Patent GB-654913-A (GB)
Title: Improvements in or relating to protective relaying systems
IPC: H02H3/30
Filed: 1948-11-10
Abstract: 654,913. Protective cut-out arrangements. WESTINGHOUSE ELECTRIC INTERNATIONAL CO. Nov. 10, 1948, No. 29244. Convention date, Nov. 28, 1947. [Class 38(v)] In a three-phase transmission system, single pole circuit-breakers in each phase at each end of a protected line section are controlled so that in the event of a phaseto-earth fault within the protected section only the circuit breakers in the faulty phase are tripped. In the event of interphase faults in the protected section, only the circuit-breakers in the faulty phase or phases are tripped, Fig. 1, or alternatively all the circuit-breakers are tripped, Fig. 2 (not shown). The equipment at one end of the protected section is indicated in the upper part of Fig. 1 and comprises a phase comparison carrier-current arrangement 14 such as that of Specification 651,998, which compares the phase angles of the fault currents at both ends of the section and in the event of an internal fault closes contacts HKB; an earth fault phase-sequence-segregating phase-selecting relay arrangement 12 having contacts SA, SB, SC; and an interphase-fault phase-selecting impedance relay arrangement having contacts ZA, ZB, ZC. The lower portion of Fig. 1 shows the tripping circuits for the trip coils TC-A, TC-B, TC.C of the single pole circuit-breakers CB-A,. CB-B, CB-C, the contacts of the various relays being shown in the positions they occupy when the relays are de-energized. Earth faults: In the event of a single phase-to-earth fault within the protected section, for example on line conductor A, the contacts HKB are closed by the internal-fault-detecting carrier-current equipment. The A-phase earth fault selector relay actuates its contacts SA and completes a circuit from a positive D.C. supply through contacts HKB, SA, SC, X3, connection 21, contacts CSB, connection TA, trip coil TC-A and the closed auxiliary circuit-breaker contacts a A to a negative D.C. supply, to trip the circuit-breaker CB-A. Interphase faults : In the event of a phase-to-phase fault within the protected section, for example between phases B and C, the impedance relay responsive to a fault between these phases, closes its contacts ZB and the carrier-current equipment closes contacts HKB to complete a circuit through contacts HKB, ZB, conductors 25, 25B, TB, trip coil TC-B to trip the circuitbreaker CB-B to isolate phase B. Two-phases-to-earth-faults : In the event of this type of fault, such as between phases B and C and earth, the closure of the contacts HKB, ZB cause the tripping of the circuit-breaker CB-B as above. The closing of the contacts ZB also causes the energization of an auxiliary relay CSB which opens its contacts CSB to remove a short circuit from a further relay CSX. The operation of the earth fault phase selecting relay arrangement 12 is such that it is the unfaulted phase selector SA which responds, the phase B and C selectors not responding. Thus the relay CSX is energized and closes its contacts CSX to connect the three upper trip circuits together so that trip coil TC-C is energized, for example, through contacts HKB, SA, SC, X3, CSX, CSA and aC, to trip the circuitbreaker CB-C. The circuit-breaker CB-A is not tripped, as the impedance of the relay CSX is such that the trip coil TC-A is in. sufficiently energized to trip the circuitbreaker when the contacts CSB are open. Three-phase faults : This type of fault within the protected section causes the tripping of all three circuit-breakers as all the contacts ZA, ZB, ZC will be closed. A delayed-action relay X3 with its three contacts X3 is provided to prevent incorrect tripping due to a momentary incorrect operation of one of the earth-fault phase-selecting relays SA, SB or SC due to transients following the opening of a circuit-breaker to clear a fault, the relay X3 opening its contacts a short time after the closing of the contacts HKB. In Fig. 2 (not shown), the phase-fault phase-selecting impedance relays have their contacts arranged so that all three circuit-breakers are tripped in the event of a phase-to-phase fault. In this embodiment these impedance relays may be of a modified type capable of adjustment to render them insensitive to nearby severe earth faults. This feature is fully described in the Specification with the aid of vector diagrams (Figs. 3 to 5, not shown).