Patent GB-801008-A (GB)
Title: Improvements in and relating to protecting systems for electric power transmission lines
IPC: H02H3/40
Filed: 1956-04-11
Abstract: 801,008. Protective cut-out arrangements. GENERAL ELECTRIC CO. April 11, 1956 [April 11, 1955], No. 11033/56. Class 38 (5). A protective arrangement for a transmission line 11 ... 13 includes a selective impedance type electronic relay 29 which responds when an operating voltage derived from the line current is greater than a restraining voltage derived from the vectorial difference between the line voltage and a voltage determined by the line current. The relay may be arranged to block the reclosing action of a circuit breaker, or to block the operation of a tripping relay arrangement to prevent unwanted tripping of a circuit breaker 14 during surge conditions. Fig. 1 shows the latter arrangement and comprises a distance relay 19 which may be a high-speed "mho " type electronic relay arranged to trip the circuit-breaker 14 via a control relay 22 when permitted to do so by the relay 29. The relay 29 is also of the " mho " type and has an air gap transformer 30 with its primary windings connected to current transformers 15, 16 and a first secondary winding 30c connected to a voltage-limiting non-linear resistor 31 and an adjustably tapped loading resistor 33, the voltage appearing across the tapped portion having a fixed relation to the line current. The second secondary winding 30d is similarly connected to resistors 32, 34 but in addition a rheostat 35 is provided to obtain a phase shift between the operating voltage produced across the tapped portion of resistor 34 and the line current. A further transformer 36 connected to voltage transformers 17, 18 has its secondary winding connected in series with the resistor 34 so that the voltage across them is the required restraining voltage. These operating and restraining voltages are limited by resistors 39 ... 41 and 43, 44 respectively and are applied to rectifiers 38, 42 which are oppositely connected and produce a resultant D.C. voltage dependent on the difference between the operating and restraining voltages. When this voltage is adequate, an electronic level detector 49 oscillates and applies a signal to a valve 52 which conducts and in turn applies a signal to a triode 56 through a delay circuit 59, 60. Hence, after a short time delay sufficient to enable the relay 19 to cause the tripping of the circuit breaker 14 under true fault conditions, the triode 56 energizes an electromagnetic relay 54, 55 to open the tripping circuit and prevent tripping. The setting and overall operating time of the relay 29 is such that in the event of a surge it will operate before the surge current becomes of sufficient magnitude to operate the faster relay 19. Under true fault conditions, relay 19 operates first and applies a signal to a triode 57 which conducts and causes the bias voltage of triode 56 to rise so that the triode 56 cannot conduct and open relay 54. Figs. 3 to 5 (not shown) illustrate various " level detector " circuits and the Specification sets out fully the theoretical considerations underlying the invention. Specification 794,370 is referred to.