Patent GB-780609-A (GB)
Title: Control of electrostatic precipitator current by electrical means
IPC: B03C3/66
Filed: 1955-01-31
Abstract: 780,609. Automatic control systems for electrostatic precipitators. AIR. PREHEATER CORPORATION. Jan. 31, 1955 [Feb. 11, 1954], No. 2808/55. Class 38 (4). [Also in Group XXXIX] In a power supply system for an electrostatic precipitator, means are provided for (1) causing a " snap action " in stepping up or stepping down the electrode voltage (2) limiting " flashovers ", (3) limiting the flashover current for the limited number that do occur. The above controls are described as applied to a known arrangement comprising a number of electrode banks formed as the segments of a circle, whereby each bank may be separately de-energized for cleaning and subsequently re-energized, Figs. 1, 2 (not shown). In the supply system for each bank, a common three-phase A.C. source 30 supplies the bank electrodes (not shown) through a transformer 32, a rectifier 34 and a screened lead 18. Series saturable reactors 36, 37, 38 in the transformer supply leads are controlled by respective D.C. windings 40, 41, 42 energized by the D.C. output of a self-saturating reactor 60 which is supplied from an A.C. source 39 and comprises load windings 61, 62 in conjunction with rectifiers 64. Parallel saturable reactors 45, 46, 47 connected across the transformer supply leads are controlled in the opposite sense to reactors 36, 37, 38 by respective D.C. control windings 50, 51, 52 energized over a rectifier bridge 54 from a winding 55 associated with series reactor 36. On initial energization or re-energization after cleaning, closure of contacts 59A causes a D.C. source 71 to energize a control winding 66, 67 for reactor 60 in a sense as to assist the constant selfexcitation, whereby the resulting decrease in impedance of the series reactors 36, 37, 38 causes an increase in electrode voltage and the corresponding impedance changes in the parallel reactors 50, 51, 52 gives a snap action to this change. Closure of contacts 59B causes a further snap increase in the electrode voltage to its final value by the shorting of a resistor in circuit with control winding 66, 67. The successive opening of contacts 59B, 59A gives a snap action to voltage decreases during de-energization. In order to limit the flashover current during energization, a voltage tapped from a resistor 65 carrying the flashover current is opposed to the source 71, whereby, if the current exceeds a value set by source 71, the energization in control winding 66, 67 of reactor 60 is in a sense as to oppose the self-excitation and the electrode voltage is thereby reduced. Flashovers are limited by applying the voltage on lead 18 to the grids of thyratrons 80, 81, whereby the voltage breakdown on flashover causes them to fire and thus short a plate transformer 82. The source 39 is thereby permitted to pass current through reactor de-magnetizing rectifiers 83 and windings 61, 62 in the opposite sense to the normal self-excitation, whereby the electrode voltage is rapidly decreased. To prevent an unduly high number of flashovers, a tapping on resistor 65 allows a condenser 92 to charge as flashovers are repeated, whereby the negative bias on the grid of a gas discharge tube is finally overcome and the tube conducts to energize a relay winding 97 in its plate circuit. Relay contacts 98 are thereby opening to put a resistor 99 in circuit with reactor control winding 66, 67 and the electrode voltage is accordingly reduced. Contacts 59C are opened to reset the tube 94.