Patent GB-276330-A (GB)
IPC: H02H3/08
Filed: 1927-08-13
Abstract: 276,330. Metropolitan-Vickers Electrical Co., Ltd., (Assignees of Breisky, J. V.). Aug. 23, 1926, [Convention date]. Cut-out arrangements. - Circuit interrupters are operated in accordance with the varying conditions of the circuit by the resulting effects produced in thermionic valves and a time-element feature, which renders the protection selective, is obtained by means of heating elements energized from the protective circuit and co-operating with high temperature-coefficient resistances in the cathode or anode circuit of the valve. In the arrangement shown in Fig. 1, a D.C. circuit 1 is protected by the breaker 1 the trip-coil 3 of which is in the anode circuit of the two-electrode valve 4. The cathode of the valve is supplied from a shunt 10 through a resistance of high negative terrperature-coetlicient 11, the resistance being adjusted by tappings to effect the initial setting. With excess current the resistance 11 heats up and after a time admits enough current to the cathode to make the valve operative and to cause the circuit-breaker to trip. The time element feature may be made more effective by employing a separate adjustable heater coil 12 for the resistance 11 connected across the shunt. By connecting the heater coil 12 across the protected circuit and supplying the cathode from an independent source and using a resistance of high positive temperature-coefficient, undervoltage protection may be provided for. By providing high positive or negative temperature-coefficient resistance in the anode circuit underload or overload protection, respectively, is obtained. An arrangement for A.C. overland protection is shown in Fig. 5. The cathode 8 of the two-electrode valve 4 is supplied from a current transformer 30 through the winding 36 on a threelimbed intermediate transformer 31. This winding is on a limb of restricted area so that, owing to saturation, the supply to the cathode is restricted. The anode supply is taken from another winding 34 on a saturable limb to ensure that the anode potential is limited. A resistance of high positive temperature-coefficient 38 is connected across the cathode and is heated by a coil 40 supplied from the current transformer 30 through a saturable transformer 41 which supplies predetermined maximum value of current and, therefore, a definite minimum time-element in the operation of the protection system. A variation of this arrangement, Fig. 6. (not shown), provides for overvoltage protection and uses a valve with two anodes so as to give full-wave rectification in the anode circuit. Protection by means of a valve, as described, may be associated with a power-directional relay of, for example, the induction-disc type, the directional relay contacts being located in the anode tripping circuit. Such an arrangement for a three-phase circuit to deal with earth or residual currents of a particular direction appearing in the system on fault is shown in Fig. 8 in which 50 is an auxiliary transformer to produce an artificial neutral and 49 is a resistance to earth from this neutral. The resistor of high temperature-coefficient of resistance is inserted in series with the anode and operating coil of the circuit interrupter. An arrangement in which the circuit-breaker is operated in accordance with the impedance between it and the fault, (that is, in accordance with the distance nf the fault) is shown in Fig. 9 in which the cathode 8 of the valve is supplied from a current transformer 30 (with a protective saturable reactance across it) and the anode circuit is energized by the voltage of the system and has a positive temperature-coefficient resistance 53 and a heater element 52 to give the time element feature for selective operation. Modifications of this impedance selective system employing. three-electrode valves are described. In the current differential system shown in Fig. 15 in connection with the protection of a transformer 61, an auxiliary threelimbed transformer 63 has a primary winding 64 connected between normally equipotential points of the balancing transformer system 60, 60. The cathode is supplied from winding 65 and the anode circuit from winding 66 through a resistance 67 having a high positive temperature-coefficient. This resistance is heated by two heating coils 68, 69 supplied respectively by the two current transformers 60. When the protected apparatus is sound but carrying heavy current, resistance 66 is heated up and its value increased so that greater unbalance is necessary to make the valve effective under these conditions.