Patent GB-378037-A (GB)
Title: Improvements in or relating to fault-responsive electric protective systems
IPC: H02H3/353
Filed: 1931-05-01
Abstract: 378,037. Protective cut-out systems. WESTINGHOUSE ELECTRIC & MANUFACTURING CO., East Pittsburg, U.S.A.- (Assignees of Tippett, L. S. C.; 11, Castlereagh Street, Sydney, Australia.) May 1, 1931, No. 12884. Convention date, May 1, 1930. [Class 38 (v).] A protective system devised more particularly for clearing single-phase earth faults in a polyphase supply has voltage relays arranged to respond to sequence-voltage components of the line voltages which are a maximum when the fault is close to the sequence-voltage network and which become smaller with an increase in distance of the fault, whereby the relay will respond whenever such fault occurs on the line within a predetermined distance from the network. The derived voltage may be equal to the zero-phase sequence component of the line voltage plus twice the negative phase-sequence component. An additional voltage component may also be applied for causing the resultant relay voltage to be substantially unaffected by variations in the earth resistance. The mathematical principles underlying the operation of the system are fully set forth in the Specification. In the circuit arrangement as shown in Fig. 1 the voltage relays 18, 19 are fed through star-connected voltage transformers 9 with earthed neutral, the relay windings 16, 17 being connected across two phases of the secondary windings in series with a condenser 21. An additional voltage component applied to the relay windings is obtained between the third phase and earth through the transformer 23, the secondary circuit 25 of which includes a resistance 26 and an inductance 27, the latter providing compensation for the effects of variable ground resistance. The relays 18, 19, which are intended to respond only to single-phase ground faults, may be combined as shown with an impedance relay system of the type shown in Specification 377,975, the latter being intended to respond to interphase faults. In this combined arrangement the sequence-voltage relays 18 are associated with the instantaneously-acting impedance relays 44, 45, 46 and the relays 19 with impedance relays 41, 42, 43 and a timing motor 36. In order to prevent the tripping of the circuitbreaker 32 in response to the picking up of the relays 18, 19 in the case of an interphase fault, the relay contacts are arranged so that the ground-detector relays are only operative provided no response of the interphase relays has occurred. The relays 19 which provide "backup" protection to the relays 18 in a manner similar to that provided by impedance relays 41, 42, 43 to relays 44, 45, 46 are for this purpose provided with more turns than the relays 18 so as to respond to lower values of voltage, the balance-points of relays 19 being thus situated at positions more distant from the relaying point than those of relay 18. An instantaneously-acting energy-directional relay 61 is provided so that tripping of the circuit-breaker occurs only with flow of energy in a predetermined direction. The voltage winding of the directional relay 61 is fed by a zero-phasesequence voltage derived from tertiary windings 66 on the transformer 9 and the current winding is by zero-phase-sequence current derived from transformers 64. Delta-connected currenttransformers 53 feed the current windings of the impedance relays.