Patent GB-284771-A (GB)
Title: Improvements in electrical protective arrangements for alternating current systems
IPC: H02H3/30
Filed: 1926-11-03
Abstract: 284,771. Brown, W. J., Field, A. B., and Metropolitan-Vickers Electrical Co., Ltd. Nov. 3, 1926. Cut-out arrangements. - For protecting polyphase transmission lines use is made of one or more of the balanced phase-sequence components, (particularly the negative phase-sequence component) into which any polyphase system unbalanced due to a fault, may be resolved, and this component together with, for example, the zero phase-sequence component (corresponding to the net star-point current) is extracted from the system at each end. A comparison between the values at each end is effected through pilot wires. Merz-Price fashion, or through a wireless or wired-wireless link in the manner described in Specifications 252,036, 252,810, 254,770, and 254,771. A differential protective arrangement using the phase-sequence components and applied to a translating device, for example, a transformer is described in Specification 258,904. The theory underlying the present arrangement as applied to a transmission line according to the invention is set out in the Specification. The arrangement at one end of a three-phase line is shown in Fig. 10. Three current transformers 11, 12, 13, star-connected, supply a wheatstonebridge arrangement of equal non-inductive and inductive resistances R, Z, the inductance Z being so chosen that the current through it, at the particular frequency of supply lags 60‹ behind the voltage. The current at the star-point connection 14 passes through an auto-transformer 15 and two-thirds of the current passes to the point 18 of the bridge and one-third to the opposite diagonal point 17. The voltage V between the points 17, 18 (or between the points 14, 17) is a measure of the negative phase-sequence component. The voltage across the ends 14, 22 of the impedance 21 in the star-point connection is a measure of the zero phase-sequence component. The points 14, 17, 21 may be connected to the corresponding points at the other end of the feeder by pilot wires through protective relays which isolate the feeder on fault. The arrangement necessary when employing a wired-wireless link instead of pilot wires is shown in Fig. 13 for one end only of the feeder and for the negative phase-sequence component at 17, 18 only. The zero phase-sequence component at 14, 22 acts through separate apparatus, not shown, similar to that shown for 17, 18. The valve 33 having tuned back-feed reaction supplies the carrierwave through the capacity coupling 41 when it is energized from the source 38. The supply circuit from the source 38 is through one or other of two relays 31, 32 controlled by the circuit 17, 18. The first relay is normally closed and is opened when the fault-indicating component increases to a certain maximum; the other is normally open and closes on fault. When the fault-indicating current from 17, 18 increases, relay 31 opens and the supply to the anode of the valve is withdrawn and the carrier wave ceases. At the same time, back contacts on the relay short-circuit the tuning condenser 37. At a slightly greater value of the fault indicating current, relay 32 closes and restores the anode supply but the frequency of the carrier-wave is now reduced by an amount F. At the other end the frequency on fault conditions (as indicated by the current 17, 18 there) is arranged to be reduced by 2F. Two relays 31, 32 are employed instead of one to give a time-interval to allow both ends to assume a settled condition before the actual comparison is made. A second contact 39 on the relay 32 applies a voltage from 17, 18 to a quenching device 40 which causes the carrier-wave to be supplied only during part of the cycle of the voltage across 17, 18, as described in Specification 254,711. The two carrier-waves, now of slightly different frequencies, are received by the amplifier 43, the sum or difference beat frequency is detected at 45, the beat frequency F is amplified at 46 (or the frequency 2F at 47), rectified at 48 or 49 to operate the cut-out relay 50. In a modification, the carrier wave imposed on the section at the near end has a permanently different frequency from that at the far end. The quenching action is instituted by relay 32 and the operation of the cut-out relays is dependent on the existence of a line frequency pulsation instead of a beat. Specification 284,773 also is referred to.