GB756760A - Precipitator flashover control through current and voltage response - Google Patents

Abstract

756,760. Protective arrangements for supply systems for electrical precipitators. AIR PREHEATER CORPORATION. Feb. 22, 1954 [May 4, 1953], No. 5177/54. Class 38 (4). Simultaneous conditions of low voltage and high current in a rectifier system 12, 14 supplying a precipitator 19-indicative of flashovercause conduction of a normally non-conducting valve 42 to operate a relay 40 which effects a further voltage reduction to an arc extinction value by de-energizing the D.C. excitation windings 20-22 of saturable reactors 16-18 in series with the A.C. supply lines 10 and simultaneously energizing the D.C. windings 30-32 of further saturable reactors 25-27 which shunt the primary windings of the rectifier transformer 12. On extinction of the arc, the relay resets and restores the normal running conditions, in which the reactors 16-18 present low impedance and the reactors 25-27 high impedance. Valve 42 is normally held off by grid bias obtained from a potentiometer 50 across the rectifier output terminals, whilst the anode circuit of the valve may, or may not, be completed at contacts 46A under normal operating conditions of the precipitator, according to the setting of a tapping B on a potentiometer 44, which carries the current of the precipitator and from which the coil 46 of a relay controlling the contacts 46A is energized. However, under the heavy current conditions accompanying flashover this relay is in its operated condition and is locked in over holding contacts 46B, so that the anode circuit of valve 42 is closed and the low hold-off bias now obtaining allows the valve to conduct and operate relay 40. A stepping switch 57 is operated by contacts 40C of relay 40 each time a flash-over occurs and, after the occurrence of a given number in any one duty cycle of the precipitator, successively energizes relays 60, 70, 71 to effect progressive reduction of working voltage by switching resistance into the excitation circuit of the series reactors 16-18. The stepping switch is reset at the end of each duty cycle by a coil 85 energized over a cam switch 82 operated by the cleaning mechanism.