GB258904A - Improvements in electrical protective arrangements - Google Patents

Abstract

258,904. Metropolitan Vickers Electrical Co, Ltd., (Assignees of Fortescue C. Le G.). Sept. 26, 1925, [Convention date]. Cut-out arrangements. - A differential-relay protective arrangement for electric translating devices (for example, rotary or stationary transformers) is differentially responsive to the respective values of a balanced rotational phasesequence component of the input and output currents. This component may be a positive-, negative- or zero-phase-sequence component. Means for segregating symmetrical components, of an unbalanced polyphase system are described in Specifications 159,147 and 161,167. In the arrangement shown, a differential induction relay 4 serves to trip a circuit-breaker 5 in the supply side when it is moved either way from a central position. The elements 6, 7 of the relay are energized in accordance with a negativephase-sequence component of the currents traversing the input and output circuits respectively. These compounds are obtained by connecting current transformers 11, 12 in two of the phases to two equal impedances 13, 14 and connecting the elements of the relay across the two impedances as shown. The impedance 14 may, in the example shown, be a resistance whereas the other is made up of resistance and reactance so that the voltage across it leads the current by 60‹. The two impedances 13, 14 are so connected to the polyphase circuit that the voltages across them are exactly opposite in phase under balanced conditions. In the event of a fault in any of the windings of the device 1 the relay will operate due to the resulting unbalancing of the negative-phase-sequence components but no operation is produced by a fault on the output circuit as this produces no unbalancing of components. In a modification in which circuitbreakers on either side of a transformer are operated in the event of any form of fault, the circuit-breakers are controlled by a zero-phasesequence differential relay energized by the vectorial sum of the three currents in current transformers in the three phases, a negative phase-sequence differential relay energized as described for the relay 4 above, and by overload-, negative- and zero-phase-sequence relays of non- differential type.