Patent GB-341986-A (GB)
Title: Improvements in or relating to electric protective systems
IPC: H02H1/00
Filed: 1929-10-29
Abstract: 341,986. Protective cut-out arrangements. REYROLLE & CO., Ltd., A., Hebburn-on-Tyne, and LEBEN, H., 8, Queen's Walk, Harrow, Middlesex. Oct. 29, 1929, Nos. 32822/29 and 14458/30. [Class 38 (v).] In an impedance &c. protective system having time-discriminating relays forming part of the system and being operative to cut out the system in the event of an interphase fault between two of the phases, lock-out relays are provided which when operated prevent incorrect cutting-out of the protected circuit by such time-discriminating relays on the occurrence of faults other than those for the clearance of which they are provided. As applied to the protective system described in Specification 341,985, three lock-out relays 230, 240, 250 are provided to prevent incorrect operation of the time-discriminating relays 50, 60 provided for dealing with interphase faults during the response of the earth-fault time-discriminating relay 40. The relay 230 is placed directly under the control of the earth-fault initiating relay 80 so that during the clearing of an earth fault the contactor 170 cannot be operated to bring into circuit an interphase fault time-discriminating relay even though the corresponding initiation relay may have operated. The second lock-out relay 240 has its contacts in series with those of the relay 230 and is set to open its contacts on a predetermined value of overload in phase 1 which value is so chosen that the relay will operate in the event of a fault on the remote side of a stardelta power transformer connected to the line beyond the protected section but will remain inoperative in the event of an interphase fault between phases 2, 3 of the protected line 1, 2, 3 itself. A further similarly arranged lock-out relay 250 is energized by currents in phase 3 and has normally closed contacts 251 controlling the energizing circuit of the D.C. contactor 160. In an alternative form of the above lock-out arrangement (Fig. 2, not shown), the three lock-out relays are replaced by two differential lock-out relays with coils operated by currents in phases 2, 3 and 1, 2 so that the contacts of these relays are closed only when currents in these phases are equal as in the case of interphase faults between them. The lock-out relay 250 may, alternatively, be retained instead of one of the differential lock-out relays. The differential lock-out relays are preferably provided with restraining forces varying with the load current as by being of the pivoted beam type in which the beam is actuated in opposite senses by two electromagnetic systems one of which tends to operate the relay and is actuated by the vectorial difference of the currents while the other tends to restrain operation and is actuated in a much smaller degree by their vectorial sum. The Specification also describes the application of this lock-out arrangement to impedance protective systems other than. the above and also arrangements in which the lock-out relays additionally fulfil the function of phase-selecting relays. In a further modification, also described, the lock-out relays, instead of actually preventing the operation of the interphase-fault-responsive time-discriminating relays during the clearing of earth faults, may vary the sensitivity of these relays as by shunting their current coils by suitable resistances so that if an interphase fault should occur during the clearing of an earth fault, the interphase fault relays may still have a limited degree of response to such a condition. Specifications 319,637 and 320,968, [both in Class 38 (v), Electric switches &c.], also are referred to.