Patent GB-374461-A (GB)
Title: Improvements in or relating to means for protecting sections of an electric line
IPC: H02H7/26
Filed: 1931-03-05
Abstract: 374,461. Protective cut-out arrangements. SIEMENS-SCHUCKERTWERKE AKT.-GES., Siemensstadt, Berlin. March 5, 1931, No. 6913. Convention date, March 6, 1930. [Class 38 (v).] Line sections are protected by currentbreakers at each end released on fault by trip coils A, A<1> each supplied with local current under the control at each end of an overload relay U in association with a directional relay R and a time-delay relay Z. Blocking relays H2, H4 are energized through pilot-wire connections (which may be telegraph or telephone wires) or by wireless link on closure of the overload or other initiating relay at the other end, and blocking relays H1, H3 are energized through the directional relays at their own ends. Protection is afforded (i) by tripping of the near-end circuit-breaker in the case of supply from the near end only and blocking out of the far end tripping means (ii) by tripping out both circuit-breakers when power flows into the section from both ends due to an internal fault (iii) by blocking the tripping means of both circuit-breakers for a through fault outside the section, the circuit-breakers being, however, tripped in this case if the outside fault persists for a predetermined time. The time relays may be graded from section to section, they may have a time characteristic dependent on the overload or may be resistance relays. Effective protection is afforded even if the pilot wires become discontinuous. With the arrangement shown, in the event of an overload in case (i) above, the source end overload relay U closes thereby energizing (a) the time-delay relay Z at that end and (b) the blocking relay H4 through the normally closed contacts of relay H1. The relay H4 opens and blocks the action of the tripping coil A<1> by opening a releasing circuit through the coil. This circuit is through the contacts of relay H4, and through the contacts of the directional relay R1 (closed only when energy is flowing into the circuit at this end). The relay R1 is made operative only when the first of the two delayed contacts z3, z4 closes. The contact arm of the directional relay assumes the contact position thereby completing a circuit through the still open contacts of relay H2 to energize the tripping coil A. The distant relay is unaffected. With an internal short-circuit (case (ii) above) both relays U operate, both blocking relays H2, H4 open at the instance of overload relays U at the other ends so that both trips are blocked for the time being. Contacts z1, z3 of the time relays close thereby bringing the directional relays into action. These both close on to their live contacts and (a) energize H1 and H3 thereby de-energizing H4 and H2 which close and remove the block on A1 and A and (b) complete circuits through the closed contacts of relays H2, H4 to energize the trip coils A, A1 and trip the circuit-breakers at both ends. With a fault outside the section (case (iii) above) both relays U close, both releasing coils are temporarily blocked by the opening of relays H2, H4, the block on the far trip is removed by the closure of the directional relay at the supply end but the release is not effected at this end because the directional relay thereat does not close. Moreover, the supply end trip is not energized because the blocking relay H2 is open at the instance of the overload relay U at the far end. When, however, at the end of a further reserve time the main delayed contacts z2, z4 of the time relays close, the trip at the source end is energized through the directional relay at that end but, as the directional relay at the far end does not close, the trip at the far end is not energized. If the pilot wires become broken, the system is still left with overload or graded overload protection and, to safeguard against incorrect operation if they become shorted, fuses S<1>, S<2> are arranged to open-circuit them in that event. The pilot wires may be fed through transformers with an independent current supplied by relays only on the occurrence of a fault in the main supply conductors.