GB862657A - Improvements in or relating to control systems for electrostatic precipitators - Google Patents

Abstract

862,657. Automatic control systems for electrostatic precipitators. RESEARCH-COTTRELL Inc. Nov. 25, 1957 [Dec. 3, 1956], No. 36642/57. Class 38 (4). In an automatic control system for an electrostatic precipitator in which the electrode voltage is controlled by a saturable reactor in series with the A.C. supply to the E.H.T. transformer and rectifier; the reactor is controlled to maintain the electrode voltage at a maximum (which can optionally be automatically controlled at a set value) subject to an override which maintains the sparking rate at a pre-set value, itself subject to an overriding control which limits the A.C. supply current to a preset value. The precipitator 1 is supplied from rectifiers 3 fed by an E.H.T. transformer 2 having a saturable reactor 6 in series with the primary. Additionally there is a surge-limiting resistor 5 which also is used to provide the current-limiting signal. The D.C. windings of the reactor 6 is energized by a phase-shift gridcontrolled rectifier 101 in turn controlled by a valve 114. Valve 114 is biased by a voltage tapped off a potentiometer 132 in such a sense as to raise the precipitator electrode voltage. This bias is opposed by either the sparking rate error signal or the current-limiting signal. The sparking rate is detected by causing a thyratron 145 to discharge a capacitor 146 each time a spark occurs. The thyratron 145 is triggered by transient voltage pulses from the primary of the E.H.T. transformer 2 coupled by transformer 140 through a high-pass filter 141, 142, 143 which removes mains frequency components. The capacitor 146 charges through resistors 147 and 148 and so the voltage across the resistor 148 rises with increased spark frequency. This voltage is integrated by resistor 149 and capacitor 150 and a selected portion of the resulting signal is indicated on a meter 156 and also opposed by a reference voltage representing the desired sparking rate tapped off a potentiometer 161. If the sparking rate signal exceeds the reference, then diode 181 contacts and the resultant error signal is applied to the control valve 114 in opposition to the bias from potentiometer 132 and so reduce the precipitator electrode voltage. The A.C. supply current is measured by the voltage drop across section BB of the resistor 5. This voltage is applied by transformer 170 and low-pass filter 171 to a bridge rectifier 174 the output of which is smoothed and applied in opposition to a pro-set reference signal tapped off a potentiometer 192. If the current signal exceeds the reference, then diode 181 conducts and the resultant error signal opposes the bias on the control valve 114 to reduce the precipitator electrode voltage. The output of the current signal generator is made approximately ten times that of the sparking rate signal generator so that it can take command. The following 10 modifications are suggested: (1) the current signal may be obtained from a current transformer instead of the resistor 5; (2) the current signal may be derived from a resistor in the earth return circuit of the E.H.T. rectifier enabling the rectifier 174 to be omitted; (3) the current and spark-rate signals may both be obtained across resistor 5 using filters to separate the signals; (4) both signals may be obtained across a resistor in the earth return circuit of the E.H.T. rectifier using filters to separate the signals ; (5) the spark-rate signals may be picked up by a tertiary winding on the E.H.T. transformer; (6) the double diode 181 may be replaced by silicon diodes; (7) the double diode 181 may be replaced by two triodes acting as anode-bend detectors; (8) the triode 114 may be replaced by a double triode with separate inputs for the current and spark rate signals; (9) the triode 114 may be replaced by a multi-grid valve; (10) the " raise voltage " bias from potentiometer 132 can be replaced by an automatic voltage control in which the primary voltage of the transformer 2 is compared with a reference voltage to provide the bias.