CA1089002A - Automatic control system for electric precipitators - Google Patents

Abstract

AUTOMATIC CONTROL SYSTEM

FOR ELECTRIC PRECIPITATORS

ABSTRACT OF THE DISCLOSURE

In an adaptive type of control for precipitators, the current reference ramp signal is compared with a precipitator current signal to produce a phase reference signal which is combined with sawtooth waves to produce a first train of pulses.
In a spark detector, signals proportional to electrode potential are differentiated, and the differentiated output is integrated.
The integrator output, if in excess of a reference potential, triggers a spark pulse of predetermined width and amplitude. The spark pulse, in addition to causing a predetermined small set back of the ramp signal, is integrated to produce a turn-off and recovery signal having a rapid excursion and subsequent slower return excursion of adjustable slope. The latter signal is com-bined with the sawtooth waves to produce a second train of pulses commencing after a predetermined turn-off interval and increasing in width at a predetermined recovery rate. Both trains of pulses are added to produce enabling signals for controlling the SCR's in the precipitator input circuit. Arc signals produced by an arc detector cause additional downward ramping of the ramp signal to minimize electrode current during arcing.

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Description

. 41-IYO-2366 '~Q~
::.
AUTOMATIC CONTROL SYSTEM
FOR ELECTRIC PRECIPITA~ORS

BACKGROUND OF~TH~ INVENTION
This invention relates to the automatic control of electric -~
energization applied to precipitators or to similar types of electrical apparatus subject to sparking.
Precipitator systems utilize adjacent electrodes having a large-potential difference between them. Gases or fluids passing through t~ese electrodes-are exposed to an electrical field~and are ionized such that undesired particles are attracted to the electrodes and thus, are removed from ~the gas or 1uid stream.
The efficiency of particle removal is directly related to the magnitude of the voltage difference between the electrodes However, an excessive potent~al results ~n sparking or ln a more .: .
severe condition termed "arcing". Sparking, ~mless quickly inhibited produces arcing. ~i -Arcing may, of course, also result from other causes,such as fai;lures of the precipitator electrodes~ During arcing, the precLpitator system does not perorm its precipitation function and additionallyl consumes undesirable and large amounts of electrical energy.
Maximwm efficiency of precipitator ~ys~ems is belleved to occur with maximum average DC electrode, i.e. ionization, current.
If the electrode potential~is substantially below the sparking level, the ionization current and, therefore, the precipitator efficlency is reduced. Conversely, if the electrode potential is too high, sparking and arcing results. Precipitator efficiency is ~similarly reduced with excessive sparking and with arcing. -~Precipitator systems~ should therefore be operated just at the ': ~ ", '" " :' . .

41-TY0-?i366 sparking threshhold. This maximizes efficiency and minimizes the production of destructive arcing.
Sparking is affected by many variable parameters, and therefore, may initiate at constantly varying magnitudes of electrode potentials and currents.
Accordingly, an adaptive spark testing process is utilized wherein electrode voltage is increased uhtil sparking is detected and is subsequently reduced. Such precipitator control systems produce an increasing ramp signal which provides for the increase9 with time, of the electrode voltage. -Responsive to spark detec-tion this ramp signal is réduced, i.e. set back~ prior to resuming its increase. Such an adaptive type of sawtooth or ramping control causes the electrode potential to continuously increase to the sparking level, to be set back upon sparking, and to ramp upward again to the sparking level.
Numerou6 parameters afect successful and efficient operation o the system. For maximum efficiency, the electrode ~ `
potential must be maintained for maximum time intervals closely adjacent to the sparking potential, and the rate and severity of sparking must be controlled.
It has been recognized that there are sparks of diferent severity. Some sparks of minimal intensity or duration may not produce arcing. However, remedial action must be taken in respect to other types of sparks. Additionally, maximum precipitator ~effici~ency is attained if the number of potentially harmful l sparks per unit of time is maintained at a low spark rate ;~ predetermined for the selected process and system. Spark rate is a function of the~slope of the upward ramp, and of the magnitude of set back. Maximum efficiency is attained by a small set back 1 . : .: .
30~ and a small slope of the upward ramp. This provides for -~

2 -,'1 : .' ,,' ' . . . . . . .. , ., - -l-IY0--2~66 continuous excursion of the electrode potential close to the ' sparking level. Proper adjustme~t of the slope of the upward ramp and of set back also provides the desired spark rate.
However, it has ~lso been found that upon detection of a potentially harmful spark, the electrode potential must be '~
reduced sufficiently in magnitude and time duration to quench -~
electrode ionization current. The ~ctual turn-off requirements ' _ depend upon the precipitator system and the type of process. If there is-insuffiGient turn-off,-eparking is æustained and arcing- ' '' is induced subsequent to set back. This objective could be met by setting back the ramp sufficiently to reduce the elec~rode voltage from a potential adjacent to the sparking level, e.g.
50 Kvs to zero and ramping upward to the sparking potential. ~
This results-in a drastic reduction of the ramp and a subsequent ' ' ramp slowly increasing from zero 'toward the sparking potentialO
At the required low spark rates, this results in a drastic reduc-tion o~ average electron potential'and current and a drastic .
l reduction in efficiency. Accordingly, it has been found desirable -'' '`~ upon spark detection to almost instantaneously reduce the electrode voltage to a minimum, e.g. zero volts. Ater a very ~' brief turn-of~ time interval, the electrode potential, and thus I
. I , the electrode current, is increased at a fairly rapld rate, the recovery rate,to a magnitude slightly below the sparking level.
Subsequently, the electrode potential is again ramped upward at the previously described slow rate until 'sparking is again encountered.
The above described turn-off interval and recovery rate "
must be accurately selected for the particular precipitator ' system and process which is utilized. This is to assure that 30~ the ionization current in the precipitator is sufficiently , : ... :
i . .
~ - 3 - :

YO^2~66 ~ 2 quenched so that arcing does not resume, and that the subsequent turn on of the AC phase control system, e.g~ solid state switching means such as back ~o back connected silicon controlled rectifiers, is not excessively fast. Conversely, a minimtun permissible turn-of and maximumi permissible rapid recovery rate improve the effi-ciency of the precipitation process.
Each of the above recited parameters can be controlled by adjustments within the control circuit. However, frequently there are undesirable interactions between them.

It is an object of the invention to provide an improved .
solid state control system for electric precipitators.
It is a urther object for such a control system to provide improved turn-off and recovery of precipitator electrode energization responsive to spark detection.
It is a further object for a precipitator-control system to provide for sensi~ive detection of sparks and for appropriate remedial control of precipitator electrode energization responsive either~to sparking or arcing.
It is another object for a control permitting adjustment of both turn-of~ time and recovery rate by means of a single adjustment.
`l It is a yet ftlrther object to provide a precipitator control system providing for separate and independent ad~ustment of control system parameters affecting accurate control of electrode potential, such as spark rate, set back, turn-off and recovery rate, and spark detection sensitivity.
,~ SUMMARY O~ THE INVENTION
y ~ The invention relates to an improved control system for controlling variable impedance means, e.g. static switching means, i , . . .
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~l-IYO-2366 .
which regulate the electrical energization of the precipitator electrodes.
According to the invention, sawtooth waveforms, of frequency synchronized with an integral multiple of the frequency of an AC input to the precipi.tator system, and phase reference signal, e.g. r~mp signals, are applied to a first time ratio modulation means to produce a first train of pulses. A
turn-off and recovery means, responsive to a spark pulse gener- ~ -ates a turn-off and recovery signal having an initial and rapid potential excursion and a-subsequent return excursion at a slower rate. Second time ratio modulation means responsiveito the latter signal and to the sawtooth waveforms produces a second train of pulses commencing after a predetermined turn-off interval and increasing in pulse width at a predetermined recovery rate, the turn-off interval and recovery rate both being a function of the rate of the re~urn excursion of the turn-off and recovery signal.
The .first and second train of pulses are combined to form a third train of pulses, and the static switching means is gated on during ~;~ time intervals defined by the pulses of this third train of pulses~20 In a preferred embodiment, the turn-off and recovery means comprises integrating means responsive to a spark pulse of predet~rmined duration, and incorporates a single adjustment afecting the .. ...
integration rate, which varies the rate o return excursion, and thus simultaneously adjusts turn-off time and recovery rate.
,1 , .
Another feature o the in~ention relates to the means for producing the above-referenc~ed phase reference ramp signal. The `
latter comprises current reference generating means wherein integrating means are connected to produce a ramp signal in one direction to gradually increase the potential applied to the ~30 precipitator electrodes.~ Upon occurrence of a spark, a spark ,........... : _ 5 _ .:::

... . . . .
.,~, ~ ,' ~I.Ql~ 41--IYO--2 3 6 6 pulse of predetermined brief time duration is applied to the current reference generating means to cause ;
a predetermined small set back. An arc detector responsive to the ratio of voltage and current in `~
the precipitator electrical system produces an arc signal while there is a prede~ermined current to voltage ration, indicative of arcing. This arc signal is supplied to the current reference generat-ing means to cause a further set back while the ` 10 arch persists. In a preferred embodiment, the set-back is limited to a predetermined small leuel ;
sufficient to maintain sufficient energization in the electrical system to permit sustained operation of the arc detection means.
A related feature of the invention pertains to the means for generating the above-referenced spark pulse.

, :.
A source adapted to be connected in a series re-sistance circuit across the precipitator electrodes, provides a voltage signal proportional to the -~
:. :. . . electrode potential. This is applied to differentiating means comprising resistance and capacitance means. The differentiating means output is applied to integrating means, whose output is supplied to one input of com*arison means. The other input of the compari~ion means is connected to a source of reference potential.

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Pulse yenerating means for generating the spark .
pulse of predetermined magnitude and duration, has its input connected to the output of the comparison means, and is triggered responsive to the integrating means output attaining a predetermined potential relative to the source of reference potential.
Adjustment means for adjusting one of the signal out-puts are provided for adjusting the spark detector sensitivity.
' "'`':' The novel features believed characteristic of this .
invention are set forth with particularity in the appended claims. The organization and manner of operation of the invention, together with further objec*s and adcantages thereof may best be under-stood by reference to the following description taken in connection with the accompanying drawings. Other : . .
copending applications relate to additional features described herein. Specifically, U.S. Patent No. 4~ 3$
dated ~be~ ~\q~ related to the current limit and overcurrent cut off system described in the last ~ -subsection of the description of the preferred em- `~
bodiment ln connection with FIGURES 8 and 9. Canadian Serial Wo. ~ filed ~ elates to the sawtooth generator described herein. ;~

~, .

IYO-2366 :~
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BRIEF DESCRIPTION OF DRAWINGS
FIGU~E 1 is a block diagram of a precipitator system incorporating a preferred embodiment of the invention; :
FIGURE 2 is a simplified schematic of designated portions of the precipitator system of FIGURE 1, including the spark detector, current reference generator, difference amplifier, sawtooth generator, turn off and recovery eircuit, and phase control comparator;
- FIGURE 3 is a graphic representation of waveform8 illustra-10 . ting the effect of the signal output of t~e current reference generator and of the signal output of the turn off and recovery.
circuit on operation of the preoipitator;
FIGURE 4 is a repres.entation of waveforms produced by the `~
; turn off and recovery circuit and illustrating their e~ect on L5 operation of the precipitator; ``.
. FIGURE S a) through ) are representations of waveforms :.
appearing at designated portions of the phase control comparator; :
~: FIGURE 6 is a graphic illustration of the time and voltage detection sensitivity of the spark detector, and the effect of 20; the spark sensitivity control;
~I FIGURE 7 is an illustration of the current and voltage responsive detection characterist:Lcs of the arc detector;
: I ....
FIGURE 8 is a simplified schematic of the current limit ~:1 and interrupting circuit; and ~,25 FIGVRE 9 is a graphic representation of the current limit and current interrupting characteristics of the circuit of FIGURE 8.
DESCRIPTION OF THE PREFERRED EMBODIMENT
....
For convenience, the~ description of the preferred embodiment ~-is divided into:the following subheadings:

' . `i, .. ~

0-236~

Z , 1. Description of Precipitator Block Diagram and of the Arc Detector, 2. Spark Detector, 3. Current Reference Generator,

4. Difference Amplifier,

5. Sawtooth Generator,

6. Turn-Off and Recovery Circuit,

7. Phase Control Comparator and Phase Control ' OscillatorJ ''~ ' '''' ''.. , - , - , .. .
10 8. Current Limit and-Interruption Circuit.

DIAGRAM AND OF_THE ARC DETECTOR . ~ .
Attention is directed to FIGURE 1, a block diagram of the precipitator system and its associated control. A precipitator 2 conventionally has wires 3 connected to a negative source of high DC potential by means of line 4 and plates 5 connected to ground `
by line 6. Power is provided to input terminals 7 and 8 rom a source, such as 440~ w lts, 60 cycles. Input terminals are coupled through contact tips 9 and 10 of a circuit breaker with contact 9 connected by line li to one end o~ the primary winding 47 of a high voltage transformer. Contact 10 is connected by line 12 to a conventional phase controI circuit illustrated as incorporating back to back connected silicon controlled rectifiers 13 and 14. The output of the phase control circuit is connected by line 45 via reactor 46 to the other side of transformer primary wlnding 47. The secondary winding 15 i9 connected to the input of a full wave ~; rectifier 16 which typically comprises a diode bridge circuit. The ~`
negative output terminal of the rectifier is connected by line 4 to wire~3 of the prec~ipitator~ and the positive ou~pu~ is connected by line 17 and series connected resistance 18 to ground.

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YQ-236~ ' "
~ 8 ~ 2 Precipitator operation is controlled by adjustment o the precipitator electrode potential and current by appropriate phase control of silicon rectifiers 13 and 14i Phase control oscillator 20 supplies firing pulses for application to the gate of silicon controlled rectifiers 13 and 14. For simplicity, iiring pulses are illustrated only as being supplied by line 19 to the gate of silicon controlled rectifier 14. It should be understood, of course, that similar ~iring pulses displaced in phase are applied .
from phase control oscilla~or -20 to the gate of silicon-controlled -rectifier 13. The system is principally controlled by a current reference signal produced by current reference generator 22 and applied by line 21 to difference amplifier 23. Although reference .-has previously been made to controlling the voltage applied to the precipitator, this voltage produces the corona current from the precipitator wires to the plate and it is desirable to control this corona current. Accordingly, the precipitator, or transformer secondary current, is controlled. Secondary current flows through resistor 18, the resulting-voltage on line 17, representative of `-~
precipitator, i.e. secondary, current is applied by line 24 to a second input of the difference ampLifier ~3. The resulting diference between the current reerence signal on line 21 and the second~ry eurrent feedback on line 24, the phase reerence signal, ; is supplied by line 25 to phase control comparator 26. Phase con-trol~sawtooth generator 27 generates sawtooth waveforms which are synchronized in time duration to the alternating current input requency, e.g 60 cycles, and which have equal and opposite excursions of predetermined magnitude from a reference potentiaL, `
e.g. zero volts. These sawtooth waveforms are supplied to the phase control comparator by line 28. The phase control comparator 3~0 ~modulates the sawtooth and phase reference signals to produce a 10 - . "~,, ,"~,, .

' ~ 0~Z , .

gating signal on output line 29 which is pr~vided to phase control oscillator 20. The phase conkrol oscillator produces firing pulses during the duration of the enabling gate signal~ supplied by the pha5e control comparator; As will be explained subsequently, turn-off and recovery generator 30 produces a turn-off and recovery signal on line 31 which is also supplled to the phase control com- ¦
parator in order to modify the enabling gate output signal on line 29.
::~ . , .
The current reference generator comprise9 an integration 10 circuit which produces a substantial-ly--linearly increasing current .~ reference signal on line 21. Aceordingly, the phase reference 1 ~;
signal on line 25 would normally be increased. The firing pulse output from phase control oscillator 20 is thus advanced so as to . . .
result in greater conduction of the silicon controlled rectifiers 13 and 14 with a resulting increase in preci~itator electrode .. . ~ . ~ .
potential and current. This ultimately results in a spark across precipitator elements 3 and 5. ~ ' Upon occurrence of the spark, the electrode voltage which l:
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may, for example, be in the range o~ 50,000 - 80,000 volts, ~20 decreases sharply to near zero volts within about 1 millisecond. ¦`~
, , . .
This drastic reduction in secondary voltage is utilized to provide a spark detection signal. Line 4, which colmects precipitator wire 3 to the negative output o rectifier 16, is also connected by -series connected resistors 32 and 33 to ground. The junction 34 of l~25 these resistors is coupled by line 35 to the input of spark detector 1 36. Resistor 3~ has a subst~ntially higher magnitude than resistor ¦~ 33 so that the output voltage on line 35 is at a relatively low potential in respect~to the negative precipitator electrode voltage. ~ :~
For example, resistor 32 may be 80 megohms whereas resistor 33 may ~.
!-for example be 10,000 ohms. Thus, for example, with a potential ' ' i . ::: :.

'.: :` ' ' ' ~[J8~0~2 . .
drop of approximately 80,000 volts re~ulting from a spark~ the signal on line 35 would have a potential of 10 volts. This tran~- -ient signal on line 35 is processed by spark detector 36 so as to produce on line 37 an output pulse having a predetermined pulse- -width and amplitude. This spark pulse is applied to the turn-off and recovery generator 30 and is ad~itionally applied ~o the current reference generator 22. During the time duration of the spark pulse, the integrator in the current reference generator integrates . ~.-in an opposite direction such that the current reference signal is reduced-to-a lesser-value. Upon termination of the spark pulse, the integrator resumes its normal upward integration and the current reference signal again increasee. Reference is made to FIGURE 3 for an illustration of how the current reference signal varies as a ;
function of a detected spark. During normal operation of the integrator, the current re~erence si~nal increases linearly at a low rate, such as or e;~ample, increasing from a minimum to maximum voltage over a time period of from l to 10 minutes. This increase of the current reference signal is illustrated by solid line portion a) of FIGURE 3 Upon occurrence of a spark, the spark detector pulse is applied to the current reference generator for a predeter~
1 mined time period, such as for example, 10 milliseconds~ In FIGURE
-~ 3 the initi.ation of the spark pulse is indicated at point b) and ;l its termination occur5 at point d). During the occurrence of the ~ spark pulse, the current reference signal is reduced and decreases ~ . .
I -25 as shown by dashed line segment c). Subsequen~ to the termination -~
~, ~ .. .
of the spark pulse at point d), integration resumes in the normal direction and the current reerence signal resumes its upward ramp, ; as shown by line segment e).~ It should be noted that the set back of the current reference~signal results in only a small voltage decrease at the precipitator electrode. For example, a one to ten . .; , : .,, :. :.
~ ~ - 12 -:'1 . .
.~: .. .
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iyo-2366 :~
, ~ q3 , percent reduction Qf the ma~imum precipitator volta~e.
In order to assure that the precipitator ionization current is adequately quenched upon occurrence of a spark, the spark pulse on line 37 is additionally applied to turn-off and recovery :
generator 30. Circuit 30 producas an output on line 31 so as to :-. . .
substantially and instantly cut off conduction of the silicon - controlled rectifiers 13 and 14 upon occurrence of a spark. Rec-tifier conduction is cut of~ for at least the duration of the spark l~
pulse and then iB increased at a pre-established rate. Line f) of FI&URE 3 illustrates the reduction o$ electrode potential resulting ¦;
in cut off of the silicon controlled rectifiers upon initiation of the spark pulse. Cut off is maintained, as indicated by line seg-ment g) to point h). At point h), conduction is gradually re-established as indicated by line segment i) unt~ the precipitator electrode voltage reaches the instantaneous level established by the current reference ramp at point j). Thereupon, the electrode voltage increases at the predetermined slow rate as shown by line segment e). ;
. .
Thus, the current reference generator 22 acts as a ramp generator producing an increasing current reference signal which is combined with the secondary current feedback si.gnal in order to produce a phase reference signal. The latter is applied to the , j phase control comparator which produces enabling pulses which are applied to the phase control oscillator to provide firing pulses ~25 ~ for advancing firing of the silicon controlled rectifiers and thus ! to increase electrode potential and ionization current. Upon occurrence of a precipitator spark, the resulting secondary voltage transient causes spark detector 36 to produce an output pulse of I predetermined magnitude and time duration. This causes the current reerence generator to be set back by a small predetermined amount . ~ .
, ....
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:YO-2366 ~ ~9 ~

and to subsequently resume its upward ramp at a low predetermined rate. The turn-off and recovery generator supplîes a signal, ~ , responsive to the spark pulse9 to the phase control comparator, which modifies the gate output of the phase control comparator on line 29 to turn the silicon rectifiers off completely for a pre- ..
determined time equalling at least the durativn of the spark pulse and then to'increase the electrode potential at a rapid rate up to . '~
the instantaneous'voltage level established by the current refer~
D
-~ ence ramp signal. The;cut off time and the time for recovery must be selected or the particular type of precipitator system and for ~ ' the type of fluid or iiquid which is beî'ng precipitated~ The '.:
~;. ' recovery slope is chosen so that on the one hand there is a rapid return to the high electrode potential established by the current reference signai. This pro~ides.for maximum precipitation efficien-, :
:15 cy.by retaining the electrode potential near the sparking level for .
'~. .a maximum period of time. On the other hand~ the recovery slope.is 1' '"
~ , . .
sufficiently low to prevent an excessively rapid turn on of thesilicon controLled rectifiers~. I have ~ound it desirable to match ;'"
;the turn-off time and the recovery time such that the.two are pro~
,: ~, ~ , : . .
~'l,20~,:', portional to one anotherj and to provide for obtaining the correct .' '.
... ratio of recovery time to turn-off'time by means of a single " .
.
adjustment.
Primary current lim,iter 38 protect's the precipitator system .'`
~ against damage résulting rom overload. The limiter assures that '''' .,25 ;~' current on the primary side of the.high voltage transformer does `i'':.;. ,not exceed a predetermined maximum value. It thus protec~s the ~
sys~tem against failure such as short circuits in the silicon~con- :':'.
trolled rectifier, reactor and trsnsformer. Additionally,~ the ... ',.
primary current limiter acts to prevent precîpitator current from ,~,i30~ ' reaching destructlv'e~ levels.l There is not necessarily a constant : ::

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ratio between the primary and secondary current, since this ratio may change as a function of the phase angle of the silicon con- ;
trolled rectifiers and of other operatlng characteristics. Accor :
dingly,.the primary current is sensed so t~at remedial action i~
taken by the silicon controlled rectifier phase control circuit toward preventing the primary current from exceeding a predetermined maximum, and if that is unsuccessful,.circuit breakers 9 and 10 are tripped. The latter action may be required in the case of short circuits in the.precipitator æystem. -Gurrent tran~former cr coupled .
to input.line 45 provid-es-a primary-curren;~-~ignal on line 3g to.
primary current l;miter 38.: If the primary current approaches the predetermiend current limit, a current limiting signal is applied by the limiter output line 43 to.the current reference generator.
This modifles the current reference signal 80 as to retard the ..
firing of the silicon controlled rectifiers 13 and 14. Thus,.there ~.
is a primary current limiter loop comprising the current transformer, ~ the primary current limiter, the current reference generator, the ``
;~ difference amplifier, phase control comparator, phase control . oscillator and the silicon controlled rectifiers. If for some ~.
reason, such as for example because o shorts, this loop is unable , .
to maintain primary current below the reference limit, the primary .
. curren~ limiter 38 actuates the solenoi~ 44 of the circuit breaker : so as to open contact~ 9 and 10..... As subsequently described, the primary current limiter circuitry provides the current limit signal . .
on llne 43 as well as tripping the circ~it breaker. This permits tripping the breaker precisely on the desired time-current trip .:
,~ : characteristic line.
. While the primary current limiter will prevent excess ...
primary current which could damage the equipment and the spark .~
~; 30 detector provides for brie~ reduction of electrode potential , .
: .

[Y0-2366 responsive to sparks it is additionally desirable to reduce elec-trode potential and precipitator current while an arc occure. Thls arcing may occur, for example, if a precipitator wire breaks and falls against the precipitator plate or, conceivably, in the event 5 the spark detector should fail to inhibit the production of an arc.
In such an event it is desirable to reduce precipitator electrode voltage until the condition clearsJ instead of driving the system ~-at the p~k~rr~-current limit. For~t.his purpose arc detector 41 . .
` provides a signal on line 42 to current reference generator 22 to ~ `
. . . . . .
modify the current reference signal and thus to retard iring of the silicon control rectifiers 13 and 14 during the duration of an :
arc. Whereas the spark detector is responsive to the transient, i.e. the rate of change ? 0~ electrode potential upon initiation o~
a spark, the arc detector 41 is continuously responsive to the precipitator system voltage and current and provides an output signal ~hile the ratio of voltage to current is indicative of the existence of arcing conditions. Current transformer CT supplies a primary current signal to the arc detector on line 39 and potential transforme~ PT provides a primary voltage signal to the arc detector .;~
on line 40. The arc detector is preferably of the type disclosed in U.S. patent 3,873,282 o David C. Finch, assigned to the assignee of thi8 app~ica~ion. Slnce the clrcuit details are illustrated in FIGURE 1 and described in colume 5 of the Finch patent they are not ., I . . .
~l ~ illustrated in the present application. The arc detector prefer-: !~ 25 ably comprise3 means for converting the CT curre.nt signal to a DC
signal ~roportional to primary current, such as a first bridge rectifier supplied with the primary current signal output of the cur~rent transformer or an ~MS to DG converter, and a second bridge ~ ' l ' rectifier supplied~with th~e~primary voltage signal output of the ;~30 potential transformer. ~The current and voltage outputs from the 1.~ ' ~ ' . ....

IYO-2366 , respectiv`e rectifiers are introduced into separate input terminals of a differential amplifier which generates either a positivé or negative voltage output responslve to the ratio of the applied voltage and of the applied current signals. A balance potent~o-meter between the output of one of the bridges and one input of~thedifference amplifier may be adjusted so that the polarity of the amplifier voltage output indicates the arcing or non-arcing , states of the precipitator. The output of the differential ampli-:~ ier-may be coupled-to a suitable_shaping.. ~ircuit which provides a zero voltage output during non-arcing states and a discreetly different voltage output during arcing states. This output is ~ provided by line 42 to current reference generator 22. The arc - detector operates on the principle that there is normally a direct relationship between electrode potential and electrode current.
Thus an increase of precipitator voltage results in an increasè
. of precipitator current. However an arcing condition is manifested by a substantial decrease in voltage accompanied by a substantial increase in precipLtator current. Reference is now made to FIGURE
7 w~erein the dash dot line illustrates a typical normal precipita-~20 tor load line. Numerous parameters associated with the precipita-tion process cause continuous variations of this load line. The solid lLne of FIGURE 7 illustrates the selected switching character-istic of the dif~erential amplifier output. This line is set suit-ably above any reasonabIy expected variations in precipLtator load I; 25 ~ lines. The area above the solid line is termed the arc region and any combination of primary voltage and primary current resulting in a point within the arc region provides an arc signal output on line ~., ~ , , 42. Since pr~imary voltage~ is plotted on the ordinate and primary current is plotted on the abscissa, the arc region encompasses 3~0 operations of the precipitator system wherein the current is :; j:
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-IYO-2366 ' 3~ n~

sufficiently high and the voltage is sufficiently low to clearly indicate the existence of arcing. The arc;detector therefore, un-like the spark detector, provides an outpu~ signal indicative o~
arcing during ~he full duration of the arc. H~wever, because'of the normally encountered variations in the precipitator load line, ' the solid line defining the arc region must be high enough, i.e.
at a high enough ratio of primary current ~o primary voltage~ to -'-' ' prevent'production o an arcing si~nal-under situations when there ~
'is no arc. Therefore, the arc-detec~or does not provide as sensi- ''-'' ,, ~ .
tive an indication of sparking as the spark detector. . ~ '' ; Initiation of an arc produces a'drastic change of precipita-tor voltage which is detected.by spark detector 36. As previously -;~
described this not only causes a setback in the current reerence ~;'`~ ' signal, but also results in the application.of a turn-of.and~
.
recovery signal from turn-off and recovery generator 30 to phase - control comparator 26 so as to cut off silicon control rectifiers- `
13 and 14 for a predetermined time interval. As was described in ~-connection with FIGURE 3 the spark pulse produced by spark-detector 36 causes the-current reference generator to modify the current ~
~20 reference signal only'by a small predetermined setback during-the -- '' time duration of the spark pulse. As illustrated-in FIGURE 3',~
dash line segment c) occurs between the initation of the spark pulse ' at poin~ b) and terminates at the termination o~ the spark pulse at ;
point d) whereupon thè current reference signal commences'its - ;~
I :
upward ramp. However, if an arc is detected, an axc signal is l~ supplied on line 42 to the current reference'generator during the ' ~'' '~ duration o~ the arc. This results in a continued decline of the ramp voltage beyond point d), as illustrated by the dsshed line which extends downwardly from point d). The ramp declines downward--ly untiI the termination of the arc signal whereupon the upward ramp . 1 :
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IY~-236~
J.~B9~Z

is resumed. However, ~he ramp declines downwardly onLy ~o a pre-determined minimum level whereupon the ramp signal remains at such ~
minimum level until the termination of t~e arc. This level, iden- i tified asi "sustained arc" in FIGURE 3 may for example represent a precipitator electrode voltage of about 10~ of the sparking level.
Thus upon occurrence of an arc the turn off signal on line 31 causes the silicon controlled rectifiers to be cut off and to be rapidly phased back up, as indicated by solid line segment i~ of , FIGURE 3, until the pre~ipitator-electrode voltage is at the level established by the ramp signa-l, i.e.- the intersection of---~ine =---segment i) and the downwardly extending dashed line. Thereupon during the duration of the arc the electrode voltage continues to decrease to the minimum sustained arc level and to remain at such ~ level until the arc terminates; -Therefore, precipitator power is- ;
; 15 reduced to minimal levels during occurrence of an arc. A timing device may be used to cut off power if the arc is not extinguished within a predetermined time interval. For purposes of circuit pro-i . .
tection, the ramp could decline, and the electrode potential couldbe reduced to zero. However, by limiting electrode potential to a predetermined minimum level, sufficient power contlnues to be pro- ~
vided to the precipitator system such that the arc detector contin- `
. i .
ues to function and the arc detector signal remain6 ~mtil the arc has actually terminated. Thus main~enance of power at a low level permits detection of when the arc clears. As subsequently described, the minimum arcing level may be attained by a suitable clamping circuit in the current reference generator. ;
For a detailed description of circuit components, attention is now directed to FIGURE 2.
;: i " , ~ } ~ :
. j --IY()-2366 ."
SPARK DETECTOR
The secondary voltage signal on line 35 is applied to the , :
input of the spark detector 36. Since a spark causes the electrode ~.
potential to drop drastically from a very high level, such as, for :
example, from between 50 Kv to 80 Kv to near zero volts, the scaled .:
down secondary voltage signal on line 35 similarly has a s~bstan~
tial excursion, such as for example, from approximately minus 10 volts to zero volts.. Upon termination of the spark or arc condition the precipitator voltage rises again to..the.sparking level, and the . secondary voltage similarly rLses.back again to its quiescent amplitude in the range of minus 10 volts. . . : .
Line 35 is coupled through capacitor 50 to the base of tran- .
sistor 51. A resistor 52 is connected from the base to a negative :...
source and a resistor 53 is connected from the transistor emitter : . .
to the negative source. The collector is connected through serially .
connected potentiometer 55 and resistor 54 to a common terminal. -.:
, ~ . . .
.~ The arm 56 of potentiometer 55 is'connected to the input of opera-tional amplifier Al which has an integrating capacitor 57 connected .
~ between its input and its output. The output of integrating ampli- . .
fier Al is connected to one input of comparison amplifier 58 whose other input is connected to a source of reference potential. The , output of amplifier 58 i~ applied to monostable multivibrator 59 which ha~ its output connected to line 37.
Capacitor 50 and resistor 52 constitute a differentiating ::
-I 25 circuit. The collector signal of buffer transistor 51 thus has an : amplitude proportional to the amplitude of the differentiated signal which in turn is:proportional to the amplitude of the vol- :;
tage differential of the precipitator electrode as well as its slope of decrease. The:output~of integrating amplifier Al there-fore is a funct~on:of the area~under the differentiated signal and :
, t~
. . `.':' ,' , - : : ., Yo~ 2366 ~j thus its amplitude is representative of both the magnitude oi the `"
voltage drop and of its duration. If the amplitude of the output of Al exceeds the reference potential, comparison amplifier 58 ' gates device 59. Monostable multivibrator 59 thus produces an out-put pulse of predetermined time, such a~ for example lO milliseconds, ' and of predetermined voltage, in response to an integra~or outpu~
having at least a predetermined magnitude. The time con~tant of ; the differentiating circuit is made short enough 90 that the inte- -grator output has insu~ficient.amplitude'to trigge~_the-multi~ibra- '; ';
tor in response to very short--duration~ parks.----These very short sparks are termed self-quenching sparks because they have insuf-- -ficient time duration to result in sustained arcing. By discrimina-ting against such self-quenching sparks, the spark detector avoids the needless turn-off of the silicon controlled rectifiers and thus 15 increases the efflciency of operation. However, the time constant of the diferentiating circuit must be long enough to allow'for the rate of fall of secondary voltage responsive to a spark. Certain .. , .,: .
types of precipitators, such as water type of precipitators have a 1-much slower rate of fall than dry types. In one embodiment of the : 1:
invention, a time constant of'one-hal of a millisecond provided i adequate results ~or both types of precipitator systems.
Thus the precipitator electrode voltage signal is differen-tiated`, the dierentiate.d signal is integrated, and the amplitude of the integrated signal output is~compa~ed by device 58 with the referenc'e potential. In the e~ent the amplitude of the integrated signal exceeds the reference potential the pulse generator prod~lces a spark pulse of predetermined magnitude and width. Spark pulses ! .
~ are produced only if the precipitator spark has at least a minimum -~
'' ~ predetermined pulse width, such as for example 10 microseconds, and 30 ~ additionally has at least a minimum pulse height, such as for '' ' ~. . :. . .
~ . .

~ 21 -,`,~ . , ;` .:
.
. . . . .

:y0-2366 ' .;1~,, .
example 20 Kv. FI~,URE 6 illustrates the effect of the spark sensitivity potentiometer 55 whose setting varies the hyperbola of sparks which will be detected to produce a spark pulse. In FIGURE ' 6'pulses located on the inside o the hyperbola are detected where- ' as those on the outside are not. Adjustment of potentiometer arm 56 to its lowermost position provides maximum sensitivity resultLng '- in the detection of pulses having minimum pulse heigh~ and minimum pulse width. Conversely, setting o'the arm to its uppermost posi-' tion provides minim~m sensitivity and a spark pulse is produced ; 10 only with sparks having a greater pulse width and pulse helght.
Thus a single control, potentiometer 55, may'be adjusted to provide ' the desired spark sensitivity for'the particular precipitation ~ ' proce~s. ' . . . . CURRENT REFERENCE GENERATOR
: .
Spark pulse line 37 and arc detector line 42 are connected ~. ,"~ , ..
to the input of current reerence generator 22, and specifically'to ~'` the input-of open collector device 65 which performs an OR function.
; The output of device 65 is connected by line 66 to the anode of diode 67~. The cathode of diode 67 is serially connected through , ~ .
I 1;20 similarly poled diodes 68 and 69 to the input of operational ampli-, 1 .
fier A2 whose output supplies the current reerence signal on line ` ' 21 to the difference amplifier. Operational amplifler A2 constitu-tes an'integrating circuit with capacitor 60 connected across its j -input and output. A "spark rate" potentiometer 63 has its end '' ;25 terminals connected~between con~non and a source of negative po'ten-tial and its wiper arm 64 is connected through resistor 62 to the input of amplifier A2. Primary current limiter line 43 is addition-ally connected to the junction of resistor 62 and o~ the input of -~2, Diode 70 has its cat~ode connected to the output to A2 and its '`
~l30 anode to the junction of line 66 and the anode of diode 67. A - ' - ~ - 22 -:: .
. ' [Y0-2366 ' iQB9~2 :
"setback" potentiometer 72 has its end terminals connected between common and a source of positive potential and its wiper arm 73 connected through resistor 71 to the junct~on of line 66 and the anode of diode 67.
During normal operation of the current reference generator, input current flows from the source of negative potential through spark rate potentiometer 63 and resistor 62 to the input of inte-grating amplifier A2 causing the output of the integrator, i.2.
line 21, to iDcrease in-a-posi~ive direction.~ The integrator has lD a long time constant which may be set, for example, to cause the !~
output to ramp up at a rate requiring up to 10 minutes or the electxode potential to risa from zero volts to the maximum level. !
Adjustment of the spark rate potentiometer 63 controls the slope of the integrator output signal and therefore controls the requency of precipitator sparking.
In the event the prinary current of the precipitator exceeds a predetermined magnitude the primary current limiter produces current flow in line 43, in opposition to the current in resistor l~ 62, so as to reduce the magnitude of the current referènce signal '20 on line 21 and to maintaiD primary current within the maximum allowable limit. During the above-described operationj components 66 through 73 do not affect operation of integrating amplifier A2.
.j .. .
Current flows from potentiometer 72 through resistor 71 and line 66 to open collector device 650 Line 66 is normally maintained at a `
~ . . ...... ... .
predetermîned low Level. ~
. Responsive to a precipitator spark a spark pulse is applied ;
by line 37 to the input of device 65 causing the output of the latter to rise to a predetermined high level for the predetermined time duration of the spark pulse. This results in current flow ,30~ ~through potentiometer 72 and~reslstor 71 being switched and diverted ~ -i ~ ; ., :

~ ~ - 23 - ~

~ : , .
,( - :

t~O~2366 from line 66 and device 65 and~instead flowing through diodes 67 through 69 to the input of A2 in a direction opposite to the . il direction of current flow through resistor 6~. As previously . .
described in connection with FIGURE 3, the current reference signal ~-:
thus commences to decrease upon initiation of the spark pulse and i~
to continue its decrease until the termination of the spark pulse.
Since this setback occurs during the predetermined time interval .:
of the spark pulse, the set~ack voltage, i.e. the difference o~
potential between points b) and d) o~ FIGURE 3, is established by the slope of the setback ramp. This;slope in turn is established by the.time constant of the integrating circuit of operational .amplifier A2. During setback current flows from a positive poten~
tial source through setback potentiometer 72 and resistor 71 . through diode 6i through 69 to the operational amplifier A2. There-fore, the setting of arm 73 of potentiometer 72 determines the ; slope of the setback, and thus the setback voltage. The time 1:
constant may be selected such that the downward ramp has a somewhat ~ .
greater slope than the upward ramp,.for example, providing for a decrease from full precipitator current-to-zero in about one second.
.20 It-may thus be seen that the current reference generator has two adjustments which act indèpendently of one another. The spark rate l.
potentiometer adjustq the slope o~ the upward ramp, and the setback :.
potentiometer ad~usts the slope o~ the downward ramp, and thereore , .:
the setback potential. l. .-.
I~ an arc occurs a signal is supplied from the arc detector i I .
~ on line 42 to device 65 for the duration of the arc. The arc signal .
;l~ produces the same results as the spark pulse signal causing the ou~
pu~ of the device 65 on~line 66 to swith to a predetermined positive level, tojdivért the current flow in potentiometer 72 and resistor .. ~.
71 through diodes 67 through 69 so as to cause a downward ramping :~
~,.

~ 24 ~
., . ~ -- . ~
.,, ~ . . .
~: , . . ..
, :, .

IYO-2366 :, action at the output of integrating amplifier A2. Actually an arc is initia~ed by a sharp reduction of precipitator electrode poten-tial which produces a spark pulse o~ line 37 which by itself is , . . .
sufficient to change the state o the output of device 65. The ; 5 difference between the spark pulse and arc signal application to ~-device 65 is that the output of device 65 instead of switching back to its low state upon termination of the spark pulse remains at its high state until the termination of the arc signal on line 42.
Therefore, the current reference signal c~ntinues to ramp down, beyond the termination of the~-spark-pulse,--until the arc terminate8.
This occurs because the high level on the output of device 65 enables continued current flow through potentiometer 72, resistor 71 and diodes 67 through 69. However as previously explained, it is desirable to limit the downward ra~ping action to some small predetermined level, such that the precipitator electrode potential is maintained at some small voltage identified in FIGURE 3 as the sustained arc level. This is-accomplished by clamping action of diodes 67 through 70. Reference is made to FIGURF. 3. Assume for purposes of e~planation that downward ramping commences at point b) and that at such point the current reference signal on line 21 is appxoximately 8 volts. Further assume that it is desired to limit the minimum current reference signal under susta-lned arc condi~ions to 1 volt. During the presence of an arcing signal on line 42 the current re~erence signal continues its downward ramp along the dashed line segment o FIGURE 3J resulting in the continued decrease of the current~reference signal voltage. Assume further that the ; anode to diode potential of ea~h of the diodes 67 through 70 is .5 volts. Thus ~he potential difference across the three diodes 67 through 69 is 1.5 volts, i.e. line 66 is 1.5 volts positive in respect~to the input of A2. When the output of integrating , ---Z

amplifier A2 has dropped to 1 volt, diode 70 clamps the output o A2 to that level, and prevents ~urther decreases in A~ output ;
voltage.
DIFFEP~EMCE AMPLIFIER :
;5 In difference amplifler 23, the current reference signal on line 21 is supplied through resistor 80 to the input 81 of opera- -tional amplifier A3. In addition, the secondary current feedback signal on line 24 is applied through resistor 85 to the same input, .
81, of A3.- Capacitor 82 is connected between input terminal 81 and output line 83 of A3. The-output of A3 i9 connected by line 83 to the input o operationa~ amplifier A4 whose output, line 25, pro-vides the phase reference-signal. Resistor 84 is connected between the input and the output of A4. Operational amplifier A3 is an integrating amplifier having a long time constant whose input essen-tially consists of the difference between the current reference signal and the secondary current eedback signals. The output of the integrating amplifier is applied through inverting amplifier ; :
, .
j~ A4 to provLde the phase reference signal. Because o the long time ~ constant of A3, the phase reference signal represents the precise ¦i20~ difference of the current reference signal and of the secondary current feedback signal. `;
I SAWTOOTH GENERATOR
~ The sawtooth generator 27 comprises main integrating ampli-... .
1~ fier A9 having resistor 97 connected ~rom a source of negative j ;
,25~ potential to its input 96 and an integrating capacitor 90 connected l, ~
1~ from its input 96 to its output, i.e. line 2~. Additionally series ; -I .
connected diode 92 and Zener diode 91 are connected in parallel with ~;
- CaQaCitor 90 . An~alternating current input is couplPd by line 93 ~ -to the input o~zero crossing detector 94, whose output is connected via~resistor 95 to the input 96 of ~9. The output of A9 is connected :, ~

IY0--2366 ~, .
by line 98 and resistor 99 to the input of operational amplifier A7. The latter has a capacitor, 100, connected across its input and output so as to ~onstitute a long time integrating amplifier.
The output of A7 is connected through resistor 101 to the input of operational amplifier A8 and resistor lQ2 is connected between the inputs and outputs of A8. The ou~put o inverting amplifier A8 is connected by resistor 103 to the input terminal 96 of A9.
As previously described in connection with FIGURE l, the i sawtooth generator provides a sawtooth waveform via line 28 to the phase control comparator--2~ This sawtooth ramp must be time -synchronized with the AC input to the precipitator and it must have equal bi-polar excursions of predetermined amplitude. Reference is made to FIGURE 5a which illustrates a sawtooth 161. The time synchronization requirement requires that the time duration bf one sawtooth precisely equals the time duration of one-half cycle of the AC signal. The time duration of one-half cycle of a signal -j having a frequency of 60 cycles is 8.33 milliseconds. For such an .
AC signal the sawtooth would have to have an 8.~3 millisecond time duration. Further the sawtooth should have equal positive and ' 20 negative maximum excursion from a re~erence axis, e.g. zero volt.
For purposes of explanation only, reference ~s made hereinafter to a sawtooth having positive excursion of plus seven volts and a , negative excursion~o~ minus seven volts.
~; AC input line 93 is coupled to the AC input, e.g. line 12 o FIGU~E 1, and this AC input is applied to zero crossing detector 94. Devices of this type, which are well known in the art, produce a pulse of short time duration each time the AC signal passes through the zero axis. This pulse is applied by resistor 95 to ~'~ ; input of integrator A9. Assumlng this pulse to be positive, the inte~rator output is driven negatively to the breakdown potential , ~ ":

';", .

-IY~-2366 of Zener diode 91, e.g. minus se~en volts. Upon termination of the zero crossing pulse the current flow from the negatlve source through resistor 97 causes the integrator output to gradually increase positively. Diode 92 prevents any positive clamping action, of the ~ener diode 91, so that integration, and increase of the output potential of A9, continues until the next zero crossing pulse occurs. At such time the integrator output potential i9 agai~
sharply reduced to the negative clamping level. I~hile this provides for time synchronization and ~or a predetermined negative clamping .::
level, there is no control of the~positive peak voltage. According- -ly, variations of AC input signal frequency and changes of component or voltage values would result in unequal positive and negative excursions of the sawtooth. The sawtooth output signal on line 28 is connected through line 98 and resistor 99 to the input o inte-grating ampliier A7 which has a very large time constant in respectto the alternating current input frequency. The output of A7 con- , ;
` stitutes the integrated value of the sawtooth. If the sawtooth has `:;
equal positive and negative excursionsJ the A7 output is zero.
..
l However, if the excursions are unèqual the-output of A7 is propor-.
tional in magnitude to the error, but inverted in polarity in view of the polarity inversion o the integrating a~pliier. The error ~ `
signal output of A7 is therefore coupled ~hrough inverting ampli-ier A8 to the input 96 of integrating amplifier A9. I, for example, the sawtooth output on line 28 has a greater positive than i 1~ 25 negative excursion, the output o A7 would constitute a negative `l signal and the output o A8 would constitute a positive error signal. This produces current flow through resistor 103 opposing l the current through resistor 97 so as to decrease the positive peak ;l amplitude of the sawtooth an to thus result in equal positive and negative excursions about the zero axis. Accordingly, integrating ?8 -IY0`-2366 :, amplifier A7 is responsive to rhe zero crossing of the AC inpu~
signal to provide an output having a predetermined-magnitude of one polarity and subsequently produces a time varying output until the subsequent zero crossing of t4e AC input. The A9 integrator output .~
is integrated by A7 to provide an error signal which is applied to ~".
the input of integrator A9 to automatically correct the excursions of the sawtooth.- The circuit automatically compensates for compon- ~:ent initial tolerance variations, time and temperature variations, . - and also compensates for 50 or.60 Hertz operation without additional adjustment.
TURN- OFF AND RECOVERY CIRCUIT
. In turn-off and recovery generator 30, the sparX detector `~
line 37 is connected by resistor 120 to the input 125 of operation- :
. al amplifier A6 whose output 126 is connected by line 31 to the ~.
15 . phase control comparator. An integrating capacitor 122 is con- : .
nected between terminals 125 and 126. Zener diodes 124 and 123 are serially connected back-to-back and have their end terminals con- ;
: . nected in parallel with capacitor 122. A variable resistor 121, . the turn-off and~recovery control, is connected between a common ;`
-i 20~ . terminal and input terminal.125.
~ The recovery and turn-off generator controls turn-of and i~ recovery of t~e silicon controlled`recti~iers upon detection o~ a spark. A8 previously described in connection with FI~U~E 3, responsive to a spark, the precipitator electrode potential and ` `
~25 current is cut off as illustrated by line f~ of FIGURE 3 for a time .` period g). Subsequent thereto, the SCR's are turned back on at a :` :
controlled rate in order to increase electrode potential as shown ; by line segment i). In order to assure that the ionization current ~ .
is fully extinguished subsequent to a spark and to avoid multiple . .~:
~sparking~, the turn-off time must be properly adjusted ~or the .. ~::

.
.. ..
:-,.
~ 29 -, ~
: ~ . .

precipitation process employed. Additionally, the recovery rate must be properly selected. I found it desirable to vary the recovery rate proportionately with the turn-off time, and accom-plish adjustment of both turn-off time and recovery time by a single control. The turn-of~ and recovery signal is supplied by .~ . .
line 31 to the phase control comparator where it is compared with a sawtooth waveform supplied by the phase control sawtooth genera-tor to produce a signal which controls turn-off and recovery of :, . . ~ . . . ....
silicon controlled rectifiers 13 and 14.- Operation of the turn-~10- off and recovery generator will now be-described in connection , with FIGURE 4~ The circuit produces on output line 31 a signa]. `~
which initiates at a predetermined potential. As described in . .
connection with the phase control comparator, it is desirable that ~ this equal the maximum potential of the sawtooth waveform, e.g.
plus seven volts. The signal subsequently descends rapidly, as . shown by line 150 to a predetermined potential, which exceeds the .. . . . . .
most negative potential of the sawtooth waveform. If, for exampleJ
the~sautooth has a~maximum negativè~potential of minus seven volts~
the~maximum negati~e potential of the turn-of~ si~nal, at point 151 20~ may~, for example,~be minus twelve volts. `After a predetermined interval, closely~approximating~the time duration of the spark pulse, the signal commences to incr~ase at point 152 and linearly increases~as~shown~by da~sh dot line 153 until it returns to the maximum potential~at point 154.~ It may be noted that this wave-¦~2S ~ form closely corresponds to the turn-off and recovery waveform characteristic previously describe~ ~n connection with FIGURE 3.
J~ The~arrangement describ~ed,~however; provides for only brief turn-o~s and a very~quick recovery~period. It is necessary to be able to~`adiust~the~syste~to~provide for~longer turn-of-f and recovery ~,30 ~ ~pcr~cds.~ ThLs is -c~ievod~by~modifylng the stope ol the Feco.ery :~

line 153. As explained below, the ælope of the recovery line may be reduced to a predetermined minimum, as, for example, shown by dash dot dot dash line 155. The turn-off and recovery waveform would then extend downwardly from line lSO to minimum voltage at 151 and after the predetermined time interval, at 152 would increase -at a slow rate along line 155 until it reached the maximum potential at point 159. As e~plained in connection with description of the phase control comparator, the last d scussed waveform will provide turn-off of maximum duration and recovery 9f minimal slope. Spe-cifically, it would provide for cut off of the precipitator elec~rode potential from point 151 of FIGURE 4 aiong dashed line 156 to point 157 and recovery would thereupon take place along dashed line 158.
Thus decreasing the slope of the recovery line lS3 o the turn-off and recovery signal on line 31 increase~ both the turn-off time and the recovery time, Operation of the turn-off and recovery circuit is initiated by a spark pulse. Assuming this to be a positive pulse, its appli-. . .
~; cation to the input of A6 through resistor 120 causes the output 126 to be rapidly decreased to the negative potential established by Zener diode 123, for example, minus twelve volts Upon termina-j tion of the spark pulse, current flow from the common terminal through adjustable resistor 121 to input 12S of A6 causes output 126 of the integrating ampliier A6 to gradually increase to the potential established by Zener diode 124,~e.g. plus seven volts.
~ 25 The rate of increase of the recovery potential is established by I variable resistor 121. Asjustment of this control provides varia-tion of turn-off and recovery, for example, from a minimum having a -il cut off of 12 milliseconds~and recovery of 10 milliseconds to a ;l~ maximum h~ing a cut off time of 50 milliseconds followed by a ~;' 30 subsequent recovery time of 100 milliseconds.

- ~ - 3 1 : ! "
', ' ' ' ' ' .' '' ~' ,,~ '' ;' L Y U ~

;
PHASE CONTROL CO~D?ARATOR AND
PHASE 'I::ONTR'OL' 'OS'CILLATOR
_ . . .
The phase control compara~or 26 receives the phaAqe reference signal on line 25, the sawtooth waveform on line 28 and after a detected spark, the turn-off and recovery signal on line 31. The S phase reference signal on line 25 is connected through resistor 110 to the input of operational amplifier A5. The sawtooth waveform -~
on line 28 is connected through resistor 111 to the input of A5 and additionally through resistor 114 to the input of operational a~plifier A10. A tur~-off and recovery_si~nal on line 31 is con-, . .
lQ nected through resistor 116 to the lliput of A10. The output of operational amplifier A5 is connected to the base of NP~ transistor Ql. The output of A10 is connected to the base of NPN transistor .
; :Q2. Emitters o both transistors are connected to a common terminal.
:. .
~ The collectors of transistors are connected together to output line ~15 29 which is connected to the phase control or gated, oscillator.
, The collector of Ql is also connected through resistor 112 to a source of positive potential, and the collector of Q2 is connected 1~ through resistor~ll5 to the positive source~
Z~ Z The phase control comparator produces an output, on line 29, ~2~0 to gate on the phase control oscillator. While such gate signals are applied to the phase control, or gated oscillator, the latter supplies ~iring pulses to controlled rectifiers 13 and 14 in a manner known in ~he art.
', The phase reference signal on line 25 and the sawtooth l~25 waveform signal on line 28 are combined by the operational amplifier ¦~ A5 input circuit such that a time ratioed pulse output is applied Z to the base of Ql. The circuit operates such that the base is Z
negative during intervals when the sum of the sawtooth waveform and the~phase reference signal exceeds zero volts. FIGURE 5a illustrates ~ ~53 ~ ~ ~

the input signals to ampliier A5 with the phase reference being illustrated as line 160 and the sawtooth signal as ~ For Sf .purpose~s of illustration the phase reference signal 161 is shown to have a zero volt amplitude. When the net voltage of the saw- ' tooth and of the phase reference signal is more positive than zero '' volts, i.e. excePds zero, such as during the'interval bétween points 162 and 163 of FIGURE 5a, the output of A5 and the base of transistor Ql are negative such that the transistor is cut off, ' and the collector of Ql is positive ~uring such interval. FIGUR~ '' 5b illustrates the collector potential o~ Ql which is low during conduction intervals 164 and 168 but is ~igh during the interval extending from 165 to 167, which interval is synchronous with the previously referenced interval between 162 to 163 of FIGURE 5a.
Under the conditions illustrated-in-FIGUR~S 5a and 5b, the collec ' 15 tor is positive during~approximatel~ one-hal~ of each sawtooth, i.e.
during each one-half AC cycle. An increase of the phase reference ' ' signal results in an increase of the duty cycle and an advanced `''-firing of the silicon controlled rectifiers. ~"' In the event a spark is detected, a turn-off and recovery -i~ 20~ signal is supplied by line 31 and reslstor 116 to the input o~ A10, where it i8 combined with the sawtooth signal applied by line 28 through xesistor 114. Reference is made to FIGURE 5c which illus~
trates the combination o the sawtoo~h and the'turn-off and l recovery signal. Prior to de~ection of a spark, line 3L is main-,1 . .
25~ tain~d at a potential approximating the maximum poten~ial,'+R, of 1;"'' .
I the sawtooth as shown at point L71. ~pon occurrence of a spark :1 . :
puLse signal, the signal on line 31 is s'narply reduced to a pre~
~ deter~ined~negative~potential -E2, which exceeds the'maximum ''~ neeative, -El, potential of the~sawtooth. As ~reviously described ' ;' in connection with FIGURE 4 and operation of the turn-off and ,1: :. .:~. .
: :,: ~ ' ' . : ' 1.~ . .

IY0-2366 ;, ' recovery circuit, the signal on line 31 remains at this maximllm negative potential or a predetermined time period approximat:ing the time duration of the spark pulse, as shown by line 172 of FIGURE 5c.. Thereupon, the signal on line 31 returns to the maximum . 5 positive potential at a rate established by the setting of the con-trol in the turn-off and recovery generator. The dash dot line identified as "a" illustrates a typical slope providing for. typical . . ~urn-off and recovery time. FIGURE 5d illustrates the collector potential of transistor Q2. This is high only during intervals 10 when the sum of the potential--of-~-the tw.rn-of--and recovery signal~
and of the instantaTieous potential.of the sawtooth waveorm is ; .
:~ . greater than zero.. Since the turn-off and recovery signal commences from a potential substantlally more negative than the sawtooth, the sum o the sawtooth and.of recovery line a) does not exceed zero volts until the time identi~ied by point 173 of FIGURE 5c. The collector potential of Q2 is switched to a high level at 174 of FIGURE 5d which ~s coincident in time with point 173 of FIGURE 5c.
Collector potential remains at a high potential to the termination ~;
.~ of that sawtooth interval whereupon the sawtooth potential rapidly . . ::
- 20 :decreases~, such that the sum of the sawtooth of lir~e a) again .:
decreases below zero at the time identified by point 175 o:~ FIGURE j:
5c. This reduction of Q2 collector potential is ~dentified b~ line 176 of FIGVRE 5d. During ~he subsequent sawtooth cycle, the sum of 11 ~
the linfe 3L voltage and of the sawtooth voltage again attains ~ero ~ .
Ii~ 25 . volts at the time identified by point 177, of FIGURE 5c, whereupon .~ the collector of Q2 is switched to a high level, at 178 of FIGURE
~'~ 5d until the~subsequent retrace o:E the sawtooth at 179 of FIGURE 5c, results in termination of the~ positive pulse on the collector of ,, ~
Q2, as indicated by line 180 of FIGURE Sd. Subsequently, at point 30~ 18L of FIGURE h~, the sum of ~the potential of line "a'l and of the f'~
:~ - 34 - :
., ~, ; : ~ , ' ~. ' , :Y0-2366 ~ ~a~30~n~
sawtooth rises to zero volts at an earlier ~ime subsequent to initiation of the sawtooth resulting in an earli~r rise of collector Q2 voltage as shown by 182 and a wider pulse. It may be seen that ' collector potential of Q2 does not ~rommence to rise until some 'time after the initiation of the spark pulse. Then, it commences to rise for one pulse interval during each sawtooth. Thepulses ter- -minate at the time of the sawtooth retrace, but'consecutively ~
commence at earlier times during the duration of the sawtooth. Thus !;' ~' consecutive pulses have increasing't.ime duration. In other words, t~e du~y cycle is gradually increase~d'until the potential of Q2 is continuously'positive. The initial time during which the collector t l' potential is entirely at zero characterizes the turn-off time, and ¦ ~-the time of increasing duty cycle represents the recovery time.
:, ~ Transistors Ql and Q2 have their collector,a çonnected in parallel ' . . ,~ . . .
''15 to line 29. Therefore, an enabling gate signal on line 29, causing ~``
a pulse output from the phase control oscillator 20 occurs only when both'the collector of Ql and the col~ector of'Q2 are positive. '~
. . ~: ., . : :
:~ 1 FIGURE 5e illustrates the enabling gate signal on line 29 for the - ~ condit~ions of Ql and Q2 illustrated by FIGURES 5b and 5d. Positive !~ ~;
~j20~ gates appear only during intervals when the collectors of both Ql and Q2 are positive. Conditions represented by FIGURE,, Se represent a relatively fast turn-of and recovery period; ' ' : .. . . .
; 1 D,~shed line b) of FIGURE, 5c illustrates a turn-of and '''l recovery signal having a slower rate o return, i.e. the recovery '~2;5 portion b) has ,a lower rate of rise than the recovery portîon a).
FIGDRE 5f illustrates the resulting collector potential on transis- , tor ~'Q2. In this case, full turn-oEf does not terminate until nearly l, ' ' f`~ the end of the fourth sawtooth when the sum of recovery signal b) ;and of the sawtooth at point 1,~3 reaches zero volts) causing the ';'l'30 ~ colLector of transistor Q2 to rise,'as indicated at 184 of FIGURE 5f. The initial collPctor pulse'terminates ¦ ' after a brief period at ~85 of FIGURE Sf, 1' ~ !^': ,. .
'' ' ' , . .

l~0-2366 the time when the retrace of the fourth sawtooth results in the sum ¦`
of the recovery signal b) and of the sawtooth dropping below zero volts at point 186. FIGURE 5f only illustrates the initial two collector pulses upon recovery. In view of their short time dura-tion, they both occur during intervals when the collector o ;
transistor ~1 is positive. Thus during the interval illustrated in FIGURE 5f, the phase comparator ou;tput ~i~nal on line 29 is coinci-dent with the duration of positive pulses on the collector o ~ transistor Q2. Subsequently.as *he--recovery signal b) approaches :.
the maximum voltage, the collector of 02 will be positive during ;~
intervals when the co~lector-of ~2-is negative. Then the-phase ----control signal will differ from the Q2 collector waveform, of . ;~
. .
~ FIGURE 5f, since the signal on line 29 will be positive only when . ..
`l collectors of both ~1 and Q2 are positive. Thus i8 can.be seen ..
that when the turn-off and recovery circuit is adjusted to have a :
~l . lower rate of return, turn-off and recovery time are simultaneously l . e~tended. ~ ::
i In summary, the phase comparator is.responsive to a phase .
reference signal supplied by line 25, whose amplitude is a functio~ .:
:of the desired precipitator electrode current. The phase reference j :
signal is time ratio modulated with sawtooth waveorms, supplied on ..
line 28, having equal bi-polar excursions of predetermined magnitude and a frequency twice that of the AC input, and synchronized there-~, with. The phase reference signal is time ratioed with the sawtooth ~ waveforms, by a first circuit comprising A5 and ~l, to produce first .
puLse width modulated si~nals switched between predetermined first I
~:¦ and second voltage levels so as to comprise a first train of pulses .. 1 ~ .
synchronous wLth~the fre~uency of the sawtooth waveforms and having a time duration, and thus a duty cycle, which is a function of the .
~30 magnitude of:the~lphase reference signal. ..

~ 36 ~:
.:.f~

`: `t ~ ' T. . . . .. ~ . .~ - ---- . - ...

[Y0-2366 , Upon detection of a spark, a spark pulse signal of pre-determined brief time duration and of predetermined amplitude is 1~
supplied to the turn-off and recovery circuit 30~ Circuit 30, ¦;
responsive to the spark pulse signal supplies a turn-off and recovery signal on line 28 to the phase eontrol comparator. The turn-off and recovery signal prefer~bly has a first and rapld -voltage variation of predetermined magnitude, substantially time coincident with detection of the~spark. Subsequen~ to a brief ; fixed interval, substantially~equal to the-time duration--of the spark pulse, the voltage of the turn-off and recovery signal has a , ~ .. - . ' :
second voltage variation, equal and opposite to the first but of a j ;
.
slower rate of c~ange. This rate of change of this recovery portion , of the signal is substantially greater than the ramping rate of the current reference signal and is adjustable to provide for simultan-eous and proportionate adjustment of the turn-off time and o the recovery time of the precipitator current subsequent to detection of a spark. ' ~ ~ Responsive to detection of a spark this turn-off and recovery ;l; signal is supplied by line 31 to the phase control comparator. It ~20 is time ratio modhlated with the sawtooth waveforms supplied on line 28, by a second circuit comprising AlO and Q2, to produce second pulse width modulated signals switched between predetermined first and second voltage levels so as to comprlse a second train of pulse 1 ciynchronous with the frequency of the sawtooth waveforms. The ;~25 magnitude~of the sawtooth bi-polar excursions and the magnitude of the turn~of~ and recovery signal voltage variations are predeter-mined such that the second train of pulses commence only ater a turn-off time interval subsequent to the initial variatio~ of the turn-off and recovery signal, i.e. subsequent to the time a spark 13 is detected. Upon initiation of the second train of pulses, the ., ~
~ !
: 1 j ~ ' , , , - ; ' ' ' :: ' ' ' '' ~

IYO-2366 ;.~, ~ 3~ ~ ~

time duration and khus the duty cycle, of the pulses increases to a predetermined maximum, i.e. 100 percent duty cycle, establi.shing the recovery rate of the precipitator electrode current. The ~ime duration of the turn-off interval (when the silicon controlled rectifiers 13 and 14 are fully cut off~ and the subsequent recovery ~'~
time interval during which the duty cyc'le of the second pulse train '-~increases from'zero to 100 percent, (when the SCR conduction angle '~' ' is increased to the level established by the phase referenee signal) -arè a function of'the rate of change of the recovery portion of the turn-off and recovery signal on line 31. Thus a single ad3ustment in the turn-off and recovery circuit 30 provides for simultaneous "~' ' and proportionate adjustment of turn off and recovery time.
The outputs, of the above reference'd first and second cir cuits, i.e. the first and second trains o ~ulses', are combined, by common connection of the Ql and Q2 collectors to line 29, to provide an enabling gate signal for application to the phase control, gated, ~ oscillator 20. The enabling gate signal permitting firing'of the 'I phase control oscillator occurs only'when pulses o both of the first and second pulse'trains have a predetermined polarity. In ':
the preferred embodiment, oscillator 20 produces firing pulses while the collectors of Ql and Q2 are both positive. Thus essentially the first and second train of pulses are applied to an AND circuit, ~' transistors Ql and Q2, to produce the enabling gatq signal.
The phase control, or gating, oscillator operates in known fashion.PreferabIy it produces firing pulses at an appropriate high ', frequency, such as for example, 10 kilohertz, during the application o enabling gate pulses on line 29. Although not illustra~ed, the AC input signal is applied to the phase control oscillator to syn-chronize firing of the two back-to-back connected silicon controlled rectiiers 13 and 14, such that each of these SCR's is fired during ~ 41~Y0-2366 ;~' alternate half wave period~ of the AC input signals. -~
Current flow in power circuits energizing a substantial ;' load can normally be maintained below destructive current levels ' by a current limit loop or feedback circuit which prevents the current in the power line from exceeding a predetermined reference current limit value. However, if this current limit regulating circuit is unabIe to maintain the current within the predetermined limit, for example, because of a short circuit, it is necessary to totally interrupt current to the load carrying circuit, such as for example, by tripping a circuit breaker '' and opening contacts at the input of the power system. It is - desirable to protect the components of eIectric circuits, includ~
ing their wiring against overload currents by utilizing a cut out or trip circuit having an inverse current function to time ' relationship. Upon the`s~uare of the load current exceeding a predetermined current limit value, tripping occurs after a time ``
interval which is inversely proportional to the square of the '-'' current value. Ideally, the product of the square of the current '~
and of the time`is equal to a constant. Referenceis made to FIGURE 9 which illustrates the desired regulating and trip ~'' '' `j functions. The vertical line identified at 100 ~ rms current, hereinafter referred to as 100 percent Ip indicates the desried curr~nt limit. As the current in the poWer system approaches this magnitude, the'regulating loop acts to limit the current to prevent its exceeding the'100 percent Ip magnitude. Because of short '~
,~ circuits, or other malfunctions, the'regulator loop circuit may be '~l unable to maintain the current within this limit. For example, in ~`the case of a precipitator system a short from the anode to the cathode of the'silicon controlled rectifiers would prevent proper regulation. In such'an event, power to the load should be com- -~ pleteIy disabled when the above described current and .,~

~ ~ 39 ~

~ 9 ~
.
time product characteristic exceeds a predetermined constant.
FIGURE 9 illustrates such a curve whlch is hyperbolic and asymptotic at its top with the 100 percent Xp line. It is asymptotic at lt~
bottom right with a lT line. Thus, if the square of the load current Ip has an extremely large magnitude, the power is interrup-ted instantly. If, however, the square of the lcad current Ip - exceeds the predetermined current limit, 1~0 percent Ip, by a lesser value, the tripping action ocurs after a predetermined time ~ , . . . .
interval. ~ -It is undesirable to have the current regulating function and the current trip function performed independently. Due to normal tolerance variations, the I2t = k function of a separate trip circuit is likely to deviate from the 100 percent Ip line, so as not to beasymptotic therewith. In order to prevent undesirable power interruption, i.e. tripping of the circuit breaker at current values below the 100 percent Ip magnitude, the I2t = k curve of a separate trip circuit was typically offset to the right of the 100 percent Ip line of FIGURE 9 by some amo~mt, such as 15 percent.
, .
~`~, This results, however, in excessive and potentially damaging current flow prior to tripping. The current limiting system disclosed herein, however, provides for a unitary regulating and current interrupt or trip circuit such ~hat the current limi~ regulation and trip circuit characteristic conform to one another in the manner described above.
Reference is made to FIGURE ~ which illustrates the current it circuit in connection with components of the previously des-cribed precipitator system, with identical reference numerals ~; being used to descrlbe common components. It should, however, be l~ ~ under~stood that ~he current limit circuit may be utili~ed in applica-'~;30 tions other than the precipitator system. Modifications may be made :": . .
~ 40 IYO-2366 ;~
~ 9 ~ ~ ~

in ~he components which are common with the above described preci-pitator system.
Alternating current inpat is applied to terminals 7 and 8.
Terminal 7 is cormected through contact 9 and l;ne 11 to one side ::
of a load. Terminal 8 is connected through another contact 10, line 12, and back-to-back connected silicon controlled rectifiers 13 and 14 to th~ other side of the load. As previously described, .::
. - firing pulses ~pplie~ by line 19' and 19'' are applied to the gates of the silicon controlled rectifiers in order to regulate their ~10 conduction angle and thus, to regulate the voltage and-the current . . .
applied to the load. The current in line 12 is sensed by current transformer CT whose output 39 is applied to a root means square to DC converter 200. Such devices are commercially available, e.g.
Analog Devices Type 440. The output o device 200, a DC signal proportional to the root means square value o the sensed current is connected by line 201 to one end o~ current limit potentiometer 202 whose other end is connected to a common terminal. The arm 203 of potentiometer 202 is connected to squaring circui~ 204. The , ~
setting of arm 203 of the primary curren~ limit potentiometer establishes the portion of the sensed RMS current signal which is applied to the squaring circuit and thus establlshes the value of the current limit, i.e. the 100 percent Ip value. The output of squaring circuit 204 is current Io which is proportional to the square of the actual root means square current flowing in the power ,~25 line, i.e. the protected line 12.
i ~ .
i A typical power function circuit, useful to perform the squaring ~unctionl and thus useful as a squaring circuit 204, is . .
illustrated in FIGURE 4, page A30-3 o~ the ~ational Semiconductor Linear ApplLcations Manual, February 1973. If this circuit is ~30 utili~ed, the input to the squaring circuit 204, from potentiometer : ` - 41 - . :

ty0~2366 ~ 9 ~

arm 203 is connected to the voltage input El of the above referenced FIGURE 4 circuit. That circuit (not illustrated herein) utilizes .
the logarithmic relationship between the emitter~base signal and the electrode current to generate an exponential function. By l .' .5 proper selection of the resistors in a'voltage divider network, R9 and RlO, the collector-emitter circuit of transistor Q4 provides an output current which is proportional to the square of the input ',~
j, signal. .This output current signal, lo~ of the squaring amplifier 204 is applied.to input 205 of integrating amplifier A20.
., . , . . i ' A current ~ref flowin~--through-Fesistor-206 to inpu~-205 ha~ l , a predetermined magnitude and is opposite in direction to the.IO j .
current applied to input 205. The magnitude of resistor 206 is '!' .
selected such that current Iref is equal and opposite to the output ',.'..
current Io f the squaring circuit when t,he current in the,protect- ~ `
ed line 12.approaches the 100 percent Ip level. .
A capacitor 208 is connected between input 205 and output ..
.` 207 of A20 so as to form an integrating amplifier. Output 207 ls ,~ connected through diode 220 and line 221 to one input of flip-flop .
~, 222. The anodP of diode 220 is connected to output 207 and the ~' -, ,~20 .cathode is connected to line 221. Normally, output 2~7 is at a .~; negative potential so that there is no current flow through diode `,, 220, An output.terminal of flip-flop 222 is connected'by line 223 to . .;:
,~ ' coil 224, of undervoltage relay 225, whose other end terminal is ,~,' '~. connected to a terminal B. During normal operation, when there has been no current flow through diode 220, t~e flip-flop is in a state , ~:
., wherein current ~lows through line 223 and solenoid 224 to terminal `,',:
:,` B to maintain armature 226, of relay 225, in a position latched , -.
; with protrusion 229 of circuit breaker member 228. Circuit brea~er ~.,' member 228 is connected to contacts 9 and 10. While the armature .~
. ~ . . .. .
is,latched to part 229, the contact 9 and 10 are maintained closed. .~:' 42 - :--. ~ .

;, .
.

:Yo-2366 0~
"

As subsequently explained,.a tripping command ~s maniested by ;~' current flow through diode 220 and line 22~ so as to cause a -' change o~ state o~ flip-flop 222. This interrupts current flow through line 223 and solenoid ~24. ~s a result, armature 2~6 is 5 . retracted by action o spring 227 so as to releasè deten~ 229 and to cause spring 231 to retract member 228 and to thus open contacts 9 and lO.
;. Output 207 of integrating amplifier A20 is additionally connected by resistor 210 to the non-inverting input 212 of ' .. ...
. - operational amplifier A21. Resistor -211 is connected from a source .. -of positive potential to input 212. Inverting input 213 of A21'is connected by resistor 230 to a common terminal and by feedback ;. resistor 214 to.output 215 of A21. In addition, the cathode of ' : -clamping diode 2~9.is connected to output 215 o~ A~l and its anode is connected to input terminal 205 of A20. Output 215 of A21 is .. .
connected through serially connected resistors 216,:diode 219 and ----. ' current limiter line' 43 to the current reerence and phase control ' circuitry 21 - 25. Filter capacitor 217 is connected across .l resistor 216 so as.to be connected between output 215 of A21 and.. _... `
20: junction.. 218 between resistor 216 and the anode'o diode 219. The . '.
currbnt reference and phase control circuits 21 - 25, which have been described in the preceeding text in connection wlth a precipi- ... .'.':' . tator syste~l, have outputs 19' and 19 " connected to the gates o ' ' . sillcon controlled rectiiers. The above reerenced circuit con~
trols the firing angle o~ the SCR's so as to retard the firing ..
' angle of silicon controlled rectifiers 13 and 14 in response to a current limiting signal applied by line 43 to the current reference :'1' l circuit. It shou~:d be noted that circuitry alternative t~ that '~:l disclosed'for devices 13, ~14 and 21 through 25 may be utilized to `~ -convert the current limit signal on line 43 to signals for '.-. ' ,"~-', ', , . ;'"'''~;:
! -~ j .

":

.Yo- 2366 ~. "
Eil9~
':
limiting the current ~low on line 12.
~uring normal operation of the power system when the current in line 12 is sufficiently below the predetermined curren~ limit, Io, the current signal representative o t~e square of the RMS
current in line 12, has a smaller magnitude than Ire~, the current in resistor 206. The summation o~ these two currents at input 205 causes integrating amplifier A20 to be clampe~ at a predetermined negative potential, e.g. minus 8 ~lt:s. Diode 220 is back-biased so that there can be no current flow on lin~ 221 and no tripping of the circuit breaker. As explained subsequently, the output 215 of non-inverting amplifier A21 i8 clamped a~ a substantially . , difere~t predetermined voltage, which is closer to zero but still is negative, e.g. -0.5 volts~ AccordinglyJ diode 219 i8 also back-biased. This prevents current flow on line 43 and thus prevents current limiting regulatory action. In summary, when the ~l load current is below the current limit such that XO is less than Iref the output potentials of A20 and of A21 are clamped to levels precluding application of a current limiting signal on line 43 and additionally precluding application of a tripping signal on linè
~20 221 When the current in the protected power line 12 attains ~i~ the predetermined current limit, the output of the ~quaring circuit 204, Io~ increases 80 as to exceed Iref. The su~mation current of -~
~1 Io and Iref at input 205 of integrating ampliier A20 therefore ~;
2~5 reverses causing output 207 of integrating ampliier A20 to inte-grate pos~tively toward zero volts. This rapidly drives the out-put of non-inverting amplifier A21 positive. Diode 219 i9 thus rapidIy forward biased such that current flows through resistor 216, diode 219 and line 43 to the current reerence and phase control circuits 21 - 25. This results in the firing pulses on line 19' and ,~ , ~ 44 o-236 II,Z~r3~

19'' being modified so as to retard firing o~ the silicon controlled rectifiers 13 and 14 to maintain the current in line 12 wit~in the .. ...
predetermined current limit value. In summary, the output of amplifier A21 is normally clamped at a slightly negative potential, such that when Io erceeds Ire~ and A20 begins to integrate posi-tively, current limiting action initiates almost instantly. The -gain of the regulating circuit, including o amplifier A21, provides or substantial regulation of silicon controlled rectifiers 13 and 14 even when there is only a small voltage deviation at output 207 10 ~ of integrating amplifier A20. For example, when positive integra-tion coZmmences~ so as to result in a small voltage change at output 207, for example, from a quiescent level of -8 volts to -7 volts, : the output of A21 changes substantially because of the amplifier gain, for example, from a quiescent level of -0.5 volts to a posi-tive voltage of 5 volts. Accordingly, the current limit regulation ; circuit comprising the current transformer, devices 200 and 204, A20, A21, diode 219, current reerence~and phase control circuit 21 - 25, -and silicon controlled recti~iers 13 and 14 normally prevent the current on protected line 12 from exceeding the predetermined `I
1 20 current limit while the output of integrating ampliier A20 is at a potential, (e.g. -7 volts) which is close to its quiescent level 1 (e.g. -8 volts), but is substantially be.low the potential at which `j the circuit breaker i9 tripped (e.g. 0 volts). ThuY, the potential difference between the clamped outputs of A20 and of A21 assures '~25 that the breaker is not tripped during normal operation of the system.
~, ,", , s\~ cQ~ '' ' ~"
lowever, a malfunction, such as a sh ~T---c~n~it~, may prevent the above described segulating system ~rom maintaining the current in protected line 12 within the current reEerence limit. In such an event9 Io exceeds Ire~ and integrating amplifier A20 continues , Z :

! 45 -:Yo - 2366- ~, .. ~ V~ , its inteErating action. This causes output 207 to further decrease from its quiescent level to a sufficiently positive potential .(e.g. o volts) to forward bias diode 220. Th~ resulting current flows through diode 220 and line 221 causes flip-flop 222 to change its state. This cuts of the current through line 223 and solenoid I
224, o undercurrent relay 225. Accordingly, armature 226 is pulled back by spring 227 releasing detent 229 and permitting member 228 to be pulled ~own so as to open contacts 9 and 10. In sum~ary, if the current in protected line 12 commences to i.ncrease-beyond-the predetermined:limit without~the current limit regulat-or being able to limit the line 12 current, Io exceeds Iref by an amount related to the square of the line current 12, and output 207 goes positive from the quiescent level at a rate determined by the magnitude of :' ' . ' 1,. ..
Io. When output~2~7 goes positive to a predetermined level, e.g.
15 0 volts, diode~220 fires causing the circuit breaker to open. Thus, triggering occurs as a function of the magnitude o~ the sensed -.
current and of the time during which an excessive current exists.
Specifically, triggering occurs as a function~of I2t, the desired trip characteristic. The precise I2t characteristic is determined .
by the magnitude of capacitor 208 in the feedback circuit of inte- ~
.
grating amplifier A20.
The following ls a more detailed descriptlon of how the circuit comprising A20 and A21 accomplishes the above described functions. Output 207 is connected by resistor 210 to input 212 of non-in~erting amplifier A21 and resistor 211 is connected from "
~l a source of positi~e potential to this input 212. This resistor `-~
network establishes the balance point, i.e. the quiescent voltage levels at outputs 207 and ~ . For example, assume that resistor 211 has a magnitude of 15 K ohms and is connected to a source of ~0 + 15 volts, in respect to the common terminal. According~y, if ~ ! ~

~ ";.` .. :' "

IYO-236f~

' iLfa~ 'O~

input 212 is at 0 volts there is a 1 milliamp current througll resistor 211. If one assumes that resistor 210 is 8 K ohmfs, the current through resistor 210 equals that through resistor 211 when the output 207 is -8 volts. Accordingly, when output 207 is at minus 8 volts, the potential drop across resis~or 210 is 8 vol~s, that across resistor 214 is 15 volts and,the input 213 o~ A21 is 0 volts. If the output 215 of A21 is at substantially Of volts, the circuit is balanced.
During quiescent conditions,-when Iref is greater~than IO, `
continued integration of integrator A20 tends to drive-the output_ 207 below -8 volts. This tends-to-drive the input 212, as well as the output 215, of A21 more negative than 0 volts. However, when the output 215 drops below 0 by more than the diode drop across clamping dïode209, e.g. -0.5 voltsj-cur-rent--flows from output 215~ - ;;
through diode 209 to the input 205 of A20. The non-inverting : input of A20 (not illustrated) is conventionally connected to : .
common,-e.g. 0 volts, thus maintaining the integrator input 205 at 0 voIts. Accordingly, current flow through diode 209 clamps t-he-~ -output 207 of A20 at a first predetermined magnitude, e.g. -8 volt-s, ~ ;~
~20 and the output 215 of A21 at a second predetermined magnitude, e.~.
-0.5 volts.
When Io exceeds Lre~, integrator A20 integrates such that its output 207 goes somewhat positive from the quiescent level. -~
Because of the resulting change in current ~low in the network com-prising resistors 210 and 211, the non-inverting input 212, and . f output 215, of amplifier A21 rise above 0 volts. Resistors 214 ~
and 230 connected to inverting input 213 cause the latter to attain ; ~ the same voltage as on ~he non-inverting input 212. Thus, as the output 207 rises ~(e.g. from -8 volts to -7~volts), output 215 of~
A21 rises above`its quiescent level (e.g. from -0.5 volts to 5 vol~s~.
~J ~ ~

. f ~ - 47 ~

, . ..

:Yo-2366 3~

A21 thus operates as non-inverting ampiifier. As the output 215 increases above 0 volts; diode 219 becomes forward-biased and ~egulating current flows on line 43 to provide the desired regula-ting action. Because of the integration action of A20, regulation tends to occur at the upper, i.e. long time, portion of the 100 percent Ip line of FIGUP~ 9. The Io current tends to equal Ire~
such that the potential at output 207 tends to remain constant after regulation has commenced.
If, because of some malfunctio~; the regulator fails to limit the current of line`12, output 207 commenceæ to integrate positive-ly to a predetermined trip potential, e.g. 0 volts, at which diode 220 is forward biased. The potential at output 207 at which the ..
circuit breaker trips (e.g. zero volts) i~ substantially diferent ! : :
from the potential at which regulation commences (e.g. near -8 ¦~
1$ volts) such that under conditions when there is no malfunction, full regulation of the power circuit current takes place without the breaker bein~ tripped. The single clamped circuit assures a ~ . . ..
precise current limit point. The trip curve such as the hyperbolic I~2t = k curve~of FIGURE 9 is maintained asymptotic to the 100 per-~2~ cent Ip current limit line, the current limit maintaine~ by the ' regulator. A single adjustment, e.g. the current limit potentio- ~
..
'; meter 202 simultaneollsly sets the trip and regulation characteris-tics. It should be noted that tripping arrangement other than devices 222, 225, 228 and 229 may be utilized.
. ~ , .
25~ hile there~is shown and described a particular embodiment ~;
of the invention, it will be obvious to those skilled in the art that various changes and modi~fications made without departing from ; ~
the invention in its broader aspects and I therefore intend in the l - ;
appended claims to cover all such chan~es and modifications that ~30 ~f~ll with p the true spirL~ and scope of ~ invention. ¦ ;~

: . :, ~: ,, , ~ 48 -

Claims (15)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:
1. In an electric precipitation system for providing electrical energy to the electrodes of precipitator means within alternating current is supplied through static switching means and suitable conversion means to said electrodes, and said switching means have their conduction interval during each cycle of the alternating current controlled by a control system to provide maximum electrical energization of said precipitator means without excessive sparking or arcing, said control system comprising:
a) a source of phase reference signal whose amplitude is related to the magnitude of desired precipitator electrode current;
b) sawtooth generating means providing a train of sawtooth waveforms whose frequency is synchronized with an integral multiple of the frequency of said alternating current;
c) first time ratio modulation means responsive to said phase reference signal and to said train of sawtooth waveforms to produce a first train of pulses whose frequency is synchronized to the frequency of said sawtooth waveforms and whose pulse width is a function of the instantaneous amplitude of said reference signal;
d) spark detection means responsive to a variation of the electrical energization of the precipitator electrodes indicative of the generation of a spark to produce at its output a spark pulse signal;
1. Claim 1 continued...
   e) turn-off and recovery means responsive to said spark pulse signal to generate a turn-off and recovery sig-nal having an initial excursion, at a first rapid rate, from a first predetermined potential to a second predetermined potential and a subsequent return excursion, at a second slower rate, to said first predetermined potential;
   f) second time ratio modulation means responsive to said turn-off and recovery signal and to said train of sawtooth waveforms to produce a second train of pulses of frequency synchronous to that of said sawtooth waveform, the pulses of said second train commencing after a predetermined turn-off interval and sequentially increasing in pulse width at a predetermined recovery rate, said turn-off interval and said recovery rate both being related to said second rate of the return excursion of the turn-off and recovery signal;
   g) AND gating means responsive to said first and second train of pulses to generate a third train of pulses;
   and h) means for gating on said static switching means during time intervals defined by the time duration of the pulses of said third train of pulses whereby said static switching means is turned off during said turn-off interval and its conduction is subsequently increased at a rate determined by said recovery rate to the level established by the amplitude of said reference signal.
2. 2. The control system of Claim 1 wherein said turn-off and recovery means comprises adjustment means for adjusting said second rate of excursion to simultaneously modify said turn-off interval and said recovery rate in direct relationship to one another.
3. 3. The precipitator control system of Claim 1 wherein said source of reference signal comprises:
   a) current reference generating means whose current reference output supplies a current reference signal having an amplitude representative of desired precipitator current;
   b) a source of precipitator electrode current signal; and c) difference amplifying means responsive to said current reference signal and to said precipitator electrode current signal to produce said phase reference signal.
4. 4. The precipitator control system of Claim 3 wherein said current reference generating means comprises:
   a) integrating means connected to cause integration in a first direction during the absence of a spark pulse to provide at said current reference output a ramp signal of first predetermined direction to gradually increase the potential applied to the electrodes of said precipitator means;
   b) means coupling said spark pulse signal to said integrating means to cause integration in a second direction during the presence of a spark pulse to produce at said current reference output a ramp signal in another direction to set back the ramp signal by a predetermined small set back potential.
5. 5. The precipitator control system of claim 4 further comprising:
   (a) arc detection means responsive to the magnitude of the potential and current in the electrical precipitation system to provide an arc signal during intervals when the magnitude of said current attains a predetermined minimum ratio in respect to the magnitude of said potential so as to be indicative of the existence of an arc; and (b) means coupling said arc signal to said integrating means providing continued integration in a second direction during the presence of said arc signal to continue production at said current reference output of a ramp signal in another direction to provide for a further set back of said ramp signal.
6. 6. The control system of claim 2 wherein said train of sawtooth waveforms has excursions between third and fourth predetermined potentials, and said second time ratio modulation means comprise:
   (a) gating means responsive to the summation of said train of sawtooth waveforms and of said turn-off and recovery signal to generate said second train of pulses; and (b) said aforesaid potentials being selected such that said turn-off interval and said recovery rate are a function of said second rate of the turn-off and recovery signal.
7. 7. The control system of claim 6 wherein said train of sawtooth waves has bi-polar excursions of equal magnitude, said first and third predetermined potentials being of equal magnitude and of a common first polarity, in respect to a reference level, said second predetermined potential being greater than said fourth potential and both being of a common second polarity, said gating means producing pulses of said second train during time intervals when said summation signal is of a preselected polarity in respect to said reference level.
8. 8. The control system of claim 2 wherein said spark detection means produces a spark pulse signal of predetermined time duration and said turn-off and recovery means comprises:
   (a) integrating means comprising an output providing said turn-off and recovery signal;
   (b) said integrating means being responsive to the application of said spark pulse to integrate in a first direction for providing at its output said initial excursion at said first rate from said first predetermined potential to said second predetermined potential, and responsive to the termination of said spark pulse to integrate in a second direction to provide at its output said return excursion, at a second slower rate, to said first predetermined potential.
9. 9. The control system of claim 8 wherein said adjust-ment means for adjusting said second rate of excursion comprises means for controlling the rate of integration in said second direction.
10. 10. The control system of claim 9 wherein the integrating means of said turn-off and recovery circuit comprises:
    (a) an operational amplifier comprising an input and an output;
    (b) integrating capacitance means coupled between said input and said output;
    (c) means for coupling said spark pulse signal to said input;
    (d) a source of DC potential;
    (e) variable impedance means coupling said source to said input and constituting said adjustment means.
11. 11. The control system of claim 10 wherein said integrating means of said turn-off and recovery circuit further comprises clamping means coupled to the output of said operational amplifier for limiting the initial excursion of said turn-off and recovery signal to said second predetermined potential and to limit said subsequent return excursion to said first predeter-mined potential.
12. 12. In an electric system for providing electrical energy to the electrodes of precipitator means, wherein electric current is supplied through variable impedance means and suitable conversion means to said electrodes, and said variable impedance means is controlled by a control system to provide maximum electrical energization of said precipitator means without excessive sparking or arcing, said control system comprising:
    (a) current reference generating means whose current reference output supplies a current reference signal having an amplitude representative of the desired precipitator electrode current;
    (b) spark detection means responsive to electrical variation in said precipitator means indicative of the occurence of a spark to produce at its output a spark pulse having a predetermined brief time duration;
    (c) arc detection means responsive to the magnitude of potential and of current in said electrical system to provide an arc signal during intervals when the magnitude of said current attains a predetermined minimum ratio in respect to the magnitude of said potential so as to be indicative of the existence of an arc;
    (d) said current reference generating means comprising integrating means connected, during the absence of a spark pulse or an arc signal, to produce at said current reference output a ramp signal of first predetermined slope in one direction, for decreasing the effective impedance of said variable impedance means and to gradually increase the potential applied to the electrodes of said precipitator means;
    
    (e) said current reference means being responsive to the spark pulse output of said spark detection means to produce at said current reference output, during duration of said spark pulse, a ramp signal of second predetermined slope in another direction for increasing the effective impedance of said variable impedance means and to gradually decrease the potential applied to the electrodes of said precipitator during the duration of the spark pulse so as to set back the ramp signal at the output of said current reference means by a predetermined small set back potential; and (f) said current reference means being further responsive to the arc signal output of said arc detection means subsequent to the termination of said spark pulse to continue to produce at said current reference output said ramp signal signal in said another direction to further increase the set back of the ramp signal to a greater set back potential for further reducing the potential applied to the electrodes of said precipitator responsive to the presence of an arc signal.
13. 13. The arrangement of claim 12 further comprising limit means for limiting the set back potential of the ramp at said current reference output during presence of an arc signal to provide for continued magnitudes of potential and current in said electrical system sufficient to permit sustained opera-tion of said arc detection means during presence of long duration arcing.
14. 14. The control system of claim 13 wherein said current reference generating means comprises:
    (a) first means for causing said integrating means to integrate in a first direction during the absence of a spark pulse and an arc signal; and (b) second means for causing said integrating means to integrate in an opposite direction during the presence of either a spark pulse or an arc signal.
15. 15. The arrangement of claim 14 wherein said limit means comprises clamping means coupled to the integrating means of said current reference generating means.