GB1266186A - - Google Patents

Abstract

1,266,186. Protective arrangements. ASSOCIATED ELECTRICAL INDUSTRIES Ltd. 1 July, 1970 [2 July, 1969; 8 Aug., 1969], Nos. 33362/69 and 39847/69. Heading H2K. A protective arrangement for a transformer system includes a detector which inhibits tripping when a transformer inrush current appears, the detector squaring the waveform of a "monitored" current quantity at a predetermined level and inhibiting tripping when the resulting rectangular pulses are not sufficiently close together. As shown the arrangement may comprise an auxiliary current transformer 12 and a circuit 13 providing a full-wave rectified signal representative of the difference between two phases of a transmission line having a substantial transformer loading. The rectified signal is applied to a high-set overcurrent detector 17 which squares the signal at a high level to effect the immediate operation of an output circuit 16 to cause the opening of circuit-breakers without any delay. The rectified signal is also applied to a low-set overcurrent detector 14 which squares the signal at a lower level, the resulting square pulses being fed to an integrator and level detector 15 to charge a capacitor, the detector 15 providing an output to the circuit 16 when the capacitor voltage exceeds a predetermined level. The detector 14 also supplies square pulses to a gap detector 18 in which a second capacitor charges in each interval between pulses. Should an interval be sufficiently long for the voltage of the second capacitor to reach a critical value, the detector 18 becomes effective to inhibit operation of the detector 15 by causing the discharge of its capacitor. In a further arrangement for the protection of a single transformer, Fig. 4, (not shown) having current transformers associated with the primary and secondary windings of the protected transformer and a comparator to provide an output to an integrator and level detector when a signal representing the difference current exceeds one representing the sum current, the gap detector is responsive to the difference signal to inhibit tripping. The high set detector also responds to the difference signal. Both arrangements use transistor circuits, Figs. 2, 5 (not shown) and the principles underlying the invention are described with the aid of graphs, Figs. 3, 6 (not shown).