Patent GB-811337-A (GB)
Title: Improvements in or relating to protector arrangements for power distribution apparatus
Inventor: RAYNER THOMAS ROBERT
IPC: H02H7/26
Filed: 1956-04-07
Abstract: 811,337. Protective cut-out systems. AUTOMATIC TELEPHONE & ELECTRIC CO. Ltd. April 7, 1956, No. 10694/56. Class 38 (5). In a protective arrangement for a polyphase power network, carrier current signals are employed for separately determining the phase difference between the currents at the two ends of each of the phase conductors in the protected section so as to effect the tripping of circuit-breakers at the two ends if the phase difference between the currents at the two ends of the phase conductors is greater than a predetermined amount. In the drawing which shows schematically part of the equipment at one end of the protected section, the output of a current transformer CT associated with phase Y is applied through a squaring circuit 10 to a modulator 11 to modulate a carrier current supplied by an oscillator 12, the modulated carrier current being amplified and filtered at 13 and 14. Similar arrangements are associated with the phases B and R and the three modulated carrier currents are of different frequencies and are applied to a further modulator 17 where group modulation of a higher frequency carrier current from an oscillator 18 is effected. When a possible fault is detected by a fault detector FCD, the detector removes a block at a gating circuit 21 and the group modulated carrier current is transmitted to the distant end via a filter 19, amplifiers 20, 22 and a hybrid connection 23. The incoming group carrier is received via 23 and together with a portion of the outgoing group carrier supplied through a pad 27, is applied to a demodulator 28. The resulting individual modulated carrier currents are separated at filters such as 30 and the signals associated with each conductor are restored to power frequency in a detector 34 before application to a comparer COMP. The comparer may comprise a normally conducting thermionic tube with an operated comparison relay in its anode circuit. Signals from the detector 34 are applied as a negative bias to the grid of this tube which is arranged so that if the signals from the two ends are in antiphase the tube is cut off and the relay releases to cause the appropriate circuit-breaker to be tripped. It is stated that as the comparison is made on an individual conductor basis there is a wide margin of at least 100 degrees in phase differences between faulty and non-faulty indications. The detector FCD may effect a changeover from a monitor relay to the comparison relay when a possible fault is detected. Intertripping equipment is provided and arranged so that the tripping of the three circuit breakers at one end causes the generation of a coded signal at INT, this signal being used at a modulator 37 to modulate a fourth low-frequency carrier current. This inter-trip carrier current is combined with the three phase-comparison carrier currents in the group modulator 17. At the distant end the inter-trip carrier current is separated at filter 33 and supplied to a demodulator 43, the resulting signal causing the unit INT to trip the circuit-breakers.