GB621253A - Improvements in or relating to electric protective arrangements - Google Patents

Abstract

621,253. Protective cut-out systems. GENERAL ELECTRIC CO., Ltd., and FITZPATRICK, J. A. Feb. 11, 1947, No. 3968. [Class 38(v)] In a system for the protection of a polyphase bus-bar system, a relay trips all the circuit breakers of the feeders connected to the busbars under bus-bar earth fault conditions. In the circuit biased against incorrect operation on through-fault conditions. In the circuit shown, five threephase feeders 7-11 are connected to a bus-bar system 1 by circuit breakers 2-6. Each conductor of each feeder has a current transformer; the secondary windings of the three transformers 12-14 associated with each feeder being star-connected with their star point 15 connected to a bus-wire 16. The other sides of the secondary windings are connected to the tapped primary winding 18 of a summation transformer 17 and the end of this winding is connected to a further buswire 19. The current which exists in the bus-wires 16, 19, under earth fault conditions in the protected zone is the algebraic sum of the currents in the fifteen current transformer windings and this current effects the energization of the operating coil 25 of the relay 26. The secondary winding 28 of each summation transformer 17 is connected to a rectifier 29; the rectifiers 29 being connected to bus-wires 31, 32 across which is connected the restraining or biasing winding 30 of the relay 26, which is thus biased against operation in proportion to the total through-current of the bus-bar system. Switches 33, 34 operated at the same time as the circuit breaker with which they are associated, isolate the individual circuits when a feeder is not in use, and these switches may be selectors working in conjunction with additional bus-wires when the protective arrangement is applied to a duplicate bus-bar system. Resistances 35 are provided between the rectifiers 29 and the bus-wire 32, so that if a rectifier becomes faulty it does not prevent sufficient bias being obtained from the other summation circuits. Further current transformers and summation circuits may be used to operate a check relay. In Fig. 1 (not shown) the rectifiers 29 are connected in series with each other. Specifications 460,046 and 540,973 are referred to.