GB598161A - Improvements in and relating to the control of electrical rectifiers - Google Patents

Abstract

598,161. Rectifying. GENERAL ELECTRIC CO., Ltd., and DALE, S. H. Oct. 10, 1945, No. 26476. [Class 38 (ii)] In a polyphase rectifier system protected against overload, the voltage developed across an impedance by harmonic frequency currents from the secondaries of current transformers in each phase which are saturated by the overload currents, is used to apply a negative voltage to the rectifier grids. The secondaries of current transformers 20, 21, 22 in the main supply lines 15, 16, 17 are connected in series across a resistor 23 in parallel with a reactor 24. At a predetermined value of supply current, the transformer cores saturate and the third harmonic currents so produced develop across resistor 23 a voltage much greater than the negligible voltage produced by the fundamental frequency current. This voltage is applied in push-pull to the grids of vapour-filled valves 27, 28 which are at a negative potential determined by the setting of potentiometer 30 connected to the mid-point of reactor 24. Valves 27, 28 are rendered conductive, and battery 32 is thus connected across resistance 18 between the cathodes of rectifiers 10, 11, 12 and the starpoint of the transformer secondary 14 supplying the rectifier grids which thus become negative and suppress the rectifier discharge. To reset the circuit, switch 33 is closed to short-circuit valves 27, 28 and then opened when the bias is removed from the rectifier grids if the fault has cleared. Valves 27, 28 may be replaced by relays having tongues biassed by springs set to operate at a selected amplitude of the harmonic voltage. The single resistor 23 may be replaced by separate resistances across each secondary connected in series, Fig. 2 (not shown). In a further modification, switch 33 is replaced by a relay operated by the anode current of valves 27, 28 which keeps opening and closing while the fault persists. Through further contacts on the relay, a condenser is charged each time this operates ; this condenser is discharging through a resistance, but after a predetermined number of relay operations, the condenser is sufficiently charged to render conductive a vapour-discharge valve to the grid of which the condenser is connected. This valve applies a permanent negative bias to the rectifier grids which are thus locked against operation, and may trip a circuit-breaker in the supply lines 15, 16, 17. A further relay may be provided whereby negative bias is applied to the rectifier grids if the heater circuit of any of the valves are broken, or until the cathodes are at their normal operating temperatures. Condensers connected between grid and cathode of valves 27, 28 may prevent operation by switching transients or H.F. potentials radiated or fed from the rectifiers.