US2841239A - System for energizing electrical precipitators - Google Patents

Description

y 1, 1958 H. JIHALL ETAL 2,841,239

SYSTEM FOR ENERGIZING ELECTRICAL PRECIPITATORS Filed Feb. 16, 1955 4 Sheets-Sheet 1 GRID PHASING 7 CONTROL ClRCUlT INVENT OR HERBERT J. HALL HARRY J. WHITE Kn/MGM ATTORNEY y 1, 1958 H. J. HALL ETAL 2,841,239

SYSTEM FOR ENERGIZING ELECTRICAL PRECIPITATORS Filed Feb. 16, 1955 4 Sheets-Sheet 2 A.C.INPUT & E VOLTAGE &

I I I] II RECTlFlER CURRENT PULSE LOAD CIRCUIT- EVERY OTHER PULSE ENERGIZES SANE PRECIPITATOR SECTION pc Um I V E i ZERO VOLTAGE J T i VOLTAGE ACROSS PRECIPI'I'ATOR SECTION- HERBERT 3'. HA HARRY J'. WHIT E BY %T/Zim ATTORNEY July 1, 1958 Filed Feb. 16. 1955 H. J. HALL ETAL 2,841,239

SYSTEM FOR ENBRGIZING ELECTRICAL PRECIPITATORS 4 Sheets-Sheet 3 SPARK SENSER ELECTRONIC INTEGRATOR M 728 l MAGNETIC AMPLIFIER 726 O3 INVENT OR HERBERT J HALL HARRY J. WHITE ATTORNEY y 1, 1958 H. J. HALI. ETAL I 2,841,239

SYSTEM FOR ENERGIZING ELECTRICAL PRECIPITATORS Filed Feb. 16. 1955 4 Sheets-Sheet 4 TO HV RECTIFIER PRECIPITATOR 804 I A, II I J AND SPARK SENSER I MAGNETIC AMPLIFIER ATTORNEY REGULATED SUPPLY ''\/vvvvvvvv- 7 SPARK 93o SENSER TO PRECIPITATOR v A INVENTOR RX A HERBERT J. HALL A FIGS-,ORB HA Y J.WHITE atent 2,841,239 Patented July 1, 1958 SYSTEM FOR ENERGIIZING ELECTRECAL PRECIPHTATURS Herbert E. Hall, Princeton, and Harry J. White, Bashing Ridge, N. 3., assignors to Research Corporation, New York, N. Y., a corporation or New York Application February 16, 1955, Serial No. 488,436

8 Claims. (Cl. 183--7) This invention relates to a system for energizing electrical precipitators and has for its primary object the provision of such an energizing system in which the wave-shape of the energizing pulses is controllable in order to improve the performance of the precipitator.

Electrical precipitator installations for large-scale industrial equipment are often very expensive. One way of reducing this expense is to operate several precipitator sections from one high voltage transformer. Since the output of the high voltage transformer is necessarily rectified for precipitator use, by employing half-wave rectification and energizing separate sections with the respective half waves of rectified high voltage, important economies can be efiected, as well as certain advantages in operation, as is well known in the art. However, in many cases, the load provided by the separate sections is not balanced and it is necessary to separately adjust the individual sections, which introduces either additional expenses or else operational difliculties, or a compromise between the various factors. It is a major object of the present invention to provide a system which is particularly adapted to the operation of sectional precipitators where unbalanced conditions may exist.

Where the material being collected by precipitation has poor electrical conductivity, experience has shown that the difliculty of collecting the material increases with 4 the bulk resistivity of such materials. This is apparently due to the fact that in such cases the sparking potential may be appreciably reduced, so that the normal degree of electrical energization for proper collection efficiency can no longer bemaintained with the conventional rectitier equipment. However, by reducing the current pulse width to efiect a more peaked waveform, it is frequently possible to operate such precipitators at a higher value of peak voltage and thereby improve the performance. it is a major object of the invention to provide means for adjustably reducing the width of the current pulse so that higher voltages may be used to suit the conditions of operation.

Further objects are to provide a precipitator energizing circuit which includes means for smoothly regulating the precipitator operating voltage and current over a wide range, which is adaptable to automatic voltage control, which is simple and relatively inexpensive, and which uses readily available commercial components.

The above and other objects and advantages are achieved according to the invention by the use of thyratron control in a novel manner. By controlling the phase angle of suitable alternating grid voltage with respect to the alternating voltages across the thyratron between anode and cathode, both the magnitude and the width of the current pulses energizing the precipitator can be smoothly varied over a wide range. Thus both the magnitude and waveshape of the voltage applied to the precipitatorv are similarly varied. The effects are illustrated and described in the following disclosure, from which it will be apparent that by individual adjustment of the grid firing angles of oppositely oriented thyratrons, as will be shown, the width and magnitude of alternate current pulses energizing the two precipitator sections can be individually adjusted to compensate for slightly unbalanced load conditions which may exist between the two sections due to variations in gas flow conditions, dust characteristics, temperature, etc. In practice, such factors frequently cause the sparking voltages of the two sections to be slightly different. For example, differences of 1 to 3 kv. are frequently encountered in industrial precipitators normally operating in the 45 to 50 kv. range.

it has been established that maximum collection efliciency in a precipitator occurs when the operating peak voltage is adjusted and maintained at the highest possible value compatible with the dust losses due to sparking. In most cases, this usually means that the voltage must be high enough to cause some appreciable sparking between the precipitator electrodestypically with an average sparking rate of the order of 50 to 100 per minute per precipitator section in the case of fly ash precipitation, for example. Thus, an automatic voltage control system based on maintaining an optimum average sparking rate is frequently used to maintain maximum precipitator performance under changing load conditions, as shown in U. S. Patents No. 2,623,608 and No. 2,575,092.

it is a further object to provide a circuit arrangement according to the invention which is under full automatic voltage control.

It has been found that pulsating waveforms derived from energizing circuits using relatively narrow current conduction periods frequently permit higher operating peak voltages within the limitations of sparking than are possible to maintain with conventional 60-cycle equipment wherein the current conduction period in each cycle is not subject to control, but is fixed, generally by load conditions, and in some cases by the characteristic of the rectifier element, e. g., mechanical rectifiers. In the present case, the current pulse width is adjustable to meet the particular load requirements.

The specific nature of the invention, as well as other objects and advantages thereof, will clearly appear from a description of a preferred embodiment as shown in the accompanying drawings, in which:

Fig. 1 is a schematic circuit diagram showing the principle of the invention;

Fig. 2 is a more detailed schematic circuit diagram showing one form of grid phase control whereby the width of the pulses can be controlled;

Figs. 3A-F show the approximate wave shapes obtainable in the new system in contrast with the conventional wave shape;

Figs. 4, 5 and 6 are schematic diagrams of modified forms of the invention;

Fig. 7 is a schematic circuit diagram showing a fully automatic voltage control system utilizing the invention;

Fig. 8 shows another form of automatic voltage control according to the invention; and

Fig. 9 shows a third form of automatic voltage control which may be used to eliminate the need for servomotor controls.

Referring to Fig. l, a source of alternating current 1 is connected in series with back-to-back connected thyratrons 2a, 2b, and a suitable current-limiting element such as resistor 3, to the primary in of a high voltage alternating-current transformer 4. Secondary winding 4b is connected in the conventional manner to a rectifier system which may be a full-wave bridge circuit 5. Although vacuum tube rectifiers are shown by way of illustration, it is understood that other rectlfiers may be used,

age'is in phase with the anode voltage.

eigjselenium rectifiers. The output of the rectifier may. beconnected, for example, to provide two unidirectional pulsating voltages for energizing twoprecipitator sections 6a, 6b, in the now well-known double-half-wave energiza:

tion systemHThegrids. of the thyratron' tubes are controlledb a'phase shift and bias arrangement indicated as item}; e r

' Fig. Zshowsthe arrangement of Fig. 1 with a typical grid control circuit for item 7- of Fig. 1. Reference characters'correspondto those in Fig. 1, except that they are raised by 200, e. g, high voltage transformer 4 of Fig. 1 is designated 204 in Fig. 2,, etc. Thyratrons 202a occurs during only approximately 80"" of the input half and ZDZb are' provided with cathode resistors 208a and j 208 b respectively, and means are provided for supplying.

alternating-current lagging-bias across these cathode resistors, to which is added vectorially an alternating-current signal-"voltage of-variable' phase and amplitude, to provide means for varying the grid firing phase angle to thereby-control the width and'magnitude of the'main current pulses energizing theprecipitator high voltage rectifier circuit. a

. Two variable auto transformerslltla' and 21% supplied with alternating current through transformer 211 provide individual control of the two thyratrons 202a and 20211 for load matching, whereby maximum energization can be applied tothe precipitator because sections 206a and 20Gb can be adjusted individually for maximum volt-. age and optimum sparking rates. .A small stabilizing automatic direct-current grid biasfor each thyratron is obtained by grid rectification across the grid-leak resistors 212a and 212]) respectively shunted by capacitors 213a and 21317. Low capacitance shunts 214a, 214b, respectively, are provided between the grids and cathodes of the thyratrons to prevent transient anode voltage disturbances from interfering with the grid control operation. Series resistors 216a and 2115b respectively may be used to limitthe grid current according to the tube ratings. Although xenon-filled thyratrons are preferred to remove effects of ambient temperature, other thyratrons, e. g.,

mercury vapor, can be used with appropriate temperaturecoritrol. arrangements. For precipitator energizing application, the impedance of. the grid circuits should be kept low, i. e., a few thousand ohms maximum. 7

Below a'repgiven values for a typical installation on a common fly-ash precipitator constructed in accordance with Fig. 2,: l

' -c. input-460 v., at: c., 1 pH Primary resistor-3.5 ohjms.

V. n'ansformer 15 kya, 3.7.5 amps R. M. S. primary,

50 kv. R. M. S secondary ThyratronsTtype EL-Clfi] rated 18 a-mp'-D.-C. each with voltage ratings: 7.

10000 v. peak forward anode voltage 1250 v. peak'inverse anode voltage Grid control circuit:

Grid cathode capacitor- 0.00l y.f Series grid resistorl0,000 ohms.

Figs. 3A-C show the wave forms when the grid 'volt- Inrthis case nearly the full pulse width is utilized and appears across the precipitator section witha slight lag as a broad Volt- 7 age pulse, the shape of the input pulse being, of course,

cycle, to produce the narrow rectified current pulse shown, with the resultant peak voltage. eifect across the pre-' cipitator section. Due to the steepness of thewave front,

for the same precipitator average current, the peak volt- T age V of Fig.3? willnot only be narrower, but will also be higher than V of Fig; 3C, which is the.'de-- sired etfect, and which is, of course, justable by means of the circuit of Fig. 2.

V In all of the following figures, the reference characters,.

where applicable, will'represent corresponding parts in all figures, except that the :first numeral will correspond to that of the figure so that both the. element and the figure in which it appears can be readily identified. For example, inFig. 4, the

designated 4'84.

Fig. 4 shows the use of a series transformer 404 with the thyratrons 402a and 4021: connected hack-to-back across the secondary winding of a series transformer 418. While retaining the current pulse width control as in Fig. l, this"arrangement permits a reduction in the average and peak currents through the thyratrons approximately as thc ratio of secondary to primary turns and also permits a partial range; control of the output load voltage and current accordingto the design of. the auxiliary transformer. It will be appreciated that the circuit of Fig. 4 provides full-range control of load'voltage somewhat modified and broadened by the impedance characteristics of the precipitator circuit. It will be seen that every other pulse is shown since this is the voltage applied to each precipitator section.the alternating pulses of'opposite direction are, of course, applied to:

the other section. V

-In Figs. 3D-F are shown the wave forms when the thyratron grid firing angles is retarded approm mately 100. The critical grid firing voltage is then not attained for the first part of the input wave pulse and firing and current fromzero to maximum. In many industrial precipitator applications, a load-voltage control range fromuabout 50'percent to 100' percent 'rated maximum is sufficient. In this circuit an auxiliary transformer 418 of low magnetizing current provides minimum output with the thyrat-ronsnon-conducting. a

Fig. 5 shows still'other' possible arrangements using only one thyratron 5'02 with atransformer 518. In this case, a D .-C. current componentflows in-the transformer secondary winding and is used to alter the series impedance in the primary circuit by varying the amount of DJ-C. core saturation. 1

Fig. 6 is similarto Fig. 4, except a center'tapped'secondary 62Gb is used-so that the tliyratron cathodes are at 'a common potential. This arrangement maybe preferred for 'certain gridcontrol circuitsofthe push-pulltype, for example. In this case, the thyratro'n' currents al/N times the primary load currentswhere N isthe ratio 0 1/2 secondary turns to primary turns.

Fig. 6 also shows the use of an input auto-transforme 621 for matching line voltage to any desired design voltage level in the primary circuit of the high-voltage transformer. If desired, a few taps 623 can be provided .on

such an input transformer to supplement the thyratron l control over a wider. total range to accommodate a larger number .of applications with a single basic energizing circuit.

In all of the above circuits, it will be understood that the grid phase control may be similar to that shown in Fig.2; and that the high-voltage transformer secondary may beconnected to the precipitator as in .Fig. 2, or in other two-section arrangements known in the art.

It should be noted that in the circuits of Figs. 4, 5

former 724 is now'provided, the output of which automatically controlled by reversible servo-motor722;

individually ad- 7 high voltagetransformer will he 7 the servo-motor 722 is controlled by magnetic amplifier 726, electronic integrator 728 and spark senser 730, all of which are fully disclosed in copending application, Serial No. 414,003, of H. J. Hall, filed March 3, 1954, now Patent No. 2,752,000, dated June 26, 1956, and are included herein only to show one manner in which the autotransformer 724 may be automatically controlled in order to maintain a desired optimum sparking rate. The arrangement of Fig. 7 thus provides not only pulse width control, but also automatic voltage control for optimum sparking. It will be understood that the actual voltage setting with the use of thyratrons as shown will usually be higher than with the conventional control systems, since as previously explained, the use of pulse width control permits higher voltages and more efficient operation. Thus, by providing an automatic voltage control system as shown, full advantage may be taken of the increased operating efliciency of the system.

Fig. 8 shows essentially the same automatic voltage control system as Fig. 7, using the specific thyratron control application as in Fig. 6. In this case, an auxiliary center tapped transformer 820a and 82012 corresponding to transformer 620a and 62% of Fig. 6 is employed and the autotransformer 824 is controlled by servo-'motor 822 in the same fashion as motor 722 in Fig. 7 controls autotransformer 724. No individual control of the thyratron 802a and 30217 is shown, as in some instances this feature of control will not be necessary and can, therefore, be omitted.

Fig. 9 shows a bridge comparator circuit using magnetic transducers 944 and 946. Standard forms of selfsaturating magnetic amplifiers 944 and 946 may be used to increase gain and sensitivity. Saturable reactors may also be used. The integrating circuit 942 is shown in the above referred to copending application, Serial No. 414,003, and is not, per se, a part of the present invention. The system shown in Fig. 9 requires that magnetic transducer 946 be provided with a direct-current input which is a function of the sparking rate of the precipitator, and circuit 942 shows one practical way of accomplishing this. The second direct-current input is provided for magnetic transducer 944, by circuit 945, the output of which may be adjusted by means of resistor 947 or in any other suitable fashion. Meter 947 is provided and its scale is calibrated in terms of the desired sparking rate, so that by setting resistor 947 the output of the bridge on leads 948, 950, will be automatically self-adjusting to maintain the desired sparking rate; at any other sparking rate, the alternating-current output adjusts its magnitude and phase according to the difierence between the desired sparking rate and the actual sparking rate. The output of the bridge on leads 948 and 950 is an alternating voltage whose magnitude and phase are sensitive to the two direct-current input signals. The magnitude depends on the difference between the input signals. The phase reverses 180 electrical degrees depending on which of the input signals is the larger.

The output from the bridge comparator may be used to energize the grid-control circuits of Figs. 7 and 8 at points AA. Circuit operation provides for retarding the thyratron grid firing angle and thereby lowering precipitator voltage when the sparking rate is too high. If the sparking rate is too low, the grid firing angle is advanced to raise the voltage accordingly. The system provides continuous control with the operating point automatically self regulating to maintain the desired optimum sparking rate in the precipitator. This has the advantage of eliminating the servo-motor and reduction gearing assembly which for a quality system might be relatively expensive. However, the servo-motor, continuous control technique is widely used in numerous industrial automatic control systems. The bridge output can also be used to energize the control phase of the two-phase servo-motor methods shown in Figs. 7 and 8. Standard 6 speed reduction gear assemblies specifically designed for servo-motor control are readily available from a number of manufacturers.

It is also obvious that the bridge comparator system of Fig. 9 can be used as a general control element for various automatic control systems. The alternating voltage output can be used directly or suitably rectified to provide a direct-current voltage or current as required. The alternating-current output is easily amplified. A direct-current output may be used, for example, to energize magnetic amplifiers controlling large amounts of power. Saturable reactor control of precipitator voltage is a possible application.

It will be apparent that the embodiments shown are only exemplary and that various modifications can be made in construction and arrangement within the scope of our invention as defined in the appended claims.

We claim:

1. A system for energizing a plurality of precipitator sections from a single alternating current power source, comprising in combination with the precipitator electrode, a high voltage transformer, rectifier means interposed between said transformer and the precipitator electrodes oriented to supply alternate half waves of rectified current alternately to different complementary sets of precipitator electrodes, thyratron control means between the alternating current power source and the primary of said transformer for controlling the primary current supply to said transformer, and means for supplying an adjustable phase-controlled firing potential to the thyratron control means to adjustably control the width of the rectified current pulses supplied to each of the difierent complementary sets of precipitate r electrodes.

2. The invention according to claim 1, said last means comprising means for supplying an alternating grid voltage of the same frequency as the power supply voltage to said thyratron means, and means for controlling the phase angle of said voltage to control the width of the rectified current pulses supplied to the precipitator electrodes.

3. The invention according to claim 2, said thyratron means comprising two oppositely oriented thyratrons for controlling the alternating supply pulses in both directions of current flow, there being a separate one of said phaseccntrol ling grid voltage means for each of said thyratrons, whereby each precipitator section may be individually controlled.

The invention according to claim 3 including further adjustable phase-shifting grid voltage means for controlling both thyratrons simultaneously in the same sense, and means for adjusting said further means in accordance with precipitator sparking conditions.

5. The invention according to claim 2, said thyratron control means comprising a control transformer having a primary winding in series with the primary winding of said high voltage transformer, a secondary winding for said control transformer, and a thyratron in circuit with said secondary winding.

6. The invention according to claim 5 including a second thyratron in circuit with said secondary winding and oppositely oriented to said first thyratron.

7. The invention according to claim 2 including means for controlling said phase angle in accordance with the sparking rate of the precipitator.

8. The invention according to claim 1, said means for supplying an adjustable phase-controlled potential comprising a bridge circuit having two balanced impedance arms and two further arms, a saturable magnetic transducer having a direct-current control winding, in each of said further arms, means for supplying one of said control windings with direct-current in accordance with the sparking rate of the precipitator and means for supplying the other control winding with adjustable direct-current, alternating-current supply input means of the same frequeney as the Piecipitato'r supply frequency connected be} 7 r 7 Ref renc s it fl' fifil of' t s paten tween the junctions of'said'impedance arms and said futl T D T E V 1 V ther arms, an output cireuit conneetedf'to t he opposite 2 65 i -Wi May, 1935 terminals of the bridge'circuit to supgly an output Whose v 1 "Lord 'June -1 1941 7 phase and magnjtude'are controlled by the sparking rate, 9 2 33 0 c 5 and means for cjcntroliing the thyratrontgrid potential in 2 7230 H 16, 4:]

accordance with said output. I 2,672,947 i Klemperer 23, 1954'

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