Electricity: electrical systems and devices – Safety and protection of systems and devices – With specific quantity comparison means
Reexamination Certificate
1999-06-30
2001-05-29
Leja, Ronald W. (Department: 2836)
Electricity: electrical systems and devices
Safety and protection of systems and devices
With specific quantity comparison means
C361S080000
Reexamination Certificate
active
06239959
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention is directed to electric power transmission system protection apparatus and, more particularly, to an improved quadrilateral characteristic for a ground distance relay.
Electric power transmission systems frequently employ so-called distance relays to determine whether a fault on the system is within a predetermined distance of a monitoring point where the relay is located. Each predetermined distance is often referred to as a separately numbered zone. The present invention is particularly concerned with distance relays which respond to single phase to ground faults within zone
1
, a separate such relay being normally provided for each phase of the polyphase power transmission system.
In a well-known form of a distance relay a tripping signal determination is made by comparing the phases of voltages derived from measurements of the system voltage and current at the monitoring point under fault conditions. For example, referring to
FIG. 1
, in the so-called mho characteristic relay of the prior art the phases of quantities V-IZ and V
pol
are compared where:
V is the phase voltage at the monitoring point;
I is the phase current at the monitoring point;
Z is a replica impedance which determines the setting of the relay, i.e., the predetermined distance or reach; and
V
pol
is V∠−90°.
Since the quantities V-IZ and V are necessarily at 90°, and hence the quantities V-IZ and V
pol
are necessarily in phase when the meeting point of the vectors V and V-IZ lies on a circle having IZ as diameter, by determining, using a phase comparator, whether V-IZ leads or lags V
pol
it can be determined whether the fault is beyond or within the distance for which the tip of the vector V lies on the circle.
In addition to the capability of discriminating with respect to distance, it is highly desirable that not all faults or fault combinations within a prescribed distance or zone necessarily lead to the tripping of a breaker. For example, a ground distance relay for phase A preferably should detect phase A-to-ground faults only, not phase B-to-ground faults, or, alternatively, phase-to-phase faults. In other words, it is generally desirable that the relays implemented in an AC power distribution system have a certain degree of discrimination not only with respect to distance, but also with respect to particular fault combinations.
An early attempt to provide a more discriminating ground distance relay is disclosed in U.S. Pat. No. 4,686,601 to the present inventor. That patent discloses a system for improved discrimination between internal and external faults as well as prevention of overreaching by the ground distance relay on single line-to-ground and double line-to-ground faults. That patent further discloses a system to prevent operation of the ground distance relay on a phase other than the phase on which a fault occurs.
U.S. Pat. No. 4,686,601 is based on a mho type characteristic like that discussed above and shown in FIG.
1
. However, many protective relay engineers prefer to use a quadrilateral (polygonal) characteristic for ground distance functions to provide more fault resistance coverage than the circular mho characteristic. Further, the system disclosed in U.S. Pat. No. 4,686,601 is not particularly sensitive to preventing the relay function from responding to faults other than single line-to-ground faults on the protected phase. In other words, such a system may respond to any other single line-to-ground fault or any fault that involves more than one phase. Such undesirable relay functions lead to unnecessary tripping of breakers which leads to unnecessary electrical transmission interruptions.
SUMMARY OF THE INVENTION
It is therefore seen as desirable to provide an improved quadrilateral characteristic ground distance relay that is able to provide improved discrimination especially with respect to double line to ground faults.
In accordance with an embodiment of the invention there is provided an improved reactance characteristic that embodies a particular phase selector that eliminates undesirable generation of a tripping signal other than for a selected phase to ground fault. This particular phase selector is I
1F
Z
1
, where Z
1
is the positive sequence replica impedance and I
1F
is the fault component of the positive sequence current component, referenced to the proper phase. Preferably, the pre-fault load current is removed from this quantity via a memory function.
Further in accordance with the preferred embodiment of the present invention, an improved restricted phase directional function is implemented, providing improved overall relay performance. A right resistive blinder function is also provided to complete the component set for the quadrilateral characteristic.
REFERENCES:
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patent: 4821137 (1989-04-01), Wilkinson
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patent: 4825323 (1989-04-01), Wilkinson
patent: 4825326 (1989-04-01), Andow et al.
patent: 4825327 (1989-04-01), Alexander et al.
patent: 4868704 (1989-09-01), Cavero
patent: 4878142 (1989-10-01), Bergman et al.
patent: 4996624 (1991-02-01), Schweitzer, III
patent: 5140492 (1992-08-01), Schweitzer, III
patent: 5365396 (1994-11-01), Roberts et al.
“Advanced Line Protection System with Single Phase Tripping”, GEK 105556, GE Power Management, Malvern, PA., No Date.
“DLP—Digital Line Protection”, GET-8037A, GE Protection and Control Business Department, Malvern, PA., No Date.
Blasey Thomas M.
General Electric Company
Horton Carl B.
Hunton & Williams
Leja Ronald W.
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