Patent GB-541172-A (GB)
Title: Improvements in or relating to electric protective systems
IPC: H02H3/28
Filed: 1939-11-27
Abstract: 541,172. Protective cut-out systems. REYROLLE & CO., Ltd., A., LEYBURN, H., and YOUNG, A. F. B. Nov. 27, 1939, No. 30888. [Class 38 (v)] Balanced protective systems for sections within which three or more feeders are connected together, employ trip relays which are electrically restrained against operation due to current-transformer differences on throughfaults, in such a way as to prevent cutting-out off non-fault current carrying feeders the relays of which may nevertheless be carrying unbalance current. In Fig. 1, two feeders are connected to sources and two to loads, the former being termed "incoming" and the latter "outgoing", but the invention is applicable to one or more of both kinds of feeder constituting the section. Current-transformers B are connected to summation transformers C, secondary windings D of which are in series in an opposedvoltage pilot circuit carrying primaries E of transformers supplying the operating windings G of trip relays for the incoming-feeder breakers. The transformers C of the outgoing feeders and of one of the incoming feeders have tertiary windings J connected in series in a further pilot circuit including the primaries of transformers feeding the restraining windings H of the relays, the windings J being so arranged that a fault entering the incoming feeder and leaving by the outgoing feeders produces voltages which reinforce one another. With this arrangement only one incoming feeder should have a tertiary winding, since otherwise, with a through fault confined to the incoming feeders; the restraining effects would cancel out. The return connection of the restraining pilot circuit is taken to the mid points M of windings E so as to traverse the operating pilot-leads in parallel, interference between the two being prevented by providing the secondaries D in two halves which are in opposition as regards the restraining currents. The relays are designed so that, on internal fault, any restraining effect which may be produced is overcome by the operating force. On external fault, any operating force that may be set up is overcome by the restraining force. Where it is impossible to avoid risk of restraining effects cancelling out owing to the division of the fault-current, as for example with three or more incoming feeders, the arithmetic suminstead of the vector sum, of the tertiary, winding currents is employed by connecting each of the tertiary windings to the pilot circuit by full-wave rectifiers, the restraining windings of the relays being connected directly in the pilot circuit, Fig. 2 (not shown). Rectification may also be employed for busbar protection using a current-balance system, two arrangements of which are outlined. In a further current-balance protective system for a busbar section, the secondaries of line-current transformers are connected in parallel with the operating winding, but each feeder has in the transformer-leads to that winding an auxiliary transformer primary, the secondaries being connected in parallel with the operating winding. The auxiliary-transformer ratios are not all the same, so that a restraining current will flow through the relay on through-fault. Alternatively, or additionally, the auxiliary transformers may have different phase-angles. Ordinary solid-core current transformers of low accuracy may be used.