Patent GB-407454-A (GB)
Title: Improvements in or relating to electric protective arrangements
IPC: H02H3/033
Filed: 1932-11-10
Abstract: 407,454. Protective cut-out systems. ELECTRICAL IMPROVEMENTS, Ltd., Carliol House, POYNDER, H. B., 14, Haldane Terrace, both in Newcastle-on-Tyne, and LISLE, G. S., 11, Lavender Gardens, Low Fell, Gateshead, Co. Durham. Nov. 10, 1932, No. 31785. [Class 38 (v).] Relates to the protection of substations wherein a number of outgoing feeders B are supplied from an incoming feeder A having lower current-carrying capacity than the sum of those of the feeders B, such conditions being met, for instance, in undeveloped districts where the transformer supplying A is not necessarily large enough to supply the total permissible load on the feeders B. To avoid the provision of overload protection for A, with the consequent disconnection of all the feeders B if such protection operates, each feeder B has overload protection, and a device operates when the load on A exceeds a given value to inject an additional current into the trip circuit of one or all of the breakers E of the feeders B to cut out that feeder or feeders even though they may not be overloaded (the load being as measured by their current transformers C). Each trip coil D (Fig. 1) is normally shunted by a fuse F, busbars G, H and a contact L, which also serves to short-circuit an autotransformer K (or a separate winding thereon) supplied from an indicating or metering transformer J in feeder A. The contacts L may be controlled from an instantaneous or time-limit relay M, or by a thermal device on the transformer supplying A. Normally, therefore, each feeder B has simple overload protection, but when A is overloaded, contacts L open and currents are injected from K into all the trip coils D in parallel and in a direction to increase the current from transformers C. The tapping of K is so chosen that, with the minimum overload on A, the current flowing in each coil D will be enough to cause tripping. While the system is in theory not discriminative, since equal currents flow through the trip coils irrespective of the load-distribution among the feeders, in practice mechanical differences would probably cause one feeder B to be cut out first, so that the remainder stay in. This arrangement can be modified to obtain discriminative operation by arranging for the trip current to be injected in the coils D in series as in Fig. 3, in which relay M controls contacts R, one set for each feeder B, each set normally short-circuiting the appropriate coil D, and all three normally short-circuiting K. Thus the tendency is for the most heavily loaded feeder to be cut out, leaving the rest in circuit, but a similar effect could be got by arranging for the contacts R to open in sequence, with a brief time-interval between. In Fig. 2, thermal devices N control contacts 0 (as described in Specification 389,893) in series with resistances P shunted across the coils D, the shunt circuits being normally completed by contacts L and busbars G, H. A modified means of operating contacts L (which is applicable also to Fig. 1) comprises the heating element Q of a thermal device operating similarly to those N. If feeders B were all equally loaded (in relation to their current transformer ratings) the current injected at the minimum overload in A would be enough to trip all the breakers. When, as is usual, the loads on B are unequal, the effect of the resistances P is to cause only the most heavily loaded to be cut out, so that the arrangement is virtually discriminative. The thermal devices Q, N, may be constructed as described in Specification 383,017, and may be applied to a system of coils which are series-connected for injection purposes. The arrangements of Figs. 1, 2 are also described and illustrated in their application to polyphase circuits, Figs. 5, 6 (not shown), and the Specification also describes and illustrates a single-phase, three-wire system equipped with an arrangement involving thermal devices, Fig. 7 (not shown).