Electric power conversion systems – Current conversion – Using semiconductor-type converter
Reexamination Certificate
2000-09-01
2001-07-24
Nguyen, Matthew (Department: 2838)
Electric power conversion systems
Current conversion
Using semiconductor-type converter
Reexamination Certificate
active
06266260
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to power conversion systems that contain energy storage devices, and more particularly to uninterruptible power supply systems capable of supplying power from an AC line voltage with or without compensation, and from a battery storage element with or without boost-conversion.
BACKGROUND OF THE INVENTION
As more and more segments of the business environment enter the information age, more and more computers and increased computing power are required. As businesses move from the old to the new economy, their reliance on the processing, transference, and storage of digital information is becoming a more critical aspect of their overall business strategy. While in the past computer crashes were seen a mere nuisance, the loss of computing power and business data may well devastate a businesses' ability to survive in today's new economy. As such, the need for reliable, uninterruptible electric power to maintain the operational status of the computing equipment and the integrity of the digital data continues to rise.
To meet these requirements, uninterruptible power supplies (UPS) have been developed. These UPS's utilize a bank of electric storage batteries and solid state conversion equipment in association with the utility line voltage to provide continuous electric power to a businesses computer system in the event of a loss or deviation of power quality from the utility. The number of batteries contained within a UPS is dependent upon the business' length of time that it needs to operate in the event of a utility power system failure. Likewise, the number of power modules included in a modular UPS, or the power rating of a power conversion module in a fixed-size UPS, is dependent on the overall total system load required to be supplied thereby. As the electrical utilization requirements of a business' computing system grow, additional power modules or additional UPS's may be purchased and integrated into an overall uninterruptible power system for the business enterprise.
While a UPS is required to supply the entire electrical requirements of a system to which it is applied during loss of utility power, and while a business may choose to operate its UPS to condition the utility line power to provide high power quality to their computing equipment, during periods of normal utility line availability operation of the UPS may provide more inefficiencies than advantages. However, since the loss or corruption of utility line voltage often may not be predicted, disconnection of the UPS may result in momentary loss of utility power to the computing equipment and corresponding, loss of computing power and electronic data.
To overcome this problem and to increase the efficiency of the UPS during periods of normal utility line voltage operation, typical UPS's include some form of bypass circuitry to route the utility line voltage directly to the UPS output to which the computing equipment is coupled. Such a configuration of a typical UPS is illustrated in FIG.
26
. As may be seen from this simplified single-line schematic, the AC line voltage input
101
is routed through a bypass circuit
103
to the UPS output
105
coupled to the load
107
. By utilizing this bypass circuit
103
losses resulting from rectification of the AC line input voltage as well as losses resulting from the generation of an AC output voltage waveform through switches
111
,
113
,
115
, and
117
may be avoided. Typically this bypass circuitry
103
comprises a back-to-back silicon controlled rectifier (SCR) circuit, although other bypass circuitry configurations are also applicable. Unfortunately, the addition of the bypass circuitry
103
adds substantial cost, thermal management problems, and volume to the UPS itself. Such disadvantages have long been accepted as a necessary evil to allow high efficiency operation during periods of normal utility line voltage operation.
Transition from this high efficiency bypass mode of operation to inverter operation requires that the inverter's bus capacitors
119
,
121
be charged. Some prior UPS systems utilize soft charging circuitry comprising additional power semiconductor devices per capacitor or per bus (not shown) to control the charge rate. Alternatively, the front end devices
111
,
113
could be modulated to bring the capacitors up to the proper voltage to allow proper output waveform generation. Unfortunately, these prior UPS systems were unable to supply power during this soft charging period. As a result, a momentary loss of the output voltage waveform could be experienced until the capacitors
119
,
121
reach their proper charge. Additionally, the required additional power devices and requisite circuitry adds costs and thermal management problems as well as volume to the UPS system, which disadvantages have heretofore merely been accepted.
To allow scalability of the power provided by a UPS, each UPS power module must be able to generate an output voltage waveform in coordination with the other UPS power modules to supply the entire connected load. In addition to providing scalability, this configuration provides redundant operation to maximize the fault tolerance of the UPS and ensure continued electrical supply to the computing equipment. To ensure that a failure within any one of the power modules of the UPS does not result in the entire UPS being taken off-line thereby resulting in a loss of electrical supply to the computer equipment, each individual power module of a typical UPS system includes in-line fault isolation circuitry
123
that operates to isolate a failed power module from the output
105
. Typically this fault isolation circuitry takes the form of in-line power semiconductors, back-to-back SCR's, electromechanical relays, etc. Unfortunately, this fault isolation circuitry
123
adds costs, thermal management problems, and volume to the UPS system.
SUMMARY OF THE INVENTION
The system and method of the present invention presents a new and improved double-conversion uninterruptible power supply utilizing a center switch circuit to disassociate the bus capacitors from the neutral connection. By coordinating operation of this center switch circuit, higher overall efficiencies are provided with a reduced part count, cost, and volume over prior UPS systems.
The double-conversion, center switch topology of the present invention overcomes the above-described and other problems existing in the art while providing three modes of operation. A first mode of operation is an economy mode, where some line conditioning is accomplished but the efficiency is maintained at a very high level. A second mode provides full double-conversion operation of the inverter, where the AC input utility power is converted to a bus DC and back again to an output AC voltage waveform. A third mode of operation is battery conversion, where the battery is utilized by the inverter topology to generate the output AC waveform to supply power to the connected loads during loss of utility line input voltage. In a preferred embodiment, the inverter does not utilize the battery as the DC bus, but instead steps up the battery voltage to a higher bus voltage to allow a simplified conversion to the output AC waveform. In one embodiment of the invention, the user may select the operating mode of the UPS system, while in an alternate embodiment the UPS can operate in a fully automatic intelligent mode, wherein the UPS determines whether to operate in the high efficiency bypass mode or as a conventional double-conversion inverter relying on either the utility line voltage or the battery.
Operation of the system of the present invention in the high efficiency mode is accomplished by opening the center switch circuit to disassociate the bus capacitors from the neutral connection and, in one embodiment, operating the inverter output switches at a rate equal to the AC line input to voltage waveform. In this way, minimal losses are incurred since the inverter is not performing any appreciable
Folts Douglas C.
Layden David L.
Stich Frederick A.
Zahrte, Sr. Donald K.
Leydig , Voit & Mayer, Ltd.
Nguyen Matthew
Powerware Corporation
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