Electricity: electrical systems and devices – Safety and protection of systems and devices – With specific quantity comparison means
Patent
1998-12-01
2000-04-04
Sherry, Michael J.
Electricity: electrical systems and devices
Safety and protection of systems and devices
With specific quantity comparison means
361 80, H02H 318
Patent
active
060468955
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to an impedance protection method for detecting short-circuits on a monitored section of an electrical power supply line in which impedance values formed from current and voltage during a short-circuit are checked to see whether they lie within a specific tripping characteristic in order to extract a tripping signal.
BACKGROUND INFORMATION
A method for detecting short circuits is described in a book entitled "Relaisschutztechnik in Elektroenergiesystemen"(Protection in Electrical Power Systems), Dr.-Ing. Heinz Clemenz, Dr.-Ing. Klaus Rothe, VEB Verlag Technik, Berlin, 1980, pages 64 through 66. In this conventional method, the absolute impedance value formed from current and voltage during a short-circuit is checked to see whether it lies within a circle with a center point defined by the start of the monitored section of a power supply line. The circle radius is determined by the size of the monitored section. Although this method may generate the tripping signal relatively quickly, it has a disadvantage in that the circular tripping characteristic adapts poorly to the conditions of an electrical power supply line; if an unfavorable value is selected for the circle radius, for example a value that encompasses all possible short-circuit faults on the monitored line section, the circular tripping characteristic may also include impedance values that cannot be attributed to a short-circuit fault but which occur under certain load conditions. On the other hand, selecting a circle radius so small that the circle cannot produce false tripping means that some of the short-circuit faults may not be detected. The conventional method therefore achieves a relatively high tripping speed, but is able to detect only some of the short-circuit faults that can occur.
A further impedance protection method is described in the operating instructions of the PD 551 impedance protection device manufactured by AEG, pages III/29 ff. This method uses a polygonal tripping characteristic. The resistance and reactance components of these impedance values are checked to see whether they lie within this polygon. The method requires a relatively large amount of time in order to form a tripping signal, since the data window needed for calculating the resistance and reactance components must be relatively large. On the other hand, the polygonal method detects a larger number of the possible short-circuit faults than does the method using the circle, since the polygonal characteristic can be better adapted to the conditions of the power supply line.
SUMMARY OF THE INVENTION
The object of the present invention is to provide an impedance protection method that detects all short-circuit faults that may occur on a monitored line section and, if necessary, to form a tripping signal, at the same time reducing the time needed to generate the tripping signal compared to the known methods.
This object is achieved with an impedance protection method according to the present invention: the monitored section of a power supply line and comparing the impedance values first determined to this first tripping characteristic to see whether they lie inside or outside this tripping characteristic; lie within the first tripping characteristic; monitored section if the impedance values first determined lie outside this tripping characteristic; determined to see whether they lie within the maximum tripping characteristic and generating the tripping signal, if necessary.
The use of a relatively small tripping characteristic as the first tripping characteristic initially provides relatively limited protection for the monitored section, since it does not detect all of the faults occurring on this section. Nevertheless, approximately 80% of the short-circuits can be detected within an advantageously short period of time because only relatively few measured current and voltage values need be used to form the impedance values, due to the imprecision tolerance. If the occurring fault could not be detected with
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H. Clemenz et al., "Relaisschutztechnik in Elektroenergiesystemen", VEB Verlag Technik, Berlin, 1980, pp. 64-71.*
R. Roeper, "Kurzschlu.beta.strome in Drehstromnetzen, zeitlicher Verlauf und Berechnung der Gro.beta.en", 6th Edition, Berlin/Munich, Siemens Aktiengesellschaft, 1984, pp. 48-53.
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"Distanzschutzeinrichtung", PD551, AEG, 1991, pp. III/33-39.*
Jurisch Andreas
Sezi Tevfik
Sherry Michael J.
Siemens AG
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