Patent GB-641303-A (GB)
Title: Improvements relating to fault detecting devices for electric circuits
IPC: H02H3/44
Filed: 1948-02-25
Abstract: 641,303. Protective cut-out arrangements. HODGKISS, J. W., WILSON, J. S., and METROPOLITAN-VICKERS ELECTRICAL CO., Ltd. Feb. 25, 1948, No. 5647 '[Class 38 (v)] A protective arrangement has a transductor which has its A.C. winding energized by a current proportional to the load current in the circuit to be protected, and the impedance of this winding is varied inversely in proportion to the load current by the D.C. control winding, which is excited by a current proportional to the load-current through a time delay network, so that the normally constant voltage across the A.C. winding momentarily increases when a sudden change in the current occurs under fault conditions, the increase in this voltage being used to operate a protective relay or equipment. Fig. 2 shows the arrangement as applied to a single phase circuit and in which the A C. winding 12 of the transductor 11 is connected in series with the A.C. terminals of a full-wave rectifier 13 supplied from a current transformer 14. The D.C. control winding 16 and a resistance 17 are connected across the D.C. terminals of the rectifier 13 which is shunted by a condenser 18. Under normal load conditions, as the current in the A.C. winding increases, the corresponding increase in the current in the D.C. winding increases the saturation of the transductor core, so that the impedance of the A.C. winding is reduced and the voltage across it remains substantially constant. Due to the effect of the condenser 18 and resistance 17, the build-up of the current in the D.C. winding will lag behind that of the A.C. winding when a fault causes a sudden increase in the current in the line 15, thus the impedance of the A.C. winding does not decrease as quickly as the current in the A.C. winding increases, and the resulting momentary increase in the voltage is used to operate a relay 19 which may be connected directly across the A.C. winding 12, Fig. 1 (not shown), or across the condenser 21 of a series resonant circuit 20, 21 which smooths the applied voltage. When the arrangement is applied to a polyphase circuit, Fig. 3, the transductors 29-31 are supplied from current transformers 23-25 via rectifiers 26-28 in a manner similar to that above, but to enable protection to be obtained from moderate interphase faults, the D.C. winding of the transductor associated with each phase is supplied from the rectifier of the next phase. Thus on balanced fault conditions protection is obtained in the manner described above, and on interphase faults at least one A.C. winding is energized without a proportionate value of direct current in the control winding, so that the necessary relay operating voltage is obtained. In this embodiment the voltages produced across the condensers 37 of the series resonant circuits 36, 37 together with the voltage across a resistor 32 are applied to a multi-circuit rectifier 38 across which relays 39 are connected, though in a modification, Fig. 4, (not shown), a summation transformer is used. The voltage across the resistance 32 is dependent on earth faults, so that application of this voltage to the relays gives protection against this type of fault. The arrangements described may be used in phase comparison protective systems in which the instantaneous direction of current flow at two points of a network are compared by means of pilot wires or by means of carrier current signals transmitted over the power line and modulated at the power frequency.