Patent GB-109009-A (GB)
Title: Protective Devices for Alternating Current Electric Systems.
Inventor: MCCOLL ALBERT EDWARD
IPC: H02H7/26
Filed: 1916-07-20
Abstract: 109,009. McColl, A. E. July 20, 1916. Protective arrangements. - Relates to the protection of parallel feeders or circuits of single-phase or polyphase systems at the end adjacent to the source of supply, and consists in the use, for this purpose, of relays which act both as balanced and overload relays. In the arrangement shown in Fig. 1, in which the relays provide instantaneous overload protection, each relay comprises an operating-coil A' and a balance coil B<6>, the iron core of the balance coil being of slightly smaller crosssection than that of the operating-coil, but wound with a larger number of turns. The relays are connected in circuit, as shown, and normally rest upon the contacts C<6>. A balancing-lead J<6> connected to equipotential points on each relay circuit normally carries no current. When, on the occurrence of a fault, the balance is disturbed, part of the excess current in the faulty feeder flows by way of the balancing-lead through the balance coils of the relays on the sound feeders, and the relay on the faulty feeder operates to trip its circuit-breaker. With a balanced overload, the relays operate because the cores of the balancing-coils become saturated at a lower value of current than the cores of the operating-coils, thereby disturbing the balance within the relay. In the case of a fault, the last feeder is cut out by reason of the different magnetic saturations of the cores. Switches L', when moved to the ' off ' position, convert the relays to overload protection only. In another arrangement, cores of equal size are used, and the relays are adapted to deal with balanced overloads by short-circuiting some of the turns of the operating-coils by fuses, leaving the restraining-coil with, say, a 10 per cent restraint, which is overcome when the balance between the feeders is disturbed to the extent of 10 per cent. with sustained balancd overloads, the fuses are blown and the relays operate, the fuses affording a time-element action. A circulating current system is shown in Fig. 4, in which the current transformers T<8> in the feeders are connected in series, the operating-coils of the relays are connected in series, and the restraining-coils also are connected in series. Leads J<8> connect equipotential points. Fuses F<8> in circuit with the restraining-coils, provide for the time-element balanced overload protection. Instead of fuses, cores having different magnetic saturations, as described with reference to Fig. 1, may be employed to give instantaneous balanced overload protection. The relays described above may be of the pivoted-beam or other balanced form. The arrangement, for wattmeter-type reverse-current relays is shown in Fig. 6, in which relays having three elements R<1>, R<2>, R<3> are employed, the function of the third element R<3> being to give overload protection. The elements R<2> may constitute the moving coils, and they are connected across the current transformers. The field coils R<1> are connected to the balancing-lead J<10>. The field coils R<3> are connected in series with coils R<2> and in such relation thereto that the relay always tends to operate when current flows in the coils R<3>, which, however, are normally shortcircuited by the fuses F<11>. In the event of a fault, current flows in the equipotential lead J<10> and operates the relay on the faulty feeder and restrains that on the 'sound feeder. Sustained balanced overload blows the fuses F", and the relays operate to cut out the feeders. This arrangement may be modified to constitute a circulating current system. The Provisional Specification describes also means for applying a percentage restraint, by the use of an additional winding, to the arrangement shown in Fig. 6, and to the arrangement based on Fig. 6 using a circulating current system. Specification 28868/11 is referred to.