GB355834A - Improvements in protective relay systems for electrical networks - Google Patents

Abstract

355,834. Protective cut-out arrangements. ALLM€NNA SVENSKA ELEKTRISKA AKTIEBOLAGET, Võsteras, Sweden. April 3, 1930, No. 10580. Convention date, Aug. 7, 1929. [Class 38 (v).] Cut-out systems employing retarded relays for the protection of a line directly connecting two feeding points of a distribution system, which points are also connected by at least one more direct line and, if desired, by not more than one indirect line are arranged so that the interruption of any of the direct lines is effected with a different retardation for different lower directional conditions in other feeders or feeder. In the system shown in Fig. 2 in which two generating stations A, B are linked by parallel feeders I, II and supply a ring-main distribution system III graded time limit relays are placed around the ring-main system with release times as indicated. The busbar and feeder protective currents of station A are shown in Fig. 1 each line having an overload relay e and directional relay d, the latter having upper contacts d<1> completed when power in the corresponding line is flowing outwardly and lower contacts d<11> for inward power flow. Normally on overload relays e, d complete the circuit of a relay c which has quick-closing upper contacts c<1> and retarded (e.g. 1 second) lower contacts c<11>. If a short circuit exists in any line, e.g. I, the power flow in at least one other line, e.g. the parallel feed 2, will be towards the bus-bars to which both feeders are connected. Thus the lower contacts d<11> of the corresponding directional relay. d will be closed and a circuit for a quick-acting relay g completed. This prepares a circuit for the tripping coil b of the circuit-breaker a in feeder I, the closure of the tripping circuit being completed by the retarded contacts c<1> of the relay c corresponding to feeder I after one second's retardation. If however the fault is not actually in line I the circuit of relay g would not be energized and tripping would not occur until a relay f with a relatively large retardation (e.g. 5 seconds) had completed its contact-closing operation. In the case of a short-circuit in line III with reverse power flow in lines I, II neither of the latter feeders are interrupted as the reverse flow of current prevents completion of the circuits of the relays c at contacts d<1>, and the relay c corresponding to feeder III completes the circuit for the corresponding tripping coil b after the completion of the time setting of its contact c<11> (i.e. 4 seconds).