GB648445A - Improvements in electrical protective systems - Google Patents

Abstract

648,445. Protective cut-out arrangements. METROPOLITAN - VICKERS ELECTRICAL CO., Ltd. July 13, 1949, No. 18527. Convention date, July 30, 1948. [Class 38(v)] In a differential protective system for a section such as a feeder, bus-bar, generator, or other machine, a relay 24 having a highimpedance operating circuit is connected in parallel with the secondary windings 17- 21 of current transformers in the conductors connected to the section, so as to prevent operation on an external fault and to protect the relay against excessive voltages during internal faults. As shown, the secondaries 17-21 are connected in parallel to points 22, 23, to which the high impedance coil 24a of relay 24 is connected, through a rectifier bridge 37 and a tuned circuit 35, 36, so as to block d.c. and harmonic components. In addition, a voltagelimiting device 38, such as a ceramic, a glow tube, or a spark gap, having a negative resistance-voltage characteristic is connected to points 22, 23. On external faults, any voltage at points 22, 23 due to current transformer saturation is insufficient to energize coil 24a, but on an internal fault, e.g. at X, the coil is energized to close its contact 25, thereby energizing the trip coil 26 of a hand-reset switch 27, having contacts in the respective trip coil circuits. Operation of switch 27 also causes a contact 34 to change over and short-circuit the operating circuit of relay 24. In a modification, Fig. 2, (not shown), the relay coil 24a is connected directly in series with device 38 across the points 22, 23, and the arrangements of Figs. 1 and 2 may be combined, Fig. 3, (not shown).