Patent GB-1272192-A (GB)
Title: Arrangement for controlling the voltage or current applied to an electrical apparatus
IPC: B03C3/68
Filed: 1970-03-05
Abstract: 1,272,192. Control systems for electrostatic precipitators. SIEMENS A.G. 5 March, 1970 [8 March, 1969], No. 10736/70. Heading H2H. An arrangement for automatically controlling the voltage of current applied to an electrical apparatus comprises a regulating means for varying the voltage or current and control means operable to control the regulating means in accordance with the voltage or current, together with current suppressing means so coupled to the apparatus to sense flash-over or short circuit therein and operable to cause the current supplied by the regulating means to be interrupted substantially without delay on sensing a flash-over or short circuit. An electrostatic precipitator F is supplied from an A.C. source U by way of back-to-back thyristors S, a smoothing choke D, transformer T and a full wave rectifier G. The conduction periods for the thyristors S are controlled by signals A, L derived by a control unit St, the input signals to the control unit being a signal B obtained by a current transformer W and representing the precipitator current and a light signal obtained from a lamp l and falling on a photosensitive device in the control unit, the signal from the lamp l being indicative of the absence of a flashover or short circuit in the precipitator F. The voltage, and hence the current, supplied to the precipitator F is normally under the control of the signal A, the signal L being a quenching or inhibiting signal, the arrangement being such that signals A and L cannot exist simultaneously. The signal A is derived by means of a capacitive device in the control unit St and the signal L from a transistor therein. When the precipitator is switched on the charge on the capacitive device increases with a corresponding increase in the operating periods of the thyristors S, the voltage and current applied to the precipitator thus increasing. During this build up the flicker of the lamp L is rendered ineffective so that it does not affect the voltage and current build up. When the voltage and current reach the required values the signal B stops the increase in the charging rate of the capacitive device and introduces a constant charging rate circuit whereupon the voltage and current at the precipitator F is maintained constant. Should the precipitator current tend to increase a discharge circuit is connected to the capacitive device whereupon the device discharges slightly thus retaining the current to its normal value. Should a flashover occur in the precipitator the lamp l is short circuited and becomes extinguished. The signal from the associated photosensitive device then disappears and the quenching signal L is derived and applied to the thyristors to cut off the precipitator supply voltage. The derivation of the signal L includes discharging of the capacitive device and the disappearance of the signal A. After a predetermined interval the signal L disappears and if the flashover has cleared the capacitive device begins to recharge thus restoring the supply to the precipitator.