GB788015A - Improvements in or relating to power line fault protection systems - Google Patents

Abstract

788,015. Protective cut-out arrangements- MARCONI'S WIRELESS TELEGRAPH CO., Ltd. May 22, 1956 [Aug. 30, 1955], No. 24906/55. Class 38 (5). To protect a section of a power line, radio means at each end of the section transmit a frequency modulated carrier wave modulated in dependence upon the phase conditions at the associated end and having a carrier frequency different from that transmitted from the other end, the equipment at each end including means for mixing the received and transmitted modulated carrier waves, the resultant of the mixing being used to prevent tripping of the section circuit-breakers under external fault conditions. Fig. 1 shows the equipment at one end of the protected section, the equipment including low-set relays 18 . . . 20 and high-set relays 15 ... 17. In the event of a fault one or more of the relays 18 ... 20 closes contacts to complete a circuit for the transmitter and one or more of the relays 15 ... 17 closes contacts to energize a timed relay 9. At the same time a voltage component produced by a summation current transformer 27 and non-linear resistor 28 is applied to a limiter circuit including diodes 29, the output of which is applied to a modulator valve 30 to modulate the frequency of a crystal-controlled oscillator 31. The frequency modulated output of 31 is frequency multiplied at 33, amplified at 34 and fed through a transformer 35 to a directional aerial 36. This aerial is also used for reception and feeds through a coaxial line filter 37 and amplifier 38 to a mixer 39 which is also fed through condenser 40 from the output circuit of the transformer 35. The output of the mixer 39 is of intermediate frequency with a frequency deviation dependent on the phase relationship of the summation transformer outputs at the two ends and is amplified at 41 before being fed to a linear detector circuit. The output of this circuit is of line frequency and of an amplitude dependent on the phase relationship. If the fault is external, the output of the detector circuit is sufficient to energize a relay 11 which opens its contacts 10 before the contacts 8 of the timed relay 9 can close and so prevents the energization of the associated circuit-breaker trip coil 7. Fig. 2 (not shown) illustrates a modification in which a resistive network takes the place of the summation transformer.