GB1470389A - Telemetry systems - Google Patents

Abstract

1470389 Electric selective signalling GENERAL ELECTRIC CO Ltd 30 Sept 1975 [3 Oct 1974] 42978/74 Heading G4H [Also in Division H3] In a frequency shift modulation telemetry system for signalling faults on high power transmission lines, two transmitters 3, 4 normally transmit respective guard frequencies but on occurrence of a fault contact 1 closes and control circuit 2 causes transmitters 3, 4 to alternate, in synchronism, between their guard frequencies and respective trip frequencies, respective receivers 5, 6 produce trains of pulses of width dependent on the periods for which the respective trip frequencies are transmitted, delay timers 12, 13 delay the leading edges of the pulses thereby delaying and shortening them by a predetermined amount, and output circuit 9 gives an output signal if outputs from timers 12, 13 coexist. Timers 12, 13 reduce the risk of false alarms. Details of the control circuit 2 are given, Fig. 2 (not shown). It includes a modulator, switching circuits, a timing circuit and means to detect the absence of the guard frequency from transmitter 4. Three modes are selectable: (a) Modulation for at least 2 sec. and as long as contact 1 is closed, (b) Modulation for 2 sec. only even if contact 1 is closed for a longer or shorter period, (c) Modulation only when contact 1 is closed. The transmission medium may be pilot wires, the power lines or a radio link. A plurality of systems, using different frequencies may share the same transmission medium. Details of the timers 12, 13 and circuit 9 are shown in Fig. 3. Pulses from receivers 5, 6 enter at C, D and go to respective RC charging circuits connected to an AND gate TR15, TR16, output from which causes protective equipment to function. A latch TR17, D13, D14 holds the output until the circuit is manually reset by a pushbutton PB or automatically reset by persistence of the guard frequencies applied to E, F.