Patent GB-543684-A (GB)
Title: Improvements in electric protective arrangements for sectionalized supply systems
IPC: H02H7/26
Filed: 1940-11-25
Abstract: 543,684. Protective cut-out systems. BRITISH THOMSON-HOUSTON CO., Ltd. Nov. 25, 1940, No. 16914. Convention date, Nov. 25, 1939. [Class 38 (v)] In a protective system for a sectionalized supply system, on failure of one or more of the breakers of a faulty section to open and isolate the fault, the protective equipment of an adjacent section or sections is modified through the medium of fault-current direction means of the faulty section and after a time-delay, so as to cut out the adjacent section and thus isolate the fault. In the case where cutting out of a section on external fault is prevented by the injection of carrier currents into the section. the time-delay means associated with the faulty section acts to suppress the transmission of such currents in the adjacent section, so enabling the protective means of that section to operate. Fig. 1 shows, a system comprising stations 1 ... 3 with ring busbars sectionalized by means of breakers, such as 3CB1 ... 3CB4 in station 3. and connected to lines, as L2-3, between each adjacent pair of breakers. In the event of a fault at X in line L2-3, it is necessary to open breakers 2CB2, 2CB3 in station 2 and 3CB2, 3CB3 in station 3, and if, say, 3CB3 fails to open, it is necessary to open 3CB4 and the breakers in line L<SP>1</SP>3-4 adjacent station 4. Fig. 2 shows the equipment for the end of the section L2-3 at its junction with station 3 and the breakers 3CB2, 3CB3, the trip coils 10 of which are under control of parallel-connected contacts 21<SP>1</SP> of fault relays and contacts 21 of a receiver relay RR, and subsequently of contacts of a seal-in relay SSR. Relay RR is polarized and normally held open against spring-action 23 by a coil 24 in series with series-connected contacts 25. A further winding 26 thereof is energized from the receiver R and holds the relay open so long as the receiver receives from either the local or remote-end transmitter T. The transmitter T is normally biased against operation under control or normally-closed, series-connected contacts 19, but this bias can also be imposed by closure of contacts 19<SP>1</SP> of a directional relay PGD when power flows into the line from the busbars RB3, that is, the bias is removed for a fault at X, Fig. 1. Low-setting and high-setting, phase-fault detector relays LPD and HPD are connected to current transformer CT so as to carry the total current in the phases of the line, and the current coils 12 of the polyphase power element of relay PGD. The neutral lead carrying the earth-fault current includes the low-setting and high-setting earth-fault current relays LGD and HGD, and coil 14 of earth-fault power element of relay PGD. Coil 15 of the latter element is fed with the residual line-voltage over potential transformer PT, and voltage coils 13 are energized from transformer PT which also feeds co-operating restraining coils 16, 17, the restraint being removed on fault by operation of series-connected contacts 20 of relays LPD, LGD which also control contacts 19. If 3CB3 fails to trip, 3CB4 and the breakers in line L'3-4 must be tripped and so cut out the transmitter of the latter which otherwise is functioning, as the fault is external to it. For this purpose a pair of faultdirectional relays FR2a, FR3b are provided with residual current and residual voltage coils 28, 27 and the fault-power direction is in this case such that FR3b operates and energizes a time-delay relay TD3b which closes contacts 30 to apply a bias to the transmitter of line L<SP>1</SP>3-4 and isolate it. The full inter-trip circuits for the entire station 3 are illustrated in Fig. 3 (not shown). A further back-up protection, which operates in the event of the protective relays themselves failing to' operate, and disconnects the faulty line and, if necessary also, the adjacent line, is afforded by a residual current relay FR with contacts controlling the trip coils 10 directly and controlling the time-delay relays TD3b over contacts 29 of the fault-directional relays FR2a, FR3b, the relay FR having a longer delay than the relays TD3b so as to give the latter time to operate normally and being, if desired, of the inverse-time limit type. The relays FR2a, FR3b, may alternatively or additionally be phase-fault relays. Specification 378,006 is referred to.