Contents
The long-used restraining principle, based on the detection of 2nd harmonics in the currents, can not always satisfactorily meet the demand of blocking the transformer differential protection for all three phases, because the line currents are not the direct image of the core saturation. The differential protection may become inadequate. A transformer can however be switched on for all cases, without the false or fail-safe tripping, when the harmonic restraint is derived from the magnetic field strength of the core.
If a delta-connected winding is present, the magnetic field strengths can not be easily recognized in the line currents, because their zero-sequence component is distorted. A new method is presented, which can estimate the fundamental and the 2nd harmonic of this zero-sequence component. The components of the magnetic field strength can therefore be determined in currents only, whose ratio is known to be characteristic for the core saturation.
The proposed method was implemented on an industrial protection system based on a microprocessor and was tested on data obtained from computer-simulation and a model transformer in a laboratory. It turned out that the proposed method performed well on all cases.
Übersicht
Die üblicherweise benützte Blockierung beim Einschalten eines Transformators beruht auf der Bewertung der zweiten Harmonischen in den Strömen. Dieses Prinzip kann nicht immer den Transformatorvergleichsschutz in allen drei Phasen blockieren. Dagegen kann ein Transformator in allen Fällen ohne Fehlauslösung eingeschaltet werden, wenn die Blockierung aus der Harmonischen der magnetischen Feldstärke des Kerns abgeleitet wird.
Eine neue Methode wird vorgestellt, wie die Homopolarkomponente der Grundschwingung und der zweiten Harmonischen näherungsweise berechnet werden kann. Die benötigten Anteile der Feldstärken können damit aus den Differenzströmen ermittelt werden.
Das vorgeschlagene Verfahren wurde auf einer industriell gefertigten Schutzkarte implementiert. Die Überprüfung des Verfahrens erfolgte mit Berechnungsverfahren und durch Messungen im Labor.
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Guo, X., Maier, H.A. & Feser, K. A new inrush detection method for transformer differential protection. Archiv f. Elektrotechnik 76, 83–91 (1992). https://doi.org/10.1007/BF01451989
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DOI: https://doi.org/10.1007/BF01451989