tap-switching transformer technology in that no over-voltage is imposed on the load when a voltage sag event is over. Moreover, the technology of the present invention is much simpler than prior approaches for AC voltage compensation in that it reduces the number of active switching elements and uses well developed autotransformer technology as the basic design element. This advantageously allows switching to occur at higher voltages with lower currents than occurs in prior designs. In addition, a single switching frequency with a single pulse width for any given voltage sag or over-voltage condition makes the control system for the present invention relatively simple to design and implement. The end result is a comparatively uncomplicated design, which exhibits lower cost and higher reliability, while providing equivalent or improved functionality in comparison to prior art voltage compensation technologies.
Generally described, the present invention includes a voltage sag and over-voltage compensation device that receives electric power from an oscillating at a system frequency, adjusts the voltage of the power, and delivers a corresponding voltage-corrected AC power supply to a load connected across an upper pole and a neutral pole of the device. The voltage sag and over-voltage compensation device includes an autotransformer having a first winding around a flux linking core and connected between the neutral pole and a center pole, and a second winding around the flux linking core and connected between the center pole and the upper pole. The autotransformer also includes a center-pole switch for selectively connecting the AC power source between the neutral and center poles when the center-pole switch is gated to a closed or “on” configuration, and for selectively disconnecting the AC power source from connection between the neutral and center poles when the center-pole switch is gated to an open or “off” configuration. The voltage sag and over-voltage compensation device also includes a control unit for selectively gating the center-pole switch between the open configuration and the closed configuration multiple times per cycle of the system frequency
Gardner Groff & Mehrman P.C.
Mehrman Michael J.
Riley Shawn
SSI Power LLC
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