Patent GB-651998-A (GB)
Title: Improvements in or relating to protective relaying systems
IPC: H02H3/30
Filed: 1948-06-23
Abstract: 651,998. Protective cut-out systems. WESTINGHOUSE ELECTRIC INTERNATIONAL CO. June 23, 1948, No. 16865. Convention date, June 30, 1947. [Class 38(v)]  In a system for protecting a section of a three-phase transmission line 1 in which carrier currents are used to effect a comparison of phase angles of the line currents at the two ends of the section, a pair of alternately firing gas-filled valves V1, V2 at each end or station produce square topped waves during alternate half cycles of the line current, one valve Vl being used to cause the transmission of restraining impulses to the opposite end during alternate half cycles, and the other valve V2 being used to apply operating impulses to a tripping relay R during the other alternate half cycles. The arrangement is such that on through faults the tripping relays R at both ends are restrained from operation, correct operation of the system being ensured by the use of two fault detecting relays FD1, FD2 at each end, one FD1 a low-set relay controlling the operation of the valves and the other FD2, a high-set relay controlling the operation of the tripping relay R. The system comprises at each end of the section, current transformers 4 supplying a network or filter HCB, which derives a single-phase component which after rectification is applied to the relays FD1, FD2. This component is also used to control the two alternately firing valves V1, V2, which are equally sensitive but otherwise function in the manner described in Specification 609,869. The impulses produced by valve V1 appear as voltage drops across a resistor R4 and control the operation of the transmitter, which comprises an oscillator OSC, amplifying valves A, A2, and an output transformer OT coupled to the transmission line C. The impulses produced by valve V2 appear as voltage drops across a resistor R5 and are applied by means of a conductor 22 and resistors R14, R12 to the grid G5 of a relay valve RT, which controls the operation of a tripping relay R having its contacts 48 in trip circuit of a circuitbreaker 3. The receiver comprises a coupling transformer RCT and a valve REC. which applies a restraining potential to the grid G5 of the relay valve RT by means of a voltage-doubling circuit 40 which operates in the manner described in the above Specification. On the occurrence of a fault, the relay FD1 operates first and in opening its contacts 25, removes a short-circuit from the valves V1, V2 which commence to fire alternately. The opening of the contacts 25 also removes a short-circuit from an auxiliary relay K to open a telemetering circuit having a key TM. The subsequent operation of the relay FD2 closes contacts 46 to complete the anode circuit of the relay valve RT, so that if the operating impulses applied to the grid G5 by the valve V1 during alternate half cycles of the line current are not opposed by the impulses received by the receiver and applied through the voltage doubler 40 to grid G5, then the valve RT conducts and energizes the relay R to cause the tripping of the circuit-breaker, this tripping operation occurring in the event of an internal fault in the protected section. The delay beween the operation of the relays FD1 and FD2 is provided to enable the transmitting and receiving circuits at each end to become fully effective before the tripping relay is rendered effective. U.S.A. Specification 2,408,868 is referred to.