US2771150A - Protective relay circuits - Google Patents

Description

Nov. 20, 1956 J, WELTS 2,771,150`

PROTECTIVE RELAY CIRCUITS Filed July 22, 1954 /A//E/Vra JfRf/IY WE .1.715

United States Patent ice PROTECTIVE VRELAY CIRCUITS leremy `Welts, 'Waltham, .Mass., assignon 'by rnesne yassgnments, to APRA Precipitator Corporation, New York, N. Y., a, corporation of Delaware Application July 22, 1954,'Serial No. 445,092

2Claims. `(Cl. 183-7) lThis invention relates generally to an electronic protective circuit, and more particularly to such a circuit designed to be 4used in conjunction with an elec-trostatic precipitator for cleaning a gaseous medium. As is well known in the art .it has been found desirable to utilize precipitators with a'high D. C. voltage applied to the ionizers in an effort to obtain the maximum possible gascleaning efficiency from the system. The use of these high voltages has been accompan-ied by the disadvantage that the ionizers may be subjected to high arc-over currents, commonly called ashovers, which-occur as a result of Velectrical breakdown in the gaseous medium passing through the ionizers and act to lower the operating eiciency ofthe system. These fiashovers may be caused, `for example, by accumulations of large quantities of dust in the system, by a change in the temperature of the gas feeding Lthrough the ionizers, by `a mechanical deficiency such as a bent electrode or the like. The precipitator system under consideration is of such design that transient iiashovers may be quickly extinguished without involving a serious reduction of the operating efficiency. However, when these disruptive discharges persist, the high voltage is effectively removed from the ionizers for a comparatively long period of time lthereby allowing a considerable quantity of gas to pass through the system without being ionized with the result that the efficiency is seriously impaired. In addition, the high short circuit current which iiows through the system during a fiashover will, if prolonged, damage the ionizers to a considerable extent.

In accordance with the invention, electronic means, including switching means, are provided with accomplish the dual purpose of protecting the ionizers and enabling the precipitator to ionize the gaseous medium more continuously even though at a voltage which provides less lthan the desired maximum efficiency. It is, therefore, the main object of this invention to furnish a protective circuit for a precipitator which will be sensitive to an excessive number of flashovers during a given time interval, while at the same time being insensitive to occasional ashovers such as may occur during normal operation of the precipitator.

This object is realized by the use of a novel integrating circuit arrangement which registers the frequency of incoming current pulses or flashovers. When the frequency of these pulses exceeds a certa-in predetermined value, a thyratron is caused to tire thereby energizing a relay in its plate circuit, said relay having contacts connected in the primary control circuit of a high voltage power supply in such a manner that the high voltage input to the ionizers is reduced.

Other objects and advantages of the invention will become apparen-t as the following description proceeds taken in conjunction with the accompanying drawing wherein the single figure represents a schematic diagram of a system embodying the invention.

Referring now to the figure, there is shown a portion of a precipitator system having an ionizing section 1 containing any desired number of ionizers 2, of which only 2,771,150 Patented Nov. 20, 1956 one is depicted for the sake of clarity vand simplicity. A suitable source of supply voltage 3, after rectification .by rectifier 4, furnishes ionizers 2 with the desired'high D. C. operating poten-tial. One side of input sec-tion 3 is provided with saturable reactor 5 having in its control circuit an energizing source 6, variable resistor 7, resistor 8, .and contacts 9. Resistor 8 may be included in or excluded from the control circuitby the operation of relay 11 in a manner which will be later explained.

Rectifier `4has oneofits output terminals connected'to one electrode of ionizers 2, the other electrode of which is grounded. The other output terminal of rectifier 4 is connected to ground through resistor 10 and Vernier I2.` A voltage regulating or 'clipping network consisting of resistor 13 and glow `lamp 14 .provides an input signal which is impressed on anintegrating circuit included within dotted area 15. A- thyratron 24, having its .grid 23 connected to the out-put of counting circuit is heated in a conventionalmanner by heating filament 25 .connected through leads 26 and y27 to the low vol-tage -sidefof stepdown transformer r28. Bleeder resistor 30 connected between a source of D. C. voltage 32 and cathode 31, `acts in conjunction with cathode resistor 29 to provide the bias for thyratron 24. A filter network comprising Iresistor'21 andcondenser Y22V prevents unwanted spurious signals from being applied to grid 23 to elimina-te accidental tiring of tyratron 24 due to a false signal.

When an arc-over occurs in the precipita-tor the high short circuit-current pulse owing through resistor 10 and Vernier 12 produces a signal voltage which yafter clipping action by resistor 13 and glow lamp 14 is impressed on integrating circuit 15. Assuming that selenium rectifier 18 is a perfect rectifier, this signal feeds into a capacitive voltage divider network comprising condenser 16, resistor 17, selenium rectifier 18, resistor 19, and condenser 20. The values of condenser 16 and condenser 20 are selected so that the voltage developed across condenser 20 is much less than the voltage developed across condenser 16. Resistors 17 and 19 are of such value that the RC time constant of resistor 19 and condenser 20 is much greater than the RC time constant or resistor 17 an-d condenser -16. With the application of each high voltage signal pulse, condensers 16 and 20 are supplied with charging current. At the end of an arc-over current pulse condensers 16 and 20 tend to discharge through their respective resistors 17 and 19, rectifier 1S preventing current flow through it toward the left thereby confining the discharge path of condenser 20 to resistor 19.

Thus it can be seen that with the proper choice of time constants the occurrence of several flashovers in quick succession will result in charging condenser 20 to a point where grid 23 to which it is connected will assume a voltage sufficient to fire thyratron 24 as determined by the characteristics of the tube. When thyratron 24 conducts, 4the current flow therethrough energizes relay 11 thereby opening contacts 33, and simultaneously closing contacts 34, the purpose of which will be pointed out below. With contacts 33 open, the addition-al resistance 8 thus inserted in the control circuit of saturable reactor 5 causes the high input voltage to ionizers 2 to be reduced.

A rotary timing switch 35 having contact makers 37, 38, and 39 sequentially arranged to alternately open and close their respective contacts 9, 40, and reset switch 36, provides a means for washing the ionizers 2, and also restoring thyratron 24 to a non-conducting condition. Assuming that thyratron 24 has tired and relay 11 is energized, the operation of the timing switch is as follows. Contact-maker 37 will disengage itself from contacts 9 thereby opening the control circuit of saturable reactor 5 to effectively remove the voltage being applied to the ionizers 2. At a later time contact-maker 38 will have engaged contacts 40 thus opening coil-actuated valve 41 to allow a washing medium which may be gaseous .to enter the deenergized ionizers 2 through nozzle 42. At substantially the same time contact-maker 39 will eng-age reset switch 36 thereby breaking Ithe plate circuit of thyratron 24. Since the accumulated char-ge on condenser 20 which caused thyra-tron 24 to fire was removed when shorting contacts 34 were closed by energization of relay 1\1, grid 23 ris thusv allowed to once more assume con-trol of the tube.

In the precipitatorunder consideration, peak flashover currents on the order of 2 to 5 amperes with a maximum frequency of 4/ sec. may be encountered. The constants for the integrating circuit may'have been so chosen that thyratron 24 will fire after eight 2.5 ampere pulses at the rate of 4/sec. have occurred. It should be understood that the constants are referred to only by way of example, and not in a limiting sense since integrating circuit 15 may be made responsive to any desired pulse frequency by proper selection of the constants.

While there has been described what is considered to be a preferred embodiment of this invention, it will be obvious to those skilled in the art that certain changes and modifications may be made without departing from the spirit of the invention as defined in the appended claims.

What is claimed is:

l. In ay gas cleaning system, means for ionizing a gaseous medlum to be cleaned, a source of input voltage for said ionizing means, integrating means adapted to register the frequency of current pulses which may occur in said ionizing means, first switching means controlled by said integrating means and adapted to operate when said current pulses exceed a predetermined frequency, means in circuit with said input voltage source and adapted to reduce said input voltage when said first switching means function, washing means connected to said ionizing means, and second switching meansadapted to sequentially remove said input voltage from said ionizing means, actuate said washing means, and restore said first switching means to its pre-functioning condition.

2. In a gas cleaning system: means for ionizing a gaseous medium to be cleaned; a source of input voltage for said ionizing means; integrating means adapted to register the frequency of current pulses which may occur in said ionizing means including a first condenser, a first resistor in series with said first condenser, a rectifier in series with a second resistor, said rectifier and said second resistor being connected in parallel with said first resistor, a second condenser connected in parallel with said second resistor; first switching means controlled by the voltage across said second condenser and adapted to operate when said current pulses exceed a predetermined frequency; means in circuit with said input Voltage source and adapted to reduce said input voltage when said first switching means function; washing means connected to said ionizing means; and second switching means adapted to sequentially remove said input voltage from said ionizing means, actuate said washing means, and restore said first switching means to its pre-operating condition.

References Cited in the file of this patent UNITED STATES PATENTS 2,666,496 Willison Jan. 19, 1954' 2,672,947 Klemperer Mar. 23, 1954 2,675,092 Hall Apr. 13, 1954 FOREIGN PATENTS 371,859' Great Britain Apr. 22, 1932

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