Data processing: generic control systems or specific application – Specific application – apparatus or process – Electrical power generation or distribution system
Reexamination Certificate
1999-08-24
2002-10-01
Black, Thomas (Department: 2121)
Data processing: generic control systems or specific application
Specific application, apparatus or process
Electrical power generation or distribution system
C700S294000, C324S521000, C324S522000, C361S080000, C361S081000
Reexamination Certificate
active
06459960
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a distance relay equipment that aims to prevent unwanted operation due to over-reach.
DESCRIPTION OF THE RELATED ART
In general, for the measured value of fault impedance in a distance relay, the impedance for the fault point is correctly found for the faulted phase, excluding the load current and fault point resistance etc.
However, as is well known, over-reach or under-reach of the set impedance occurs in respect of the other phases (phases associated with the faulted phase).
Here, “over-reach” means that the operating range of a distance relay reaches out of the distance which shows a limit of operation or the range of an impedance.
“Under-reach” means that a distance relay detects an internal fault in the operating range of the distance relay, excluding the output of operation.
A typical example of this tendency is the over-reach of the phase of lead in a short-circuit distance relay (ZIS) element described at p252-p254 of “Protective Relay Techniques, Chapter 3” published on Apr. 15, 1981 by Tokyo Denki University Publications Office.
Conventionally, as a countermeasure for such over-reach, as described in “Backup Protective Relay Systems” p.41, Volume 37, No. 1, published on Jun. 5, 1981 by Denki Kyodo Kenkyu (Electrical Joint Research), systems of preventing unwanted operation output of a ZIS element of phase of lead during occurrence of a fault, using the operating condition of an over current relay (OCR) shown in
FIG. 15
, or systems wherein the over-reach zone is restricted by a combination of blinder elements shown in
FIG. 16
are widely employed.
In FIGS.
15
(
a
), (
b
), the OCR combines with a first-stage X
1
of a distance relay in ab-phase.
In FIG.
15
(
b
), a circle shows characteristic of an mho relay element that finds a direction of fault, and X
1
and X
2
paralleling to an R axis shows reactance relay element that finds the distance where fault happens.
Here, in FIG.
15
(
b
), X means “reactance”, R means “resistance”.
Therefore, the operation of over-reach of a distance relay in ab-phase is locked by in-operation of OCR in a phase when a fault happens in bc-phase.
In
FIG. 16
, the operation of over-reach of a distance relay in ab-phase is locked when Zab is in a blinder operation zone. In
FIG. 16
, X means “reactance”, R means “resistance”.
Here, “blinder elements” mean discriminating filter to protect the unwanted operation of the distance relay.
However, when the OCR operation condition is employed in combination with a ZIS element, there is the problem that the fault detection current sensitivity of the ZIS element depends on the magnitude of the load current.
Specifically, in the case of a heavy load system, it is necessary to raise the operation sensitivity of the OCR setting to a level such that this is normally not operated.
But, as a result, inevitably, the operation detection sensitivity of the ZIS element when a fault occurs is adversely affected. Namely, distinguishing the fault is adversely affected, making it difficult to distinguish the fault current and load.
Also, in the case of a countermeasure using the blinder element, the cooperation of the setting value such as to exclude the load impedance and setting with the object of preventing over-reach on fault occurrence can be difficult to operate, making it difficult to select an optimum setting value for use.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a distance relay of high reliability by adopting a principal that does not depend on the magnitude of the load current for preventing unwanted operation of a distance relay element due to over-reach.
It is a further object of the present invention to provide a distance relay wherein special consideration of setting in use is unnecessary, and wherein high reliability is provided, unaffected by system conditions.
These objects can be achieved according to this invention by providing, in first aspect, a distance relay equipment for detecting a protective region having a fault, among a protective zone that includes a plurality of protective regions, based on a status of a power system, comprising:
an operation decision unit having short-circuit fault distance relay elements that input respectively an electrical quantity from an AC power system of three phases a, b and c, the operation decision unit calculating a fault impedance from each of the electrical quantity to compare said fault impedance, which correspond to ab-phase, bc-phase and ca-phase, with a set impedance, to produce a judging signal of each ZIS element (ZIS element-ab, ZIS element-bc and ZIS element-ca), depending on said result of said comparison;
a calculating unit for calculating a phase-to-phase voltage amplitudes(|Vab|, |Vbc|, |Vca|) in said ab-phase, bc-phase and ca-phase ;
a minimum phase voltage selection unit for selecting a minimum voltage phase (V
&Dgr;
min) for the phase-to-phase voltage amplitudes(|Vab|, |Vbc|, |Vca|) determined by said calculating unit;
a fault phase element output unit for having an AND function, which is set to each of the phases (ab-phase, bc-phase, ca-phase) of said minimum voltage phase (V
&Dgr;
min) selected by said minimum phase voltage selection unit and said judging signal (ZIS element-ab, ZIS element-bc and ZIS element-ca), and outputs an operation signal, to separate, the protective region having the fault, from the protective zone, of said ZIS element-ab, said ZIS element-bc and said ZIS element-ca, if said AND condition is satisfied; and
an operation output unit for judging whether or not two or more operated phases of the ZIS elements are present, to output an operation signal of said ZIS elements to separate the protective region having the fault from the other protective zones.
In second aspect, there is provided a distance relay equipment according to the first aspect, wherein said minimum phase voltage selection unit comprises:
a first unit for determining a first value equal to the absolute value of the difference between the absolute value of a first phase voltage and the absolute value of a second phase voltage;
a second unit for determining a second value equal to the absolute value of the difference between the absolute value of a first phase voltage and the absolute value of a third phase voltage;
a selection unit for selecting the first phase voltage if
i) the first and second values are greater than a predetermined threshold,
ii) the absolute value of the difference between the absolute value of the third phase voltage and the absolute value of the second phase voltage is less than the predetermined threshold, and
iii) the difference between the absolute value of the third phase voltage and the absolute value of the first phase voltage is greater than the predetermined threshold;
the selection unit selecting the second phase voltage if
i) the first value is greater than the predetermined threshold,
ii) the second value is less than the predetermined threshold, and
iii) the difference between the absolute value of the first phase voltage and the absolute value of the second phase voltage is greater than the predetermined threshold; and
the selection unit selecting the second phase voltage if
i)the first value is less than the predetermined threshold,
ii) the second value is greater than the predetermined threshold, and
iii)the difference between the absolute value of the first phase voltage and the absolute value of the third phase voltage is greater than the predetermined threshold.
In third aspect, there is provided a distance relay equipment according to the second aspect, further comprising:
a third unit for selecting the minimum voltage phase among said first phase voltage, said second phase voltage and said third phase voltage if said selection unit does not select one of the phase voltages.
In fourth aspect, there is provided a distance relay equipment according to the second aspect, wherein said predetermined threshold is about 0.05 pu.
In fifth as
Amou Hidenori
Kurosawa Yasuhiro
Shuto Itsuo
Barnes Crystal J
Black Thomas
Foley & Lardner
Kabushiki Kaisha Toshiba
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