GB547260A - Improvements in bus-bar protective systems - Google Patents

Abstract

547,260. Protective cut-out systems. BRITISH THOMSON-HOUSTON CO., Ltd. March 21, 1941, No. 3855. Convention date, March 22, 1940. [Class 38 (v)] In a protective system for busbar sections wherein the input to the bars is balanced against the output in a comparison circuit, and an additional safeguard against operation by external faults due to current-transformer saturation is provided by a relay in the neutral-to-earth connection of apparatus connected to the bars so that operation of the system depends on occurrence of an earth fault, the latter relay is arranged to discriminate between single and double line-to-earth faults and . to prevent operation by the latter, the inter-phase component of which may cause saturation and hence unwanted tripping. Since the external single line-to-earth fault current is limited by the impedence of the earth circuit, operation of the system on such earth faults is precluded, whereas, with the usual isolated-phase busbar arrangements, the possibility of a double lineto-earth fault in the busbar section is very remote. The busbar section 13 is supplied by a generator 19 and transformer 21 and feeds the circuits 27 ... 29. Current-transformer groups 37, 40 in the outgoing and supply circuits have overload coils, as 36, and earth-leakage coils, as 35, capable of tripping the associated breakers 22, 30. These breakers, together with the busbarsectionalizing breakers 15, 16, can also be tripped under control of a hand-reset relay, the coil 51 of which is in a circuit containing contacts 49, 57, controlled by relays 41, 56, respectively, in the balancing circuit and neutral-earth circuit 25 of the generator 19. The individual leakage circuits of the current-transformer groups 37, 40, and the similar groups 34 in adjacent busbar sections 12, 14, include the transformers 42, 46, 45, the parallel-connected secondaries of which supply relay coil 41, which is thus responsive to unbalance due to internal faults or to currenttransformer saturation on heavy external faults. It is shown in the Specification that only half as much current flows in the earth-circuit 25 on double as on single line-to-earth faults, hence the relay 56 is arranged to operate on a little over half the maximum earth-current that can flow in circuit 25, so providing the necessary discrimination. High-impedance, single-phase, short-circuits to earth on the section 13 would not operate relay 56, therefore such faults are disconnected after a time-delay by a low-setting relay 59 having contacts 60 in parallel with those 57. Where the busbars are not of the isolatedphase type, additional interphase fault relays may be provided. Where the resistor 26 is replaced by a reactor and the D.C. component, due to the larger time-constant of the circuit, assumes large value, relay coil 56 is supplied through a 1:1 air-gap current transformer, so as to eliminate that component, Fig. 2 (not shown).