Electricity: electrical systems and devices – Safety and protection of systems and devices – Ground fault protection
Patent
1997-07-21
1999-08-31
Leja, Ronald W.
Electricity: electrical systems and devices
Safety and protection of systems and devices
Ground fault protection
361 79, H02H 300
Patent
active
059461748
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND INFORMATION
The present invention is directed to a process for detecting ground faults that occur in an electric power transmission line that has a plurality of phase conductors. In particular, upon the response of a protective device, the currents and voltages from these conductors are monitored with respect to a plurality of threshold values. A fault identification signal is generated when at least one conductor current exceeds one of the threshold values or when at least one conductor voltage drops below another one of the threshold values. The fault indication signal is indicative of a single-pole or multi-pole fault.
Previously proposed systems of this type have been discussed, for example, by H. Titze, "Faults and Fault Protection in Three-Phase Power Systems", 1953 Vol. 2, p. 48, and by H. Clemens and K. Rothe, "Protection Methods in Electric Power Systems", 1991, p. 211. Disadvantages that are found in such previously proposed systems include an inability to reliably distinguish single-pole from multipole ground faults and an inability to pinpoint which phase conductor has experienced the detected fault.
SUMMARY OF THE INVENTION
An object of the invention is to provide a process for detecting a ground fault in an electric power transmission line making it possible not only to reliably distinguish between a single-pole and a multipole ground fault, but also to reliably determine the phase conductor(s) affected by the ground fault.
To achieve this object, a measured quantity corresponding to the current of the zero phase sequence system and another measured quantity corresponding to the current of the negative phase sequence system are formed according to this invention from the derived currents when a fault detection signal is not available. An auxiliary signal is generated in the case of a ground fault of one phase conductor when the phase angle between the two measured quantities is less than a predefined threshold phase angle. The currents derived from the currents in the other two phase conductors are monitored to detect whether each of these derived currents is less than a fraction of the measured ground current, and when the derived currents drop below the fraction of the ground current, another auxiliary signal is generated. The auxiliary signals are tested in a logic circuit, which emits a fault identification signal indicating the ground fault of one of the phase conductors when both auxiliary signals are present.
While the use (see article Detection of high-resistance ground faults in the journal "ABB Technik" 1/90) of negative and zero phase-sequence system components for a sensitive ground fault protection is well known, the components of such a protection are used for precisely determining the direction of the ground fault, i.e., to precisely determine whether a fault is upstream or downstream from a directional ground fault relay.
The basic advantage of the process according to this invention consists of the fact that upon response of the protective device due to a ground fault, the phase conductors having a ground fault can be very reliably determined. This is due to the fact that when a fault identification signal is not available, it is checked whether the phase angle between one measured quantity and the other measured quantity is below a predefined threshold phase angle. In addition to this criterion, it is also checked whether the currents in the other phase conductors are less than a fraction of the ground current detected; if both criteria are met, the auxiliary signals are generated. If both auxiliary signals are present, a fault identification signal identifying the ground fault of one of the phase conductors is emitted.
The invention is based on the fact, among others, that in a power transmission line with three phase conductors a criterion for phase selection in the event of a ground fault can be derived from the phase angle between a measured quantity corresponding to the current of the negative phase sequence system and another measured quantity correspond
REFERENCES:
patent: 4536815 (1985-08-01), Li et al.
patent: 4559491 (1985-12-01), Saha
H. Titze, "Fehler und Fehlerschutz in elektrischen Drehstromanlagen", 1953, 2. Band,Seite 48, No Month.
H. CLemens, K. Rothe, "Schutztechnik in Elektroenergiesystemen", 1991, Seite 211, No Month.
Aufsatz, "Erfassung hochohmiger Erdfehler" in der Zeitschrift ABB-Technik, Jan. 1990 EV ST 3.
R. Roeper, "Kurzschlussstrome in Drehstromnetzen", 1984, 6. Auflage, Seiten 48 bis 53, No Month.
Leja Ronald W.
Siemens Aktiengesellschaft
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