Electricity: electrical systems and devices – Safety and protection of systems and devices – Feeder protection in distribution networks
Reexamination Certificate
2000-04-24
2002-05-21
Jackson, Stephen W. (Department: 2836)
Electricity: electrical systems and devices
Safety and protection of systems and devices
Feeder protection in distribution networks
C361S062000, C361S066000, C361S093100
Reexamination Certificate
active
06392856
ABSTRACT:
BACKGROUND
This invention relates to electric power utility networks including generating systems, transmission systems, and distribution systems serving loads. The power flowing on these networks is primarily in the form of alternating current and as such is familiar to those skilled in the art.
To remain competitive, electrical utility companies continually strive to improve system operation and reliability while reducing costs. To meet these challenges, the utility companies are developing techniques for increasing the life of installed equipment, as well as, diagnosing and monitoring their utility networks. Developing these techniques is becoming increasingly important as the size and demands made on the utility power grid continue to increase.
A utility power grid is generally considered to include both transmission line and distribution line networks for carrying voltages greater than and less than about 25 kV, respectively.
Referring to
FIG. 1
, a portion of a utility power network is shown to include a transmission network
10
having generators
12
, substations
14
, and switching stations
16
, all of which are interconnected via transmission lines
18
. Transmission lines
18
, in general, carry voltages in excess of 25 kilovolts (kV). With reference to
FIG. 1
, the voltage on a particular transmission line is approximately proportional to the thickness of the associated line in the figure. The actual transmission system voltages are indicated in the accompanying key located at the lower right.
Referring to
FIG. 2
, an exploded portion
10
a
of the utility power network of
FIG. 1
includes distribution lines
20
coupled to a transmission line
18
through step-down transformers
22
. Each distribution line carries power to loads
24
at voltage levels less than those levels associated with transmission lines (e.g., 25 kV or less).
The utility power grid is susceptible to faults or contingencies which are a critical problem for the utility industry. In particular, when a fault occurs on the transmission grid, momentary voltage depressions are experienced, which may be problematic to loads connected to the grid.
Large industrial plants with loads above a few megawatts are typically serviced at medium voltage, 4,106V and above, and may have more than one source substation, or have more than one feeder line between the utility and their main transformer(s). While this configuration greatly improves the overall continuity of the power supply, it exposes the plant loads to short duration voltage sags caused by faults or weather related events on the parallel feeders or substations, or on the transmission system. These sags generally fall in the range of 0.2 P.U. to 0.8 P.U. of nominal voltage, and <1 second duration, although there are considerable differences from one location to another.
Most industrial sag events last less then 20 cycles or so. Yet for modern manufacturing facilities, this is more than enough to cause interrupts, especially in automated manufacturing operations, to the point where the user feels a clear cost for these power quality (PQ) events. But with load levels in the megawatt, or 10's of megawatts range, the cost of available devices to address the sag issue had formerly been far too high to contemplate.
To better understand the dynamics of a fault on a utility power system, the sequence of events on the system due to a 3-phase fault on the transmission system will now be described. For example, referring again to
FIG. 1
, assume the fault occurs on a portion of the transmission network remote from a segment
70
. Segment
70
lies between a substation
14
a
and a switching station
16
of transmission line network
10
. Referring to
FIG. 3
, the voltage profile as a function of time at substation
14
a
due to the fault is shown. In this particular case, the voltage drops from a nominal 115 kV to about 90 kV. It is important to appreciate that if the fault were to occur more closely to segment
70
or on the segment itself, the drop in voltage is generally much more severe, and the voltage on the line can approach zero.
In general, such a fault appears as an extremely large load materializing instantly on the transmission system. Further details as to the events which typically occur on the transmission system in response to the appearance of this very large load, are described in U.S. application Ser. No. 09/449,435.
As discussed above, faults occurring on the utility power network have dramatic effects on the loads connected to the distribution network. Indeed, momentary voltage sags at a factory or manufacturing facility can cause production losses, scrap product, missed schedules, overtime and added maintenance, all of which add significant cost. For example, a single power failure at a semiconductor processing facility can result in the scrapping of $250,000 in semiconductor integrated circuits. Moreover, regardless of how well electric utility companies serve such factories, such events are inevitable.
Various equipment and device solutions have been developed to address these momentary voltage sags. In general, such equipment and devices mitigate the effects of these sags by injecting real and/or reactive power into the system.
Two such devices used to address grid instability problems and associated sags are the superconducting magnetic energy storage (SMES) and the PQIVR system, both products of American Superconductor Corporation, Westborough, Massachusetts. The PQIVR system focuses on maintaining power quality for a particular load and integrates energy storage and power electronics to boost utility sag events by 10-50%, at any load level to keep the load operational. A PQIVR can include a superconducting magnet which stores energy used to bridge voltage sags. When a sag is detected, the PQIVR immediately rebuilds the voltage so that the load sees only smooth, uninterrupted power. In some embodiments, the PQIVR can bridge multiple, rapid-fire events and, following any magnitude of carryover, recharge rapidly.
A SMES device is similar to the PQIVR in that it stores electrical energy in a superconducting magnet. However, unlike the PQIVR, the SMES focuses on stabilizing the entire utility power grid instead of concentrating on one industrial customer. In particular, the SMES provides power to the distribution network to stabilize the utility power network in response to a detected fault after the load is isolated from the grid. Because the SMES, like a battery, is a DC device, a power conditioning system is generally required in order to interface it to an AC utility grid. Thus, the power conditioning system generally includes DC/AC converters as well as other filtering and control circuitry.
SUMMARY
The invention features an approach for providing voltage protection to a load connected to a distribution network of a utility power system or network by boosting the voltage on a distribution line, during a momentary voltage sag caused by a fault or other contingency. By “utility power system or network”, it is meant those systems or networks having at least one distribution line network coupled to a higher voltage transmission line network designed to carry a nominal voltage under normal operating conditions. The distribution line network generally includes at least one distribution line having a load and carries voltages at levels lower than those on the transmission network.
One general aspect of the invention relates to a method of providing voltage protection from a voltage recovery system to a load connected to a distribution network of a utility power network. The method includes selecting a voltage protection characteristic required by the load; and determining, on the basis of electrical characteristics of the voltage recovery system and the distribution network, whether the voltage recovery system is capable of providing the required voltage protection characteristic.
Embodiments of this aspect of the invention may include one or more of the following features.
Determining whether the voltage recovery system is
Kehrli Arnold P.
Nelson Bradley D.
American Superconductor Corporation
Fish & Richardson P.C.
Jackson Stephen W.
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