Automatic Scr Precipitator Control

Abstract

An automatic precipitator energization control circuit wherein precipitator voltage is rapidly reduced to zero for an independently selective period of duration upon the occurrence of each spark or flashover in the precipitator and gradually increased to a voltage level determined with reference to the voltage level of the precipitator at the time the spark occurred. The deleterious effects of multiple sparks or spark bursts are eliminated, a soft start is assured, and true r.m.s. current is sensed to provide a current limit to override the spark control circuitry to limit the input power to the unit and to clear a short circuit condition in the precipitator from dirt buildup or the like.

Description

BACKGROUND OF THE INVENTION

The automatic control of the voltage of an electrostatic precipitator has long been a source of concern in the art. It is desirable to maintain the direct current voltage on the precipitator electrodes as high as is possible, consistent with the avoidance of excessive current drain due to arcing or sparking. The invention is described in a preferred system application, providing a continually increasing voltage on the precipitator electrodes and reducing that voltage a predetermined amount on the occurrence of each spark in the precipitator.

Means have been provided in the prior art for comparing the line current with a desired or reference value and for adjusting the impedance of a current-dependent saturable reactor between the AC source and the precipitator upon the detection of an excessive current flow. An associated problem has been the accumulation of dirt on the precipitator electrodes to the point where a "bridge over" occurs, practically shorting out the electrodes. Reactor controlled systems cannot clear such a fault because they cannot reduce the voltage applied to the precipitator to zero. In these prior art systems it has been necessary for the operator to manually turn off the precipitator control to remove the voltage from the precipitator and then to re-energize the system. The sudden inrush of current upon initial re-energization of the precipitator has on occasion magnetized the precipitator transformer and tripped out the various protective devices in the circuit.

Other prior art systems have been responsive to the rate of sparking in the precipitator. Here, the voltage on the precipitator electrodes is not immediately responsive to the conditions which actually exist in the precipitator and adjustments are consequently often made after the condition requiring the adjustment has passed.

As taught in the U.S. patent to W. J. Brown, U.S. Pat. No. 3,040,496, the occurrence of a spark in the precipitator may develop into an arc which would cause deterioration and eventual destruction of an electrode and would also decrease the potential difference between the electrodes thereby decreasing the effectiveness of separation of the particles from the gaseous medium. It is thus taught that it is therefore desirable to reduce the power supplied to the electrodes immediately when a spark occurs in order to prevent further arcing and then to restore it to a value at which separation is effective.

Brown teaches the momentary reduction of precipitator current to minimum upon the occurrence of a spark to inhibit the formation of subsequent arcs in the precipitator. The precipitator current in the Brown system is held to approximately zero for about three half cycles, of the AC supply cycle. Thereafter the passage of current to the precipitator is restored to a value slightly less than that prevailing before the occurrence of the spark.

Such arcing may occur in the form of spark bursts, whereby the initial spark is followed by several more spaced apart in time by one, or a few, half cycles of the supply frequency (if full-wave energization is used), or cycles thereof (with half-wave rectification). This is particularly prevalent with salt cake and basic oxygen furnace applications.

It has been found that the structure disclosed by Brown, while a significant advancement in the art, does not provide the desired versatility and reliability in controlling the precipitator current upon the occurrence of a spark or flashover in the precipitator. It is desirable to maintain the precipitator current supply in an off condition only for so long a period as is absolutely required to prevent successive spark bursts. These conditions naturally vary greatly with the application of the precipitator.

The application of the precipitator for the separation of particles from the gaseous medium in salt cake and basic oxygen furnace applications, requires precipitator current to be reduced to zero for a longer period of time than would normally be required for most other applications. Naturally, when the precipitator current is in the off condition, the precipitator is no longer collecting or removing the particles from the gaseous medium being cleaned or freed from the undesired particles suspended therein.

Automatic precipitator control circuits are also generally provided with a current limiting circuit in addition to the spark control circuit. It often occurs that the particle build-up on the precipitator plates may create an electrical short without the presence of a spark. When a spark signal is not present, the control will respond to the current limit circuit to limit the maximum current flow permitted to the precipitator to prevent the average precipitator current from exceeding a preselected safe limit. Such current limit controls are disclosed in the aforementioned Brown patent and in the U.S. patent to H. J. Hall, et al., Pat. No. 3,147,094. However, such existing current limit circuits have been found to not adequately provide means whereby the short circuit condition in the precipitator will be cleared automatically to prevent recurrence of the condition whereby a short is present without a spark signal.

The conventional current limit circuits such as disclosed in Hall, et al., which sense the alternating current supply, also tend to be peak responsive (responding to the peak current), and are not considered acceptable for accurate automatic precipitator power control. This is particularly true where silicon controlled rectifiers in a back to back relationship are employed in the AC supply circuit in order to regulate the current supplied to the precipitator such as disclosed by Australian Pat. No. 248,429. This is due to the fact that SCR wave forms are more complex than those in the conventional control devices, such as saturable reactors, such that the relationship of 0.707 times the peak current no longer holds true in order to determine the r.m.s. current.

It is therefore the objective of the present invention to provide a novel automatic energization control for an electrostatic precipitator in which the aforementioned deficiencies of the prior art control systems are remedied and whereby a novel automatic energization control is provided which is much more reliable than those of the prior art and which permits the achievement of a much higher overall precipitator input power than heretofore found possible.

SUMMARY OF THE INVENTION

The electrical precipitator system of the present invention employs, in addition to a spark control circuit which serves to reduce the conduction angle of the SCR gates in the AC supply circuit upon the occurrence of a spark in the precipitator and thereafter slowly increase the magnitude of the power supplied to the precipitator, a fast response control circuit which is also responsive to a spark in the precipitator to override the spark control circuit and immediately reduce the magnitude of the power supplied to the precipitator to zero for a short interval, two to three cycles of the AC power supply for example, after which control is returned to the spark control circuit. The fast response control circuit is provided with a variable parameter which is controllable to vary the duration of this interval, depending upon the particular condition to which the precipitator is applied, without affecting the parameter values of the spark control circuit which independently regulates the regrowth of rise of the precipitator power supply to operational levels.

The spark control circuit consists generally of an RC storage network having variable parameters to vary the magnitude of the stored energy and the RC discharge time constant. The spark control circuit is utilized to regulate the magnitude of a control signal fed to an SCR gate driving circuit which in turn reduces the conduction angle of the SCR's in proportion to the magnitude reduction of the control signal. The RC storage network of the spark control circuit responds to the occurrence of a spark in the precipitator as it is fed from a pulse forming network which generates a pulse upon the occurrence of each spark in the precipitator.

The fast response control circuit also consists of an RC storage network in the preferred embodiment, which is fed from the same or similar pulse forming circuit as is the spark control circuit. The fast response control circuit is also operative to regulate the magnitude of the control signal fed to the SCR gate driving circuit.

However, the RC time constant of the fast response control circuit is much faster than that of the spark control circuit in order to permit the fast response control circuit to immediately override the spark control circuit and reduce the precipitator power supply to zero for a short interval, after which the control of the power supply is returned to the spark control circuit to gradually restore precipitator power to operational levels due to the slower RC time constant characteristics of the spark control circuit.

The present invention is particularly unique in the fact that a variable parameter is provided in the fast response control circuit in order to vary the interval for which the precipitator power supply will remain at zero without in any way affecting the parameter values of the spark control circuit or RC storage network thereof. This feature permits extensive versatility in application of the electrical precipitator system of the present invention since the precipitator systems heretofore in use do not adequately serve to prevent a rapid succession of sparks or arcing in the precipitator for all applications due to the large variety of particle substances to be precipitated from the polluted gases given off from the many different types of industrial facilities.

In the preferred embodiment the variable parameter of the fast response control circuit is operable to vary the magnitude of the pulse applied thereto. Thus, by varying the amount of energy stored within the RC network of the fast response control circuit, the discharge time (precipitator off time) is proportionately changed even though the RC time constant remains the same. The present invention thereby permits the variation of the time interval for which the precipitator power supply remains at zero without in any way affecting the RC time constant of the spark control circuit which regulates the rise time of the precipitator power to operational levels.

The precipitator system of the present invention is further characterized by a current limit circuit which senses the true r.m.s. value of the precipitator power supply and, in a similar manner to the spark control circuit, reduces the supplied precipitator power to zero when the AC power supply current reaches a predetermined limit. In the preferred embodiment, the current limit adjust in operative to vary the magnitude of the sensed current which is utilized to control the conduction angle of the SCR's to limit input power to the power supply.

The current limit circuit employs a fast response incandescent bulb in combination with a photodiode in order to measure the true r.m.s. value of the current flow into the primary of the high voltage power supply. The use of an incandescent bulb permits the sensing of true r.m.s. current as opposed to peak current. The conventional current limit circuits in present-day precipitator systems are peak responsive and are therefore not considered acceptable for use with SCR wave forms which are more complex as previously explained and in addition are not considered adequate for saturable reactor controls.

Although incandescent bulbs in combination with photodiodes have been previously employed in regulating circuits (see U.S. Pat. Nos. 2,429,614; 2,779,897; 2,808,559; and 3,312,895), they have heretofore, to my knowledge, never been employed in a precipitator system for the purpose of measuring a true r.m.s. current for the accurate control of the precipitator power supply.

The current limit circuit operates to reduce the conduction angle of the SCR gates when an increase of precipitator current supply is experienced beyond a preselected limit and further functions to reduce precipitator voltage to zero upon the occurrence of a short circuit in the precipitator caused by dirt build-up or the like. It is not uncommon that the current will exceed the selected limit in the absence of spark signals and therefore the SCR control circuitry must respond to the current limit alone to limit the rise of current.

The reduction of the SCR conduction angle to zero permits the short circuit condition to clear itself in the case of dirt build-up in the precipitator after which the spark control circuit automatically reenergizes the precipitator. Older controls do not have the ability to reduce precipitator power input to zero and thus, once this short circuit or bridge over condition occurs, the operator must manually turn the control unit off to clear the short circuit condition in the precipitator.

The system of the present invention is also unique in that the RC network of the spark control circuit and the fast response control circuit are fed to a very high input impedance field effect transistor in order to control the magnitude of the control signal regulating the SCR gate driving circuit. The use of a field effect transistor in this particular combination provides a highly stable control circuit as the parameter values of the RC storage networks of the spark control circuit and fast response control circuit will not change their values due to temperature change and variable load conditions.

The aforementioned combinations of network circuits thus provide a unique precipitator system which is considerably more reliable than those of the prior art.

Other objects and advantages appear hereinafter in the following description and claims.

The accompanying drawings show, for the purpose of exemplification without limiting the invention or the claims thereto, certain practical embodiments illustrating the principles of this invention wherein:

FIG. 1 is a schematic diagram illustrating the power circuit of the precipitator and the phase control circuit in block form.

FIG. 2 is a schematic diagram of the spark responsive control circuitry of FIG. 1.

FIG. 3 is a schematic diagram of the phase control circuit of FIG. 1 with the SCR gate driving circuit illustrated in block form.

FIG. 4 is a schematic diagram of the current limiting control circuit of FIG. 1.

FIG. 5 is a schematic drawing of the SCR gate driving circuit of FIG. 3.

FIG. 6 illustrates a typical graph of the primary precipitator transformer voltage versus time in the system of the present invention showing the immediate reduction of precipitator voltage upon the occurrence of a spark and the gradual increasing thereof to desired operational levels.

Referring now to the drawings, FIG. 1 illustrates the basic power circuit in which alternating current from a 440 volt, single phase, 60 cycle source 10 is converted to a high direct current in a transformer-rectifier combination consisting of a high voltage transformer 12 and diode bridge rectifier 14, which is applied to the electrodes of precipitator 16. Connected in the line between the AC source 10 and the primary winding 17 of transformer 12 is a surge limiting coil 18 and a gating network 20.

Gating network 20 includes a pair of silicon controlled rectifiers SCR1 and SCR2 connected in a parallel, back to back configuration. As is well known, the operation of the SCR's allows proportional control based on the conduction angle thereof as determined by the phase of the signal applied to the trigger or gate electrodes.

Also paralleling SCR1 and SCR2 is a volt trap 22 having a lower rating than the rating of the silicon controlled rectifiers SCR1 and SCR2 to provide protection for the excessive transients which occasionally occur due to the highly transient nature of the load. All transients in excess of the rating of the volt trap 22 are dissipated in the form of heat.

In addition, an RC network which includes resistor 24 and capacitor 26 is connected across SCR1 and SCR2 The function of the RC network is to limit the rate of change in the positive going voltage supplied to the anode of a PNPN device, the SCR's, which might otherwise permit the triggering of the SCR's upon initial closure of a circuit breaker or the occurrence of some other excessive transient in the SCR circuitry.

A current sensing transformer 28 is used to detect the line current and to generate a signal which is applied to phase control circuit 30 for the purpose of controlling the phase of the SCR triggering signal. The occurrence of a spark is detected in resistor 32 and the signal appearing at terminal 34 is also fed to the phase control circuit 30 for control of the SCR triggering signal. The outputs of phase control circuit 30 are applied to the trigger electrodes of SCR1 and SCR2 to control the conduction thereof relative to the signal applied across the anode and cathode junction. The voltage applied to the precipitator is thus subject to control by the line current and by occurrence of sparking in the precipitator.

SPARK CONTROL CIRCUIT

The occurrence of a spark in the precipitator generates a damped high frequency oscillation of approximately 15 microseconds in duration. The amplitude of the oscillation is initially of the order of five to 10 times the normal current appearing in the secondary winding 35 of transformer 12. The increased voltage drop across resistor 32 is thus responsive to a spark in the precipitator.

Referring now to FIG. 2, the spark responsive signal appearing across resistor 32 of FIG. 1 as taken from terminal 34, is fed to a high pass filter network including capacitor 36, and resistors 38 and 39, which block the 120 cycle ripple in the DC output of rectifier 14 in the precipitator 16 supply. Capacitor 36 also prevent the direct current components of the signal taken from terminal 34 from entering the phase control circuit 30. A Zener diode Z1 shunts resistor 39 and is included in the circuit to clip the magnitude of the incoming current peaks to prevent circuit component damage.

The output of the filter network is then rectified by diode 40 and fed to a pulse forming network comprising capacitor 42 and resistor 44 connected in parallel across the line. The signal appearing at terminal 46 is a negative pulse which is transmitted through a capacitor 48 to a monostable multivibrator MV1 and through a capacitor 50 to a second monostable multivibrator MV2. Multivibrator MV2 is identical in every respect in its circuit and operation to multivibrator MV1 and consequently the operation thereof will not be separately discussed.

The negative pulse appearing at terminal 46 is fed through capacitors 48 and 52 to the base electrode of NPN transistor Q2 of multivibrator MV1. Transistor Q2 is appropriately biased to be in saturation and NPN transistor Q1 is cut off when the monostable multivibrator MV1 is in its single stable state. The negative pulse applied to the base electrode of transistor Q2 drives it into cutoff as transistor Q1 saturates. The time constant of the multivibrator MV1 is approximately 3 milliseconds and is determined by the values of resistor 54 and capacitor 56 connected in parallel between the collector electrode of transistor Q2 and the base electrode of transistor Q1.

A positive pulse thus appears at the collector electrode of transistor Q2 upon the occurrence of a spark in the precipitator, that is, when Q2 goes into cutoff. This positive pulse is applied to the base electrode of PNP transistor Q3. Transistor Q3 is a single current amplifying stage and the output thereof as taken from the emitter electrode is a positive going pulse of approximately 3 m.s. duration. The positive pulse output of transistor Q3 is applied to terminal 62 to control the power limit of the precipitator in a manner hereinafter to be described.

Thus, each time a spark in the precipitator 16 is detected by resistor 32, a positive pulse of a fixed amplitude and duration is applied to terminal 62 from multivibrator MV1. Similarly, a positive pulse of a fixed amplitude and duration appears at the output of monostable multivibrator MV2 upon the occasion of a spark in the precipitator. The output of multivibrator MV2 is not amplified, but is applied directly to terminal 64 to commence a fast response portion of the spark control. The operation of the spark responsive control circuit in response to the positive pulses applied to terminals 62 and 64 to control the voltage of precipitator 16 will be discussed in detail infra in connection with FIG. 3.

Referring to FIG. 3, the detection of a spark in the precipitator 16 results, as explained with reference to FIG. 2, in the application of a positive pulse of a fixed magnitude and duration to terminal 62 and thus a finite increment of the charge to capacitor 108 through potentiometer 58 and diode 60. Potentiometer 58 in FIG. 2 is employed to vary the amount of energy stored in capacitor 108 with with each spark, and potentiometer 116 of FIG. 4 is employed to vary the rate of discharge. Capacitors 110 and 112 assist in the filtering of stray 60 cycle pickup.

The charge on capacitor 108 determines the conduction of field effect transistor Q6, an increase in charge increasing the back bias and reducing the load current through resistor 124 and the base current of Q7. Each spark therefore reduces the emitter to collector current of Q7 which in turn reduces the DC signal current supplied to the SCR gate driving circuit 120, and lowers the precipitator voltage proportionately. Condenser 108 then resumes its discharge through resistors 116 and 118 to develop the characteristic rising voltage of the precipitator to operational levels.

Field effect transistor Q6 is a silicon unit with very high input impedance and is provided because of its high stability over a wide temperature range. Thus the storage circuit parameters are prevented from changing value due to temperature changes and due to variable applied loading.

Potentiometer 58 determines the amount of reduction of the precipitator voltage and potentiometer 116 controls the subsequent rate of rise of precipitator voltage to operational values which permits extreme versatility for the many possible different precipitator applications. The rate at which the precipitator is recharged, is selected to avoid overcurrent surges through the SCR's during the recharge period.

In addition to its function as a 60 cycle filter, capacitor 112 controls the fast response spark control program. Capacitor 112 receives from terminal 64, through potentiometer 66 and diode 68, the 3-millisecond positive pulse output of multivibrator MV2 of FIG. 2. Capacitor 112 charges approximately 400 times as fast as capacitor 108 and may assume source voltage in response to each spark in the precipitator 16. Capacitor 112 thus immediately assumes control of the precipitator and effectively disconnects capacitor 108 from its discharge resistor 118 by operation of diode 106.

During this effectively disconnected period, capacitor 108 receives the positive pulse from MV1, increasing its charge and limiting the precipitator voltage control of capacitor 112. The program period is quite extended, about 23/4 cycles of the supply voltage. This approximately 80-millisecond period is a significant portion of the discharge time constant of capacitor 108.

Potentiometer 66 is adjusted to back bias Q6 to cutoff for each pulse applied to capacitor 112.

Thus, the charging of capacitor 112 back biases transistor Q6 to cutoff upon the occurrence of each spark. Each spark therefore reduces the emitter to collector current of transistor Q7 which in turn reduces the DC signal current to SCR gate driving circuit 120. The net result is an immediate reduction of precipitator voltage to zero.

Potentiometer 66 determines the duration for which the precipitator voltage will remain zero by varying the total amount of stored energy in capacitor 112 for discharge at a fixed RC time constant rate. Variation in the value of potentiometer 66 does not therefore effect the parameter values of the spark control circuit.

Proper adjustment of potentiometer 66 thus reduces the voltage of precipitator 16 to zero very rapidly for a selected duration, with consideration that the phase control circuit 30 may impose slight inherent delay. Full response occurs in less than three-quarters of a cycle of the AC source 10. The charge on capacitor 112 overbiases the control network to hold the precipitator voltage at zero for approximately two cycles, as preset by adjustment of potentiometer 66, before allowing the gradual elevation of the precipitator voltage during approximately two supply cycles to the new level as determined by the charge on capacitor 108. Bursting effects, the occurrence of several sparks in rapid succession, often within a single half-cycle period, are thus eliminated.

FIG. 6 represents precipitator primary current as a function of time with sparks occurring at times T.sub.1, T.sub.4 and T.sub.7. The time duration of zero conduction angle between times T.sub.1 and T.sub.2 is controlled by potentiometer 66 as previously stated. The precipitator recharge time duration from time T.sub.2 to T.sub.3 is fixed at a constant rate due to the RC storage network comprising components 118 and 112 which is part of the fast spark quench circuit. Precipitator recharge time from T.sub.3 to T.sub.4 is controlled by variable resistor 116, and the RC network comprising 116, 118 and 108 which recharges the precipitator at a much slower rate. Another spark occurs at T.sub.4 and the same explanation applies. Potentiometer 58 determines the current level which will be assumed at time T.sub.3 after spark quench. If desired, capacitor 112 can be made an adjustable capacitor to provide a control of the time required to raise the current level from T.sub.2 to T.sub.3.

Potentiometer 58 is adjusted to provide the desired value or level to which the precipitator primary current will be restored to upon termination of the spark quenching period from T.sub.1 to T.sub.2, or T.sub.4 to T.sub.6, or T.sub.7 to T.sub.9. This value or level is normally and preferably selected such that the current is slightly less than that prevailing just before or at the time of occurrence of the spark to prevent successive spark bursts. However, the control range of potentiometer 58 is sufficiently wide to permit this current level at times T.sub.3, T.sub.6 and T.sub.9 to be selected anywhere from zero to the maximum allowable current limit as preset by the current limit adjust, if so desired. Thus, one could set potentiometer 58 such that the current level at, say, time T.sub.6 is greater than that at time T.sub.4 rather than slightly less if such were desired.

To sequentially relate the events of FIG. 6 to the circuitry of FIG. 3 from time zero to T.sub.1, the primary supply current is maintained at the current limit I regulated by the current limit control circuit. When a spark occurs at time T.sub.1, a control pulse of fixed duration, and of a magnitude regulated by potentiometer 58, is applied to capacitor 108 for storage and discharge through resistances 116 and 118. At the same time T.sub.1, a similar pulse is applied to capacitor 112 (having its magnitude regulated by potentiometer 66) for storage and discharge through resistor 118.

Since the RC time constant of the fast response storage network (R 118 and C 112) is much faster than that of the spark control RC network (R 116, R 118 and C 108), Q6 is quickly driven to cutoff from period T.sub.1 to T.sub.2, whereupon capacitor 112 continues to discharge through period T.sub.2 to T.sub.3. At time T.sub.3 capacitor 112 is sufficiently discharged such that the slower RC time constant of storage network R 116, R 118 and C 108 is permitted to assume control to gradually increase the current supply toward the preset limit I during period T.sub.3 to T.sub.4. Potentiometer 58 is preadjusted such that the primary current level at time T.sub.3 will be slightly less than that prevailing at time T.sub.1 which in this instance is I.

When the second spark occurs at time T.sub.4, the same sequence of events occurs, however, capacitor 108 has not yet completed the discharge of its stored energy at time T.sub.4, at which time additional energy is applied to the capacitor by generation of a new control pulse passed through potentiometer 58. As the total amount of energy stored in capacitor 108 at time T.sub.6 is greater than that at time T.sub.3, the primary current level at time T.sub.6 will be slightly less than that at time T.sub.3 as shown. The resultant effect is that the current level at time T.sub.6 will be always slightly less than that prevailing at time T.sub.4.

The time period from T.sub.6 to T.sub.7 is sufficiently long before the occurrence of the next spark at time T.sub.7, to permit complete discharge of capacitor 108 and thereby restoration of the primary current level to I, the selected current limit. Thus, just prior to the occurrence of a spark in the precipitator at time T.sub.7, the stored energy in capacitor 108 is depleted and thus no remaining charge is present at time T.sub.7 when a new control pulse is applied thereto as was the situation at time T.sub.4 ; therefore, the current level at time T.sub.9 will be slightly higher than that at time T.sub.6 as indicated, but will be still slightly less than the current level prevailing at time T.sub.7 as was the condition with the first spark occurrence. In this manner, after each spark quenching period, the precipitator voltage is always restored to a value slightly less than that prevailing just prior to the occurrence of the spark, providing of course, that the value of potentiometer 58 has not been varied in the interim.

CURRENT LIMIT CIRCUIT

Referring to the current limit circuit of FIG. 4, the line current as sensed in current transformer 28 of FIG. 1 is applied to terminal 70. The voltage appearing across resistor 72 is reduced in a current limit adjust consisting of coarse adjustment potentiometer 74 and fine adjustment potentiometer 76, and is applied through the secondary winding 78 of transformer 82 to the filament of a fast response incandescent bulb 80. Transformer 82 provides a fixed bias to the bulb 80 for the purpose of preheating the filament and thus improving the response time thereof. Condenser 83 is selected for phase control to provide phase coincidence between the preheat current and the signal current in the filament.

Bulb 80 is of the incandescent type because of the necessity of sensing true r.m.s. current rather than peak current. A commercially available Penlite Inc. Model No. 60-25 bulb has been found to be satisfactory, the bulb having a stated response time of approximately 8 milliseconds wherein one-half of the maximum intensity of illumination is obtained. The effective response time of bulb 80 is, however, substantially less than that stated due to the preheating of the filament. A Zener diode Z2 may be placed across the filaments of bulb 80 to clip the excessive current peaks which may occur upon sparking of the precipitator 16.

A photodiode PD1 is located in a light receiving relationship to bulb 80. Photodiode PD1 and resistors 84 and 86 form a light sensing network, the output of which is a DC voltage directly proportional to the intensity of the illumination from bulb 80. The current through the sensing network increases as the resistance of the photodiode PD1 decreases thus increasing the voltage applied to the base electrode of NPN transistor Q4. Transistor Q4 and NPN transistor Q5 and appropriate biasing resistors 88, 90, 92 and 94 form a two-stage amplifier 95. Resistors 88 and 92 provide temperature stabilization and resistor 96 further stabilizes the amplifier 95 by providing a feedback path from the collector electrode to the base electrode of transistor Q4. The output of amplifier 95 is taken from the collector electrode of transistor Q5 and follows the potential applied to the base electrode of transistor Q4. The output voltage is applied across Zener diode Z3 through diode 98 to terminal 100. Zener diode Z3 is provided to prevent the circuitry from overdriving and thereby delaying the re-energizing of the precipitator when the current limiting circuit reduces the conduction angle of SCR1 and SCR2 to zero upon dirt buildup or "bridging over" between the plates and wires of the precipitator 16.

Referring again to FIG. 3, the output at terminal 100 of the current limiting circuit of FIG. 4 also serves to deliver a charge to capacitor 108. As earlier explained, any increase in the voltage across capacitor 108 results in reduction of voltage of precipitator 16 current by reducing the conduction angle of SCR1 and SCR2 and thus the line current. The current limiting control circuitry thus overrides the spark control circuitry, immediately reducing the SCR conduction angle until the overcurrent conditions terminate. When a dirt buildup short circuit condition occurs in the precipitator, such that the desired maximum current drain permissible is exceeded (this limit is not adjustable as the aforementioned current limit and is preestablished in the circuit design of the current limit circuit), the SCR conduction angles are immediately reduced to zero allowing self-clearing of the short circuit condition and thereafter, the gradual restoration of precipitator voltage. The same reactions also occur when other short circuit conditions are present, such as a broken wire lying across the precipitator plates; however, in this event, precipitator voltage will remain zero until the condition is manually cleared. Data acquired in an experimental installation indicates that current fluctuations may be held to within .+-. 2.5 percent by means of the present invention in the absence of sparking in the precipitator.

SOFT START

To prevent full line voltage form being applied to the precipitator 16 upon the energization thereof which might magnetize the precipitator transformer 12 and trip out the various protective devices in the circuit, the conduction angle of SCR1 and SCR2 is held to zero during energization and gradually increases up to the normal operating point. Referring again to FIG. 3, power is applied from terminal 102 through the normally closed contacts of relay 104 to terminal 106 to charge capacitor 108. As we have seen, the charging of capacitor 108 applies a back bias to field effect transistor Q6 which holds the SCR conduction angle to zero by maintaining PNP transistor Q7 in cutoff. When the line circuit breaker 110 in FIG. 1 is closed, the voltage applied across transformer 111 operates coil 114 of relay 104. Operation of relay 104 removes the DC voltage applied to capacitor 108 and the back bias is slowly removed from transistor Q6 as capacitor 108 slowly discharges through potentiometer 116 and resistor 118. Until capacitor 108 is discharged, transistor Q6 is back biased maintaining Q7 in cutoff and the conduction angle of SCR1 and SCR2 at zero degrees. Only after the initial discharge of capacitor 108 is the precipitator voltage subject to the control of the spark and r.m.s. line current responsive circuits.

SCR GATE DRIVING CIRCUIT

The operation of the SCR gate driving circuit 120 of FIG. 4 may more easily be explained with reference to FIG. 5. The unit is available commercially from the Sprague Electric Company, Special Components Division, North Adams, Massachusetts under the name VecTrol VS6732.

The DC input to SCR gate driving circuit 120 determines the phase shift of the triggering pulses to SCR1 and SCR2 with respect to AC source 10. A voltage appearing at terminals 125 and 126 of FIG. 3 is applied to like numbered terminals in the circuit of FIG. 5. Line voltage from source 10 is applied to the primary winding 127 transformer T1. When the instantaneous polarity of terminal 128 is positive, the potential at terminal 130 of secondary winding 132 is also positive. This signal is passed through diodes 134 and 136 to charge capacitors 138 and 140 respectively to the same potential. This same positive signal is applied through resistor 142 to the anode and directly from terminal 143 to the cathode of SCR3. Since the cathode of SCR3 is maintained at the same potential as the anode, SCR3 is prevented from firing and the conduction angle of SCR1 remains at zero degrees.

Concurrently with the above, the corresponding negative potential at terminal 144 of secondary winding 132 is fed through resistor 146 to the gate electrode of SCR3. The application of a negative potential to the gate electrode of SCR3 prevents the firing thereof in response to any transients which may be induced in the circuit. Diodes 134 and 136 prevent the capacitors 138 and 140 from discharging when the polarity of the voltage of winding 132 is reversed.

During the negative cycle, however, capacitor 140 begins to discharge through resistor 148, and capacitor 138 through resistors 150 and 152. The RC time constant of capacitor 140 is much smaller than that of capacitor 138. The discharging of capacitor 140 prior to capacitor 138 allows the voltage on the anode of SCR3 to become positive with respect to the voltage applied to the cathode thereof. SCR3 is thus enabled and will conduct upon the occurrence of a pulse on the gate electrode.

The negative bias on the gate electrode is removed upon the reversal of polarity. The voltage appearing at terminals 125 and 126 of the circuit of FIG. 3 is applied to like terminals of transformer T2. The inductance of the windings 158 and 160 of saturable reactor 162 is reduced with an increase in the current in the input winding 164 which accompanies the absence of sparking or a current limit signal. The phase of the signal generated across winding 166 of transformer T2 is thus shifted when the saturable reactor 162 fires advancing the firing angle of SCR3. Resistor 168 and capacitor 170 give the SCR3 triggering pulse a fast rise time and diode 172 blocks a reversal of current in winding 166 of transformer T2. Diode 174 is used to prevent the cathode of SCR3 from becoming positive with respect to the trigger electrode.

The operation of the lower half of the circuit of FIG. 5 and the firing of SCR4 is identical to the upper half as explained supra and will not be further discussed, firing of SCR4 occurring 180.degree. later in all cases.

The triggering of SCR3 and SCR4 controls the voltage across terminals 176 and 178 and terminals 180 and 182 respectively. These terminals are connected to the trigger electrodes of SCR1 and SCR2 in the gating circuitry 20 of FIG. 1 to control the voltage of precipitator 16.

Claims

I claim:

1. A precipitator control circuit comprising,

controllable gate means operable for controlling electrical power to a precipitator,

spark sensing means connected for sensing the voltage drop of electrical power supplied through said gate means to a precipitator being controlled upon the occurrence of a flashover,

wave forming means operable to form at least one control pulse of predetermined magnitude and duration in response to a flashover sensed by said spark sensing means,

fast response means connected to receive and store energy of said formed control pulse and discharge the same at a predetermined fast rate,

variable impedance means operable to control the amount of energy of said formed control pulse to be stored in said fast response means,

slow response means connected to receive and store energy of said formed control pulse and discharge the same at a predetermined slower rate,

control means responsive to the discharge of said fast response means to correspondingly regulate said gate means to substantially cut off the electrical power supplied therethrough to a precipitator being controlled,

said control means also being responsive to the discharge of said slow response means to regulate said gate means by correspondingly controlling the rise time of electrical power supplied therethrough to operating levels subsequent to cutoff thereof in response to said fast response discharge.

2. The precipitator control circuit of claim 1 including a current sensing means operable for sensing a current rise in excess of a predetermined operational level of electrical power supplied through said gate means and to form a current control signal of corresponding magnitude and duration and apply the same to said slow response means to correspondingly reduce power being supplied through said gate means.

3. The precipitator control circuit of claim 2 wherein said current control signal is regulated by said current sensing means to be of sufficient magnitude to initially reduce power being supplied through said gate means to substantially zero.

4. The precipitator control circuit of claim 2 wherein said current sensing means senses the r.m.s. value of said current.

5. The precipitator control circuit of claim 4 wherein said current sensing means includes a variable current limit adjust to regulate the magnitude of the sensed current and an incandescent bulb-photodiode combination to measure the r.m.s. value of the sensed current.

6. The precipitator control circuit of claim 1 wherein said fast response means and slow response means primarily consist of RC networks connected to a common input for said control means.

7. The precipitator control circuit of claim 6 wherein said slow response RC network is provided with a variable impedance input to permit adjustment of the amount of energy of said formed control pulse to be stored therein.

8. The precipitator control circuit of claim 7 characterized by variable impedance means in said RC Networks to permit variation of the RC discharge time constant.

9. The precipitator control circuit of claim 1 wherein said wave forming means includes a monostable multivibrator.