Electrical transmission or interconnection systems – Plural supply circuits or sources – Substitute or emergency source
Reexamination Certificate
2001-02-13
2002-10-15
Jackson, Stephen W. (Department: 2836)
Electrical transmission or interconnection systems
Plural supply circuits or sources
Substitute or emergency source
C307S043000, C307S044000, C307S045000, C307S070000, C307S080000
Reexamination Certificate
active
06465910
ABSTRACT:
TECHNICAL FIELD
This invention relates generally to power systems, and more particularly to power systems for providing an assured, or uninterruptible, supply of electrical power to one or more critical loads. More particularly still, the invention relates to such power systems employing fuel cells as a source of electrical power.
BACKGROUND ART
By far, the most common source of electrical power for a great variety of loads is via the extensive power grid provided by the various electric utilities. The electrical power available on the utility grid is generally fairly reliable as to continuity and adherence to established standards of voltage, frequency, phase, etc. However, from time to time discontinuities and/or departure from those standards do occur. If they are brief or modest, most loads are relatively insensitive to those events. On the other hand, there are a growing number of loads which are relatively intolerant of even brief aberrations in the power supplied by the utility grid, with the principal example being computers and various types of electronic data processing devices. Even brief interruptions in the standardized supply of electric power by the utility grid may cause the computer to malfunction, with sometimes costly, and always bothersome, consequences.
In defining this concern, the Computer Business Equipment Manufacturers Association (formerly CBEMA, and now ITI) has developed a set of Power Acceptability Curves which establish the standards, or at least provide guidance, for determining the power norms which will assure continued operation of those types of loads. In that regard, a standard has been adopted indicating that a computer can tolerate a one half cycle or 8.3 ms power interruption. The power available on the utility grids is not presently capable of meeting this standard on a substantially continuous basis. Accordingly, it has been and is, necessary to provide supplemental power sources if it is important to assure that critical loads have a substantially continuous or uninterrupted supply of electrical power. For purposes of this application, a supply of power with interruptions or transfers of no greater than 8.3 ms duration, may be referred to as being “seamless”, “substantially continuous”, or “substantially uninterrupted”.
Referring to
FIG. 1
, there is illustrated one existing form of uninterruptible power supply (UPS), a so-called “on-line” or “double conversion” type, used to supply a critical load in those instances when the utility grid supply is interrupted or is outside of specified limits. The utility grid power supply normally appears on conductor
110
, and is passed via normally-closed contacts of a 3-pole transfer switch
112
to a rectifier
120
, which supplies the critical loads
114
via an inverter
122
. However, to provide continued power in and during those intervals when the utility grid power is not within the specified limits, a backup battery
116
is provided to supply immediate power of limited duration, and an emergency electrical generator
118
is then connected to the other contact of transfer switch
112
to follow-up with a longer term temporary supply. To accommodate the use of battery
116
in a system which relies on AC power for the loads
114
, it is necessary to provide the rectifier
120
to charge battery
116
and the inverter
122
to convert the DC supply from the battery to the necessary AC supply for the loads. A high speed switch
124
connected between the transfer switch
112
and the loads
114
operates as a bypass switch to provide temporary power if the inverter
122
or rectifier
120
must be serviced. Because the grid and loads are not normally directly connected, but rather the power to the loads is required to pass through a pair of converters with the aid of the UPS battery, this type of UPS is termed an “in-line” or “double conversion” type. This arrangement, though effective, requires a number of costly components that are in use only during the intervals when the utility grid power is unsatisfactory.
Another arrangement of a power system for providing substantially uninterrupted power to critical loads is described in PCT application US99/10833 for “Power System”, published on Nov. 25, 1999 as WO 99/60687. Referring to
FIG. 2
in the present application, the relevant portions of the invention described in that PCT application are depicted in a very simplified, generalized form, with elements being numbered such that their last 2 digits are the same as their functionally equivalent counterparts in FIG.
1
. The critical loads
214
receive substantially uninterrupted power from a motor-generator
230
within an uninterruptable power system module
231
, which module also contains transfer switches, rectifiers and inverters. Several alternative electrical power sources are provided to maximize the continued powering of the motor-generator
230
. One such power source may be the utility grid
210
. Another source may be the fuel cell generator power plant
218
. A transfer switching arrangement
212
enables one or the other of the utility grid
210
and the fuel cell
218
to normally provide the power to drive the motor-generator
230
. This type of uninterruptible power supply is also of the “on-line” or “double conversion” type inasmuch as the grid is not directly connected to the loads
214
, but acts through the rectifier and inverter converters and the flywheel and/or fuel cells to energize motor-generator
230
which in turn provides uninterrupted power. In fact, the fuel cell
218
is configured to operate in a grid connect (G/C) mode with the utility grid
210
for system economy, so in grid connected mode both the grid and the fuel cell supply the “grid” terminals of the transfer switch. In the event of failure of the grid supply
210
, the fuel cell
218
is intended to serve as the continuing power source for the motor-generator
230
. However, in such event, the fuel cell
218
must reconfigure from a “grid connect” (G/C) mode of operation to a “grid independent” (G/I) mode. The power conditioning system (PCS) portion of the fuel cell
218
includes associated inverters, switching transistors and breakers (not shown) that effect the conversion of DC power to AC power and that govern the fundamental G/C and G/I modes of fuel cell operation. That mode transition (from G/C to G/I) has typically required the fuel cell
218
and transfer switch
212
to interrupt power generation for up to 5 seconds. Such interruption is not “seamless”, and would be of unacceptable duration for critical computer loads
214
. Accordingly, a backup flywheel power source
216
provides immediate power of limited duration(similar to the battery source
116
in
FIG. 1
) to the motor-generator
230
at least during such mode conversions. That backup power source
216
is a flywheel
236
driving a bi-directional AC/DC converter
238
. The converter
238
keeps the flywheel spinning during normal operation, and discharges the flywheel
236
during backup operation. The various transfer switches used in the transfer switching arrangement
212
and in the uninterruptable power system module
231
may be electro-mechanical, static, or a combination thereof, and serve to effect the various power switching functions.
While the Power System of the abovementioned PCT application may provide a substantially uninterrupted source of power to various critical loads and may advantageously employ fuel cells as one of the main sources of the power, it nevertheless requires the use of considerable additional equipment that is complex and costly. For example, the separate motor-generator
230
, and the backup power source
216
which includes the flywheel
236
/converter
238
combination, represent necessary, but expensive, components in order to assure the degree of power continuity sought and required.
Another type of UPS is of the “stand-by” type wherein the grid is directly connected to the loads and a stand-by UPS remains idle, even if connected to the loads, until a switch disconnects the grid from t
Fragola, Jr. Francis A.
Healy Herbert C.
Ross Ricky M.
Young Douglas Gibbons
DeBeradinis Robert L
Jackson Stephen W.
Schneeberger Stephen A.
UTC Fuel Cells LLC
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