Patent GB-794370-A (GB)
Title: Improvements in and relating to protective systems for electric transmission lines
IPC: H02H1/00
Filed: 1955-11-29
Abstract: 794,370. Protective cut-out systems. GENERAL ELECTRIC CO. Nov. 29, 1955 [Nov. 29, 1954], No. 34171/55. Addition to 794,134. Class 38 (5). [Also in Group XXXIX] The protective system of the parent Specification is provided with auxiliary means for transmitting additional signals for telemetering and/or intelligence intercommunication, such as telephony signals, using the same carriercurrent frequency and transmission channel as used for the blocking signals of the protective system, the arrangement being such that the additional signals do not interfere with the operation of the protective system. Fig. 3 shows the arrangement of the control relay CR of the protective system with the addition of the auxiliary means AUX. The control relay comprises an oscillator 61 which in the event of a fault is caused to oscillate by a " starting " signal received over a conductor 40 from a ground fault starting relay or a phase fault starting relay (not shown). The output of the oscillator is applied by means of a coupling transformer 62 to the control grid 69c of an intermediate amplifier 69 which only conducts when the starting signal is applied to its screen grid 69d. The amplified alternating voltage from the amplifier 69 is further amplified by a transformer 70, push-pull amplifier 77, 78, and a transformer 79 and is rectified at 84 to produce a signal to energize the carrier-current transmitter (not shown) over conductor 86. If the fault is internal to the protected section of the power transmission line, a signal is received from a ground fault or phase fault tripping relay over a conductor 41 or 45 to sustain the oscillations of the oscillator 61. This signal is also supplied to the screen grid 94d of an amplifier 94 having its control grid 94c supplied by the oscillator 61. The amplified voltage output of the amplifier 94 is applied through a voltage doubler circuit 102 ... 105 to cathode follower triode valves 106, 107 which provide a tripping signal over conductor 110 for the associated circuit-breaker and is also applied through a voltage doubler circuit 101, 111 ... 115 to cut-off the intermediate amplifier 69 and the push-pull amplifier 77, 78 to stop the carrier-current transmission. The control relay also includes a transient blocking circuit including valves 121, 122 to prevent unwanted tripping due to a system disturbance when an external fault is cleared. The auxiliary means comprises a test switch 140 which when closed applies a positive potential over a resistance 141 and a conductor 142 to the input circuit of the oscillator 61 and the screen grid 69d of the amplifier 69 to start carrier-current transmission. Alternatively, a relay 145 may be energized by D.C. pulses so that the opening and closing of contacts 146 cause the production of a pulsating carriercurrent transmission for telemetering. By closing a switch 147, relatively low voltages are applied over a conductor 149 and a resistor 150 and a conductor 142 and a resistor 148 to the input circuit of the oscillator 61 and the screen grid of the amplifier 69. This causes the amplifier 69 to produce a voltage of relatively low amplitude resulting in the energization of the carrier current transmitted at a lower level than during fault conditions. Suitable modulating and demodulating means (not shown) enable the carrier-current to be used for telephone purposes. As the transmission of carrier current is used for blocking purposes and as the operation of a tripping relay cuts off the amplifiers 69, 77, 78 to stop the transmission, the use of any function of the auxiliary means cannot interfere with the normal operation of the protective system.