Electrical transmission or interconnection systems – Plural supply circuits or sources – Substitute or emergency source
Reexamination Certificate
2000-12-28
2001-10-16
Tso, Edward H. (Department: 2838)
Electrical transmission or interconnection systems
Plural supply circuits or sources
Substitute or emergency source
Reexamination Certificate
active
06304006
ABSTRACT:
FIELD OF THE INVENTION
The present invention generally relates to the field of power transmission and distribution systems. More particularly, the present invention relates to utilizing uninterruptible power supplies and a generator in a power system with a critical load.
BACKGROUND OF THE INVENTION
FIG. 1
is a one-line diagram of a typical arrangement of a main ac supply (e.g., power grid)
10
, an on-line Uninterruptible Power Supply (UPS)
12
, a load
14
, and a battery bank
16
. The on-line UPS typically contains a controlled rectifier DR
1
, a dc-to-ac inverter A
1
, a static bypass switch S
1
, and isolation transformers T
1
and T
2
(T
2
is typically a nine-winding transformer, with a three-phase primary for S
1
, a three-phase primary for A
1
and a three-phase secondary for the load, T
2
is typically a three-winding transformer for single phase). The dc bus
12
-
1
is designed specifically to support the bank of batteries
16
. The bus can provide charging current and also draw current from the batteries in the event of a power outage on the main ac supply
10
. A UPS usually contains a static (electronic) bypass switch S
1
so that the output of inverter A
1
can be switched to the main supply
10
in the event of a fault or high inrush load.
FIG. 2
depicts an example of a conventional system employing an auxiliary generator
18
in combination with a UPS
12
. Battery supplied UPSs provide backup power for short periods of time, typically on the order of minutes. The backup time can be substantially increased by adding a generator to the system. This is typically accomplished using an automatic transfer switch S
2
, at the input of the UPS
12
, as shown in FIG.
2
. When the main ac power supply
10
fails, the batteries
16
supply power to the UPS. If the power remains down for a predetermined period of time, the automatic transfer switch S
2
will start the auxiliary generator
18
and switch the UPS
12
input to the auxiliary generator output through S
2
.
The use of an auxiliary generator in this manner has some drawbacks. Typically the auxiliary generator remains idle most of the time. Further, when an auxiliary generator is connected to and supplying power to a load (e.g., load
14
), the auxiliary generator may not be operating at an efficient power output level. Many times it would be desirable to increase the power output level, allowing the auxiliary generator to supply power to both the load and the grid. However, the solution shown in
FIG. 2
does not allow the auxiliary generator to feed power back into the grid.
FIG. 3
depicts an example of a conventional system employing an auxiliary generator
18
in combination with a UPS
12
and with the ability to supply power from the auxiliary generator
18
back into the grid through synchronizing equipment
19
.
FIG. 3
contains elements similar to those described above with respect to
FIG. 2
, and their description is omitted for brevity. The system of
FIG. 3
includes a feedback switch S
3
connected to the auxiliary generator
18
. The feedback switch S
3
is connected to synchronizing equipment
19
, which is connected to the grid.
When the auxiliary generator is connected to the grid, this is referred to as grid parallel mode. The economics of operating the auxiliary generator in grid parallel mode are determined by a number of factors including the real-time price of fuel and electricity, as well as any arrangements the user has with the utility for peak shaving. The synchronizing equipment
19
allows the auxiliary generator to operate in grid parallel mode and may prevent backfeed of power in the event of a grid failure. However, the synchronizing equipment
19
can be expensive.
In view of the above problems, there is a recognized need a for an energy management system that can supply backup power from a generator or from a dc storage device and also output that power back into the grid, without requiring synchronization or costly backfeed prevention equipment.
SUMMARY OF THE PRESENT INVENTION
A power system is provided in which a grid supplies electrical power to a load and in which backup power is provided from one of a generator and a dc storage device. The power system includes a standalone inverter. The output of the standalone inverter is connected to the load. The power system includes a grid parallel inverter. The output of the grid parallel inverter is connected to the grid. A dc bus is electrically connected to the input of the standalone inverter and to the input of the grid parallel inverter.
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US. application No. 09/363,530, Jungreis, filed Jul. 29, 1999.
ABB T&D Technology Ltd.
Tso Edward H.
Woodcock Washburn Kurtz Mackiewicz & Norris LLP
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