Patent GB-352500-A (GB)
Title: Improvements in or relating to electric protective systems
IPC: H02H1/00
Filed: 1930-04-10
Abstract: 352,500. Protective cut-out systems. REYROLLE & CO., Ltd., A., Hebburn-on- Tyne, and LEBEN, H., 8, Queen's Walk, Harrow, Middlesex. April 10, 1930, Nos. 11393 and 27456. [Class 38 (v).] In the protection of transmission lines by means of distance relays the time of operation of which is proportional to the impedance of the line, phaseselecting means are employed to ensure that the relay coils are energized from the faulty phase or phases. The phase-selecting means comprises three phaseselecting relays B, C, D of the electromagnetic " lift-and-fall type. Each is energized (in an earth-fault arrangement, Fig. 1) in accordance with the earth voltage on the several phases through the voltage transformer E, the contacts of the relays being interconnected (for example, the normally closed upper contacts B<1> of relay B with the lower contacts C<2> of relay C which are closed due to the low voltage or relay C on fault) so that the operation of each relay is rendered ineffective in the event of the simultaneous operation of the next relay in a predetermined phase-sequence. The phase-selecting relays may directly control the energizing circuits of the time-discriminating distance relay but, in the arrangement shown, they energize auxiliary D.C. contactors G<1>, G<2>, G3 from an auxiliary supply F and through an initiating relay H which closes on the occurrence of an earth fault. The system may be adapted to deal with interphase faults by energizing the phase-selecting relays from the interphase voltage derived from the secondary of a voltage transformer in the line, but, in this case, each relay is provided with a further set of normally-open contacts connected in series in an alternative energizing current for one of the auxiliary D.C. contactors in order to provide for a three-phase fault. The earth-fault and interphase fault arrangements described may be employed in combination, but the earth-fault arrangement may be associated with the interphase fault-selecting arrangement described in Specification 341,985 in which differential or balanced-type selecting relays are used. The timediscriminating or distance relay is preferably that described in Specification 319,637, [Class 38 (v), Electric switches &c.], having current and voltage coils and a variable resistance driven by timing mechanism in the circuit of the voltage coils. In this case, the connections of the distance relay to the appropriate phase are controlled by contacts on the three D.C. contactors which have additional contacts to control the tripping circuit and for starting up the timing mechanism controlling the variable resistance. The system is dependent on the normally occurring drop of voltage on the faulty phase and a resulting increase of voltage on the leading phase. Where the sequence is other than this the phase-selecting relay contacts are connected in the opposite phase-sequence. The system as above described is designed for a circuit fed by a generator having its neutral point earthed through a resistance. In the case of a supply from a generator directly earthed, the arrangement is modified so that the fault current is utilized to assist in preventing the operation of the relay in the next phase sequence. This arrangement is shown in Fig. 4, in which each phase-selecting relay is provided with an auxiliary winding B<4>, C<4>, D<4> energized by the current from a current transformer Q<1>, Q<2>, Q<3> in the phase associated with the preceding relay in the sequence.