GB443218A - Improvements in or relating to electric protective arrangements - Google Patents

Abstract

443,218. Protective cut-out systems. REYROLLE & CO., Ltd., A., Hebburn-on- Tyne, LEESON, B. H., Waverley, Tynemouth, LEYBURN, H., 212, Wingrove Road, Newcastle-on-Tyne, and FREELAND, C., 74, Hawthorne Avenue, Cleadon Park, South Shields. Aug. 22, 1934, No. 24208. [Class 38 (v)] In a Merz-Price protective system, ordinary solid-core type, main current transformers B .. b2, accurately balanced against one another up to a predetermined relatively low overload value (such as twice normal load), are connected to the pilot circuit D, D<1> through auxiliary transformers C, c designed to become saturated at an overload value not greater than that to which the transformers B .. b<2> are balanced, and tuning means is provided in association with each protective relay E, e in the pilot circuit for preventing operation of the relay except by currents at or near the fundamental system frequency. The transformers B &c. are connected to tappings on the primaries of the transformers C, c so as to afford discrimination between earth and interphase faults, and the secondaries C<2>, c<2> are connected in opposition so that normally no current flows in the pilot circuit. The relays E, e are shunted by condensers G, g of such capacity that currents of fundamental frequency flow through the relays, whilst those of higher frequencies leave the relays unaffected. With heavy straight-through faults, the pilot circuit will only be traversed by capacity currents and by unbalance currents, which comprise a small percentage of fundamental-frequency current and a larger percentage of third and higher harmonics set up by the saturation of C, c. The latter currents are byepassed, and the former are insufficient to operate the relays and in some circumstances may even be arranged to decrease with increased primary load. Further stability may be provided by restraining coils E<2>, e<2> energized either as shown or from the secondaries C<2>, c<2>, a resistance being added where necessary to provide sufficient impedance. Where it is desired to earth the relays, they may be transformer-connected to the pilot circuit, the shunts being placed across the primaries of the transformers, Fig. 2 (not shown). When applied to a "split-pilot" protective system, Fig. 3 (not shown), with a connection between the mid points of the unsplit pilot and one of the split pilots, the condensers are connected across the split pilots at each end and the restraining coils may be directly or transformer-connected in the pilot circuit or energized as shown in the Figure. Condensers may also shunt relays connected in a pilot lead across equipotential points of a circulating - current system fed from two auxiliary transformers.