GB2144003A - Power supply for electrostatic precipitator - Google Patents

Abstract

The supply has a converter 10 coupled to a primary power source 12 for producing a converted voltage with a different frequency content to that of the source 12, the output of the converter 10 being connected to a step-up transformer 14 and a rectifier 20A to provide a high voltage fed to the precipitator 28A, 28B. The converter 10 may be an adjustable frequency, adjustable duty-cycle A.C. to A.C. converter supplied from a 60 Hz source and giving an output at a different frequency (Figs. 1, 2). The source 12 may be D.C. in some embodiments. In a further embodiment, the voltage applied to the precipitator is controlled by a micro-computer responsive to various current and voltage sensors, including means for sensing sparking and back corona. The conduction angle of thyristor Q1, Q2 is incremented from half-cycle to half-cycle of a 60 Hz supply 12 until the sensed conditions indicate that the conduction angle should be decremented or that the thyristor Q1, Q2 should be turned off. Every nth half cycle, e.g. every sixth half cycle, the conduction angle is increased to prevail throughout the majority of that half cycle, so that a high peak voltage is produced every nth half-cycle, these pulse peaks constituting the different frequency content (Figs. 6, 7). MOSFETS or a saturable reactor may be used instead of thyristors Q1, Q2. <IMAGE>

Description

SPECIFICATION Converting power supply for an electrostatic precipitator The present invention relates to electrostatic precipitators, and in particular, to controllable power supplies for electrostatic precipitators.

A conventional technique for cleansing stack gas is the use of an electrostatic precipitator which induces a high electrostatic field in the path of the gas flow. The high potential and electrode configuration ionizes the gas; charging particulate contaminants, and causing them to migrate to the precipitator plates.

Contaminants adhering to the plates are dislodged by rapping and captured in a hopper.

A conventional power supply for a precipitator employs a high voltage transformer operating through a full wave rectifier to provide a high potential to the precipitator. It is known to vary the potential in the precipitator by employing a controllable semiconductor or saturable reactor in the high voltage transformer primary. These devices are used to control the extent of conduction in the primary voltage stage and, therefore, regulate the high voltage potential in the precipitator.

A disadvantage with power supplies of this type is that they are often regulated to produce a certain degree of sparking which may lead to back corona effects. As result, a large discharge current is passed through the precipitator without performing any useful work.

Furthermore, the rectified potential applied to a precipitator often has a superimposed ripple voltage which is of a frequency corresponding to the power line frequency. This ripple frequency is often of a repetition rate which is disadvantageous for efficiently operating a precipitator.

In practical precipitator power supplies, the high electrostatic potential within the precipitator will occasionally cause a spark. A precursor of this spark can be a back-corona effect, wherein ions of the wrong potential tend to migrate within the field. This back-corona effect produces a negative resistance which tends to hasten a voltage breakdown or sparking condition. The instant prior to sparking exhibits a potential distribution wherein a significant potential gradient exists across any dust layer on the precipitator plates. A spark often dislodges a portion of the dust layer and creates a discontinuity in the potential gradients in the vicinity of a recent spark. It has been found that this discontinuity tends to foster further back-corona effects and sparking.

An important consideration in running a precipitator efficiently is keeping the average potential in the precipitator sufficiently high to cause a high extent of precipitation but not so high as to cause a rapid rate of sparking. It has been found that the potential across the dust layer in a precipitator does not necessarily initiate a spark instantaneously. Therefore, the possibility exists of briefly applying a high electrostatic potential during a transient period of time sufficiently short and infrequent so as to avoid excessive voltage across the dust layer. Since the dust layer is not excessively stressed by this high potential, there is a reduced likelihood of sparking.

It is also known (U.S. Patent No.

4,290,003) to provide a power supply which has a pair of anti-parallel SCRs (thyristors) coupled through a high voltage transformer to a rectifier bridge. The thyristors can be controlled by a microcomputer that can sense various operational parameters of the precipitator and its power supply. In response to various changes in precipitator parameters, this known system can adjust the extent of drive through the power supply in anticipation of imminent sparking, thereby reducing the likelihood of sparking. As such, the microcomputer-controlled power supply can operate quickly and accurately and achieve control not readily obtainable with older voltage controllers.

It is also known to produce a high voltage by constructing a high frequency power oscillator which drives a high voltage transformer.

Since the high voltage transformer operates at a relatively high frequency it can have a relatively small core, which tends to reduce fabrication costs.

Known AC to AC (alternating current to alternating current) converters have employed an AC to DC (alternating to direct current) converter to produce a local supply voltage.

This local supply voltage powers a chopper circuit which operates at a rate controlled by a free running multivibrator. The repetition rate of the multivibrator can be adjusted so that the output of the chopper is a variable frequency, usually a square wave A.C. source.

Such chopper circuits have had various applications in systems requiring a higher frequency, alternating current power supply.

Thus there is a need for a highly efficient power supply for an electrostatic precipitator which has characteristics most suited to running an electrostatic precipitator efficiently: one in which the ripple frequency is adjustable to provide for matching the precipitator requirements.

In accordance with the illustrative embodiments, demonstrating features and advantages of the present invention, there is provided a power supply for powering an electrostatic precipitator from a primary power source. The supply has a converter means adapted to be coupled to the primary power source for producing a converted voltage with a different frequency content. The supply also has a high voltage means coupled to and driven by the converter means to produce, from its converted voltage, a high voltage influenced by the different frequency content for performing precipitation. Thus the different frequency content affects the efficiency of precipitation.

Also in accordance with a corresponding method of the same invention, an electrostatic precipitator can be powered from a primary power source to affect precipitation efficiency.

The method includes the step of converting power from the primary power source into a converted voltage having a different frequency content. Another step is stepping up the converted voltage to a high voltage. Another step of the method is applying the high voltage to the precipitator and allowing the different frequency content to affect precipitator efficiency.

By employing such apparatus and methods, improved electrostatic precipitation is achieved. In the preferred embodiment, an AC to AC converter converts a line voltage at, for example, 60 Hz into an alternating current at a different frequency. This converted alternating current is stepped up through a high voltage transformer and applied to a full wave bridge. The bridge produces a pulsating DC voltage for creating a high electrostatic potential within the precipitator. In one embodiment, the AC to AC conversion, is achieved by a multivibrator driven by a clock to operate as a conventional chopper circuit. The chopper circuit chops a DC voltage produced by a full wave rectifying bridge powered by the line voltage.

In a preferred embodiment, the AC to AC converter has an adjustable repetition rate and duty cycle. Each of these parameters can be set to produce the most efficient transfer of energy to the electrostatic precipitator. It is anticipated that each precipitator, depending upon its physical structure and upon its layer of internal dust, will operate most efficiently at a particular frequency and duty cycle.

By employing apparatus and methods according to the above, an improved and highly efficient precipitation is achieved. In a preferred embodiment, a pair of anti-parallel thyristors are coupled to a transformerArectifier set. The thyristors are controlled by a microcomputer which sets the angle of conduction of the thyristors according to its internal program. The internal program regulates the conduction angle in accordance with measurements of various operational parameters. In this preferred embodiment, the conduction angle is also periodically increased. For example, the conduction angle can be increased for one-half cycle by a factor of eight at every sixth half cycle. Of course, other increase factors and duty cycles can be chosen depending upon the particular exhaust being treated.

While it is preferred to program the periodic increase in conduction angle through a program contained in memory, a discrete, hardwired apparatus for producing the same periodic increase in conduction angle is also disclosed. In one embodiment, a counter detects every nth half cycle. During that nth half cycle, the conduction angle is increased by a predetermined factor. In this embodiment the conduction angle is established by a digital counter which divides each half cycle into a given number of pulses. The reaching of a predetermined count indicates the elapsing of a corresponding portion of the conduction angle. A clock involved in this counting of divisions of the conduction angle, can be changed in frequency to effectively multiply the conduction angle by a predetermined factor.

The above brief description as well as other features and advantages of the present invention will be more fully appreciated by reference to the following detailed description of a presently preferred but nonetheless illustrative embodiment in accordance with the present invention when taken in conjunction with the accompanying drawings, wherein: Fig. 1 is a schematic diagram of a power supply and electrostatic precipitator according to the principles of the present invention:: Fig. 2 is a schematic diagram of a power supply which is an alternate to that of Fig. 1; Fig. 3 is a simplified equivalent circuit for modelling the characteristics of an electrostatic precipitator; Fig. 4 is a voltage-current characteristic of an electrostatic precipitator showing low, modest and high dust layers; Fig. 5 is a simplified schematic diagram of an electrostatic precipitator system according to the principles of the present invention; Fig. 6 is a flowchart associated with the microcomputer of Fig. 5; Fig. 7 is a timing diagram illustrating the changing voltages associated with the apparatus of Fig. 5; Figs. 8A and 8B are potential diagrams showing the potential distribution within an electrostatic precipitator for a relatively clean and dusty precipitator plate, respectively; ; Fig. 9 is a simplified schematic diagram of a portion of a converter which is an alternate to that disclosed in Fig. 5.

Referring to Fig. 1, the power supply for an electrostatic precipitator is shown employing an AC to AC converter means 1 0. Converter 10 receives an input potential from primary power lines 12, which carry an alternating current at, for example, 60 Hz. Converter 10 produces an output voltage at a different frequency which is applied to the primary of a high voltage transformer 14 (herein referred to as a high voltage means). The frequency applied to transformer 14 can be set by adjusting the potential on line 1 6 of converter 10 by setting the wiper of potentiometer 1 8.

Converter 10 can be any one of various conventional AC to AC converters which may convert an AC potential into a DC potential which is then chopped at an adjustable repetition rate. In one embodiment, a commercially available, variable frequency speed control is employed, such as a Parajust Y manufactured by Powermetrics of Orange, Connecticut.

The secondary of transformer 14 is coupled to two opposite nodes of bridge circuit 20, employing four semiconductor rectifiers in a full wave bridge configuration. The other two nodes of bridge 20 are connected respectively to ground and to one terminal of inductive choke 22 to supply it with a negative potential. The other terminal of choke 22 is commonly connected to electrodes 24A and 24B.

These electrodes are encompassed by grounded electrode plates 26 to form an electostatic precipitator.

Referring to Fig. 2, an alternate precipitator system is illustrated. The apparatus of Fig. 2 may be substituted for the AC to AC conversion means 10 of Fig. 1. An input transformer 30 has its primary connected to an AC primary power source at terminals 1 2. Its secondary is connected to two opposite terminals of full wave bridge 34 (similar to previously mentioned bridge 20). The other terminals of bridge 34 are connected to ground and to the collector of NPN transistor 01, which is part of a chopper circuit. Bridge 34 is referred to as a rectifier means. The ripple on the collector of transistor Q1 is filtered somewhat by grounded filtering capacitor C3. The emitter of transistor 01, terminal T1, is connected to one terminal of filtering capacitor C1, whose other terminal is grounded.The base of transistor Q1 is connected to an output of multivibrator 36, the well known circuit which has two states. Multivibrator 36 is configured as a one shot so that it has only one stable state.

The amount of time multivibrator 36 stays in the unstable state is controlled by a second means shown herein as variable resistor 38 connected to an input of multivibrator 36 which shunts the resistive/capacitive timing circuit of multivibrator 36. The multivibrator is driven into its unstable state by a clock circuit, which may be a conventional crystal oscillator 40. The frequency of oscillation of clock 40 is controlled by a first means shown herein as variable capacitor C2, connecting between an input of clock 40 and ground. The collector of transistor Q1 also connects to the power input terminals of multivibrator 36 and clock 40.

The operation of the circuit of Fig. 2 is such that a alternating current conveyed through transformer 30 produces from bridge circuit 34 a full wave rectified signal which is stored on capacitor C3. The multivibrator is pulsed at a repetition rate determined by the setting of capacitor C2 of clock 40. The duty cycle of multivibrator 36 is controlled by the variable resistor 38. When the multivibrator is in its unstable state, it produces a high signal on the base of transistor 01. In response, transistor Ol conducts to apply a positive current through terminal T1 for a time period determined by the setting of the variable resistor 38. At the end of the unstable state when the multivibrator reverses its state, the base of transitor 01 becomes relatively negative so that all junctions of transitor Q1 are reversed biased.Accordingly, no current is conducted through transistor Q1 so that the voltage across the capacitor C1 quickly falls to zero because of the load (not shown) on terminal T1. Of course, capacitor C1 removes low energy, high frequency transients from the output on terminal T1 to leave a relatively noiseless signal.

Refering to Fig. 1, the AC to AC converter 10 can operate similarly to the chopper circuit described in connection with Fig. 2. Specifically, the primary power source applied to terminal 1 2 can be converted into a pulsating or alternating current across the primary of transformer 14. This signal can be adjusted in frequency by adjusting potentiometer 1 8. Of course, in some embodiments, converter 10 may also have an internal duty cycle controller.

The time varying signal on the primary of transformer 14 is stepped up and rectified through full wave bridge 20 to produce a high negative voltage at the choke 22. Choke 22 operates in a conventional manner to reduce fast current transients which may adversely affect precipitator operations. Consequently, a high negative voltage appears on electrodes 24A and 24B. In a conventional manner, this voltage produces a high electrostatic field which ionizes particulate contaminants so that they are captured on plates 26.

The operation of the precipitator can be more fully understood by considering the equivalent circuit of Fig. 3. The potential across the precipitator is designated herein as Vin. This potential is deemed to be applied across a spark gap 50 which is in parallel with a storage capacitance 52. In parallel with capcitance 52 is the series combination of variable resistance 56 and a residual potential source (battery 54). The signficance of this equivalent circuit is that the precipitator stores energy through its natural capacitance but which may be discharged through spark gap 50 when voltage Vin is excessive. This discharge however is never so complete that all energy is removed.

The effective v-i (voltage/current) characteristic of the precipitator is graphically illustrated in Fig. 4. Graphs G1, G2, and G3 show the v-i characteristic for progressively thicker layers of dust on the precipitator plate.

When the plate is relatively clean the v-i characteristic has an offset voltage V1 at which conduction commences followed by a nonlinear but positive resistance interval. The moderately dusty operation of graph G2 shows a reduced offset voltage but decreased resistance. The graph G3 indicates a relatively thick layer of dust where the current increases monotonically until a threshold voltage is exceeded. After that the precipitator exhibits a negative impedance characteristic indicating inefficient operation.

In this preferred embodiment, the frequency and duty cycle of the AC to AC convertor 10 is adjusted to provide for a maximum transfer of useful energy into the precipitator. This maximum transfer of energy can be deter mined by any appropriate means of measuring precipitator efficiency. For example the frequency and duty cycle of converter 10 can be separately adjusted to provide a relatively low spark rate for a predetermined potential in the precipitator. Alternatively the power delivered to the precipitator can be measured and maximized by adjusting the frequency of duty cycle of converter 10. Also the output frequency of converter 10 can be adjusted to set the precipitator potential at the maximum usable value.

In Fig. 5 a voltage controller, part of a converter means, is shown as block 10 connected between input line P2 and output line P4. Block 10 has a pair of oppositely poled thyristors (SCR's) Q1 and Q2 connected in anti-parallel between lines P2 and P4 so that they can support an alternating current. By triggering thyristors Q2 and Q2 to conduct through a desired phase angle, the extent of conduction therethrough can be controlled in a well-known manner. A typical thyristor connection is shown in Fig. 7 of U. S. Patent 4,290,003. While a thyristor controller is shown herein it is apparent that other controllers employing elements such as a MOSFET transistor or a saturable reactor may be employed instead.Alternatively, if power source 1 2 were a direct current source, controller could be an appropriate chopper circuit.

A high voltage means is shown herein as a transformer-rectifier set T1, 20A A conductive element is shown as limiting inductor 1 7 which is in series circuit across the primary power input terminals 12 together with controller 10 and primary 14 of high voltage transformer T1. The high voltage means includes transformer T1 and full wave bridge 20A comprising diodes CR1, CR2, CR3 and CR4. The anode of diode CR1 and the cathode of diode CR2 are connected to one terminal of secondary 1 9 its other terminal being connected to the cathode of diode CR3 and the anode of diode CR4. The cathodes of diodes CR1 and CR4 are shunted to ground by resistors R4 and R6, respectively. The anodes of diodes CR2 and CR3 are connected to the junction of surge-limiting inductors L2 and L4.Having a high turns ratio, transformer T1 produces a negative, direct current voltage at the junction of inductors L2 and L4 which is of a high magnitude.

While conductive element 1 7 is shown as a current limiting reactor, it is apparent that a resistive element may be employed in other embodiments. Being inductive, element 1 7 has the advantage of being responsive to transient phenomena indicative of imminent sparking.

The non-common terminals of inductors L2 and L4 are separately connected to high tension electrodes 24B and 24A, respectively, of precipitators 28B and 28A. Precipitators 28B and 28A are constructed in a well-known manner and are disposed in the path of the exhaust from a machine or a process. Sufficiently high electric fields within precipitators 28B and 28A will ionize and deflect particles in the exhaust thereby cleansing it.

Element 1 7 is connected in parallel with primary 1 30 of transformer T2 and its secondary 1 32 is connected between ground and the input of absoluting buffer ABS1. Absoluting buffer ABS1 (which may be constructed as shown in Fig. 6 of U. S. Patent 4,290,003) produces a unipolar signal having a magnitude proportional to the absolute value of its input, although a Hall-effect device or other apparatus may be used instead.

A signal proportional to the primary current of transformer T1 is provided by current transformer T3 which is inductively coupled to the line between input 12 and element 1 7. Current transformer T3 is coupled to absoluting buffer ABS2 by means of isolation transformer T4. Absoluting buffer ABS2 is identical in construction to buffer ABS 1. The primaries of transformers T1 and T5 are connected in parallel. The secondary of transformer T5 drives the input of absoluting buffer ABS3 whose construction is identical to that of buffer ABS 1. It is apparent that buffers ABS2 and ABS3 are driven by voltages proportional to the primary current and voltage, respectively, of high voltage transformer T1.The secondary current of transformer T1 flows through either resistor R4 or R6, this current alternating therebetween for successive halfcycles. This secondary current signal is transmitted by non-inverting buffer amplifiers 1 34 and 1 36 which are separately connected to the ungrounded terminals of resistors R4 and R6, respectively.

A high voltage sensing means is shown herein as a pair of dividers, although a Halleffect device or other apparatus may be used instead. Connected between high tension electrode 24B and ground is one such voltage divider, comprising serially connected resistors R8 and R10. Similarly, a divider, comprising serially connected resistors R12 and R14, is connected between high tension electrode 24A and ground. The junction of resistors R8 and R10 is connected to the input of inverting buffer ampllifier 1 38 to drive it with a voltage proportional to the operating potential of precipitator 28A.Similarly inverting buffer amplifier 140, being connected to the junction of resistors Ri 2 and R 14, is driven with a voltage proportional to the operating potential of precipitator 28A.

For many applications, it will be convenient to locate the just described apparatus of Fig.

5 near precipitators 28B and 28A. Frequently such equipment will be located adjacent to one or more smoke stacks. Since the balance of equipment can be located at a place conveniently accessible to an operator, such partitioning is indicated by dotted partition line RF.

The outputs of buffers ABS2 ABS 3, 134, 136, 138 and 140 interface with inputs lN2, IN3, IN4, IN5, IN6 and IN7, respectively, of subsystem 142. The output of buffer ABS1 is coupled to signal conditioning circuit 143 whose output is connected to input IN1 of subsystem 142. Circuit 143 is preferably a low pass filter, however, in some embodiments an integrator may be employed instead.

While supplying seven different inputs to subsystem in this manner provides reasonably detailed information on precipitator performance, it is expected that in other embodiments a different number of inputs may be employed. Subsystem 1 42 is part of a command means and includes triggered monitors, such as the one shown in Fig. 8A of U.S.

Patent 4,290,003. The command means (part of the converter means) also includes microcomputer COM. Microcomputer COM may be constructed substantially as described in U.S.

Patent 4,290,003. The coupling between subsystem 1 42 and microcomputer COM is shown as a broad arrow to suggest the existence of more than one data line and the directional flow of information. Microcomputer COM is operative to repetitively strobe inputs IN1-lN7 so that these inputs are effectively multiplexed into microcomputer COM. Microcomputer COM is also operative to transmit a control signal to subsystem 144. Subsystem 144 (an example of one being given hereinafter) is arranged to convert the control signal produced by microcomputer 1 42 into a pair of timing signals which are transmitted along lines 146 to controller 10 to control its conduction angle. Obviously subsystem 144 provides a suitable interface between controller 10 and command means 142.Accordingly, the structure of subsystem 1 44 would be significantly different if instead of thyristors, controller 10 employed a MOSFET transistor, saturable reactor or other device. An operator may provide input to microcomputer COM by operating switches in control accessory CNL.

Microcomputer COM can display information to an operator by means of display accessory DISP. Elements CNL and DISP may be constructed substantially as shown in Fig. 6 of U.S. Patent 4,290,003.

Microcomputer COM, which provides overall system control and timing may take any one of several forms. it is preferable that microcomputer COM be constructed with a commercially available microprocessor, however, many alternate structures will be readily apparent to persons skilled in the art. In fact in some embodiments, analog circuitry may be employed. For example, selectable storage capacitors may be charged to potentials representing the signals on inputs IN1-17 at various instants of time. These stored charges may be selectively coupled to a combining network to produce a control signal.

Microcomputer COM establishes the rates and sequence in which each of the inputs IN1-lN7 transmits its respective signal to command means COM. In this embodiment this rate will be normally twice the power line frequency but subject to substantial increase under predetermined conditions. It is apparent that other rates may be employed to suit the characteristics of a specific voltage controller and precipitator.

Primary power lines 1 2 are also connected to a pair of inputs of shaper 84. Shaper 84, an amplifier, rapidly saturates to produce a square wave output synchronously with the power line frequency. This output of shaper 84 is applied as one comparison input to phase comparator PC. Phase comparator PC has another input 60 which is nominally at 60 Hz. In a well known fashion, a phase differential between line 60 and the output of shaper 84 produces an error signal from phase comparator PC. This error signal is applied to voltage controlled oscillator VC to regulate its frequency of oscillation. Oscillator VC has an integrating type of control so that the oscillator continues to change frequency until the output of the phase comparator is zero.It will be appreciated that the signal on line 60 is square wave synchronous with the 60 Hz power line frequency applied to terminals 1 2 of shaper 84. Voltage controlled oscillator VCO nominally produces on output line 245760 a signal operating at 245.76 kHz. This output is applied to the input of divider DIV, a decade divider which is operable to divide the 245.76 kHz signal into outputs nominally at 30.72 kHz, 240 Hz, 120 Hz and 60 Hz on lines 30720, 240, 120, and 60, respectively.

Line 60 is provided as an input to microcomputer COM on one of its sense lines. Thus the microcomputer can determine the current phasing of the power line. Also, a signal indicating that 75% of the current half cycle has expired is produced on line 72, another sense input of microcomputer COM. Signals on line 1 20 and line 240 at twice and four times line frequency are fed into separate inputs of NOR gate 45. It will be appreciated that the resulting pulse on line 72 indicates the prevalence of the last 25% of each half cycle. This signal is employed in the manner described hereinafter.

In order to facilitate an understanding of the apparatus of Fig. 5 its operation will be briefly described under the conditions where sparking is imminent, where it has occurred, where back-corona is present and normal conditions.

Assume the apparatus of Fig. 5 has been recently energized and is producing a relatively low voltage on electrodes 24B and 24A.

Microcomputer COM addresses and receives data from inputs IN6 and IN7 for every half cycle of the power line input 1 2. This data, including the voltage on electrodes 24B and 24A, is received after approximately 75% of a half cycle has elapsed. Such timing allows microcomputer COM to fairly assess the conditions presently existing during each half cycle and to adjust the control signal of line 146 in advance of the succeeding half cycle.

For awhile, the control signal is periodically advanced every half cycle to increase the voltage of electrodes 24B and 24A. The incrementation of the control signal of line 46 may in some embodiments be scaled down as the voltages of electrodes 24B and 24A approach their rated values. It is assumed in this example that sufficient voltage will cause a condition such that sparking is imminent.

Assume now that during the next half cycle the coronas in precipitators 28B and 28A distend and form projections or "flares." Such distension is the precursor of sparking and it produces a distinctive increase in precipitator current. This increase in precipitator current produces an increased voltage drop across element 17. Since the current perturbation caused by this corona distension contains substantial high frequency components, inductor 1 7 is especially sensitive thereto. In addition, since corona distension is likely to occur in the latter part of a half cycle of power input 12, the fact that microcomputer COM takes its measurement during that time makes it particularly sensitive to this phenomenon.

Upon receiving a measurement from input INI after the elapse of at least 75% of the then-existing half cycle, microcomputer COM compares this latest measurement against a preset threshold (for example, 2 volts). Referring to the flow diagram of Fig. 6, this sequence is shown as several branches. At branch 200 the system waits for a phasing signal at terminal 72 (Fig. 5) indicating elapse of at least 75% of the half cycle. It is preferable to allow as much as possible of the current half cycle to elapse in order to allow calculations to occur during the quiescent period athalf cycle when the thyristors 10 are off. At branch 202 the signal from input IN1 is stored and at branch 204 the threshold comparison is performed.If the threshold is exceeded the control signal (line 46 of Fig. 5) is decremented as shown at branch 206 by a factor of approximately 1 %. This decrement is chosen to suit the characteristics and response time of the precipitator being controlled.

After this operation (or assuming branch 206 was skipped because the threshold of branch 204 was not exceeded) the recently measured value of input IN1 has subtracted from it the previous value of IN1, as shown at branch 208. This difference is compared to a preset limit (for example 10%) as shown at branch 210 and if the limit is exceeded, the control signal is decremented, otherwise it is incremented. This decrementation and incrementation is shown at branches 212 and 214, respectively. The extent of decrementation is chosen to suit the characteristics and response time of the precipitator. The extent of incrementation at branch 214 is less than the decrement occurring at branch 206. This relation will ensure that if decrementation occurs, its effect will not be overcome by the incrementation at branch 21 4.

The result of the foregoing steps is that if element 1 7 (Fig. 5) indicates imminent sparking the control signal (line 146 of Fig. 5) is decreased, otherwise it is increased. Thus the high voltage applied to precipitators 28B and 28A is at a relatively large value, just below the point at which sparking occurs. In this embodiment the control signal is varied by a fixed amount, although in other embodiments, the amount of change can be obtained according to a table, a formula or according to other measured parameters.

The foregoing described an operation in which sparking was prevented. In the event, however, that some massive disturbance produces a spark anyway, the following describes the system response thereto.

Assume that in the middle of a half cycle of power input 1 2 (Fig. 5) sparking commences in precipitator 28A. As a result, the voltage on high tension electrode 24B abruptly falls.

The relatively small voltage consequently produced at input IN6 is detected by microcomputer COM shortly thereafter. The latest value of IN6 is compared to the value occurring one-half cycle earlier, and if it exceeds a predetermined limit (for example, 25%) command means COM responds to this emergent condition by bringing the control signal on line 46 to a minimum value. This feature is also illustrated in the flow diagram of Fig. 6 which shows that immediately after the operation of previously described branch 212 or 214, the recent values of high voltage, obtained from inputs IN16 and IN6, are stored into memory (step 216). These recent values have subtracted from them the corresponding value of high voltage stored from the previous half cycle (step 218). If these differences are both greater than or equal to zero, no further adjustments to the control signal occur and the routine recycles as described hereinafter.

If either of these differences are negative, indicating a fall in the high voltage, a comparison is made to a preset spark limit to determine if a spark has occurred. If the limit has been exceeded the following occurs as indicated by branches 220, 222, 224, and 226 of the flow diagram (Fig. 6).

The control signal is reset to zero in an attempt to disable controller 10 (Fig. 5). However, if the thyristors of controller 1 2 are already conducting they will continue to conduct at least until the end of the half cycle of power input 1 2. Since a spark appears to have commenced, microcomputer COM begins demanding data from input IN6 and IN7 at a relatively high rate. This elevated rate is important since controller 10 must remain off so long as sparking persists. The existence of the spark is noted by the continuing voltage depression at inputs IN6 and IN7. Alternatively, increased current measurement at inputs IN4 and IN5 indicate continuing sparking.Therefore the inputs lN4, IN5, IN6 and IN7 are monitored on a "real time" basis until they reach a value which insures spark extinction.

The time required to extinguish a spark can vary upon each occurrence thereof. For these reasons microcomputer COM disables controller 10 until spark extinction. After extinction the control signal is restored but at a value perhaps smaller (for example 0 to 48 reduction) than that existing in the half cycle in which sparking occurred. In this fashion the likelihood of repeated sparking is avoided.

Assuming the high voltages subside to below a quench value shortly after the commencement of a succeeding half cycle of power input 12, the operation associated with branch 228 (Fig. 6) occurs. This operation is the transmission of the restored control signal, followed by a return to the beginning of the sequence of operations, as indicated by branch 234. Having restored the control signal, one of the thyristors of controller 10 (Fig.5) again conducts at a time (phase angle) determined by the control signal.

The foregoing sequence of operations just described in connection with Fig. 6 constitutes one microcomputer programming cycle.

Accordingly, the microcomputer awaits the next occurrence of a phasing signal at the elapse of at least 75% of the current half cycle of power input 12, as indicated by branch 200.

The above sequence comprised a power cycle wherein sparking had occurred and wherein branch 222 (Fig. 6) was executed instead of branch 230. Accordingly, after microcomputer COM (Fig. 5) determines that the decrement in the high voltage measurements of inputs IN6 and IN7 does not indicate sparking, the operation illustrated branch 230 (Fig. 6) commences. This operation consists of determining whether this moderate decrease in high voltage exceeds a threshold (for example 5%) which would indicate a backcorona effect. If this corona limit is exceeded the control signal is decremented a predetermined amount (for example 1 %) as indicated in branch 232. This decrement is greater than the increment which may be produced by the operation associated with branch 214.While the variation just described for the signal was a fixed decrement, in other embodiments a table, a formula or the value of the measured inputs IN 1IN may be employed to determine the variation of the control signal during the occurrence of a back-corona effect.

With the foregoing approach the voltages on electrodes 24B and 24A are periodically increased until the back-corona occurs. Upon occurrence of the back-corona, the conduction angle of controller 10 is decreased. In this manner, electrode voltage is kept around a peak which represents relatively high efficiency. It is apparent that if a back-corona did not occur and if the voltage to current characteristics of precipitators 28B and 28A were monotonic, then the precipitator voltage would increase until sparking was imminent.

In the event that the corona limit of step 230 is not exceeded, programming step 236 is executed. The program of microcomputer COM continually counts the number of half cycles of power source 1 2 elapsing. In one embodiment the program awaits the arrival of every sixth half cycle. For a 60 Hz line frequency this means the program awaits passage of successive fifty millisecond intervals.

On the sixth half cycle, the program does not skip immediately to step 228 but first executes programming step 238. In step 238 the control signal is boosted by a predetermined amount. In one constructed embodiment, the conduction angle represented by the control signal, is increased by a factor of 8. For example, were the conduction angle currently at 20 , it would now be increased to 160 . It will be understood that the number of half cycles that microcomputer COM awaits before boosting the control signal, can be varied depending upon the installation. For some applications where the tendency to spark is relatively high, the number of half cycles awaited can be increased. Similarly, where a likelihood of sparking is high, the amount by which the conduction angle is increased can be moderated.Also, while a multiplication of the conduction angle is described, in other embodiments the conduction angle can be increased by a predetermined amount. Of course, in embodiments in which the amplitude of the signal applied to high voltage transformer T1 (Fig. 5) is directly controlled, the adjustments will be made to amplitude and not to conduction angle.

The absolute value of the voltage applied to the primary of high voltage transformer T1, which is the input IN3 of interface 142 (Fig.

5), is graphically illustrated in the time-plot 3a of Fig. 7. As shown therein the voltage falls within a full-wave rectified sinusoid and its conduction angle for most half cycles is relatively small. However, as discussed above (step 238 of Fig. 6), at half cycle HC1, the conduction angle is greatly increased to prevail throughout the majority of the half cycle.

Referring to plot 3b of Fig. 7, current IN2 sensed at the interface 1 42 (Fig. 5), greatly increases during this interval. Simultaneously, the voltage IN6 measured at interface 142 similarly rises to a relatively high peak during this interval. Thereafter, the voltage displays the typical ripple effect common in AC rectified circuits.

In step 238 certain parameters associated with steps 204, 210, 220 and 230 are changed to avoid interpretation of the following boosted half cycle as a faulty condition.

These changed parameters apply for only the next half cycle when boosted values are measured, except as otherwise noted. Specifically, in the next execution of steps 204 and 210 any violation of the inductor voltage is ignored since this voltage will appear excessively high.

Similarly the limit tests of steps 220 and 230 will be suppressed. Instead of suppressing the subsequent limit, tests can be conducted with more liberal limits. Alternatively, the tested values can be established at not their actual measured value but at their previously unboosted values.

Significant to note is the fact that the voltage IN6 stays normally at a modest value except for a periodic peaking which occurs herein at every sixth half cycle. This peaking is beneficial since it increases the extent of precipitation. The resulting brief, but relatively high potential, ionizing field within the precipitator helps to insure a more thorough precipitation phenomena. However, the moderation of this potential immediately thereafter allows voltages to stabilize.

Referring to Fig. 8A, a plot of potential between precipitator electrode PE and precipitator plate PL is shown to rise from a maximum negative potential of 45 kV to zero volts in exponential fashion. When a dust layer DL is present on the plate PL (Fig. 8B), the potential distribution changes significantly. A substantial amount of the voltage drop occurs across the dust layer DL. This phenomena makes more likely the occurrence of a spark affecting this dust layer. Occurrence of a spark at the dust layer will dislodge a portion of the dust layer and open a low-potential window onto the plate. It has been found that the discontinuity represented by this window tends to further encourage back-corona effects.

However, the apparatus disclosed herein pulses the electrostatic potential to a relatively high value. In response, the potentials across dust layer DL do not immediately rise. In fact the total potential from electrode PE to plate PL begins to fall before the potential of dust layer DL can rise significantly. Thus most of this higher potential exists between the electrode PE and the inside surface of dust layer D, that is, in the area where it can be most effective. Accordingly, the foregoing modulation of precipitator power produces a highly efficient transfer of energy into precipitation without causing sparking which may disturb the dust layer DL.

Referring to Fig. 9, a simplified digital to analog angle converter is shown which may be used in the converter 44 of Fig. 5. In this embodiment, the feature of increasing the electrostatic precipitator potential once during every nth cycle is achieved by hardware. In contrast, the previously described system achieved the increase of potential through software. Accordingly, a divide by n divider 500 is shown herein as a means for counting the passage of a predetermined number of half cycles. To this end, half cycles are detected at input terminal 1 20 of divider 500 which connects to the 1 20 Hz output of divider DIV (Fig. 5). The output of divider 500 produces a rising pulse edge once every nth half cycle. This divided output is applied to the input of one shot 502 to provide a pulse which is approximately one half cycle in duration.The output of one shot 502 is applied to a normal input of NAND gate 504 and an inverting input of NAND gate 506.

The other normal inputs of NAND gates 504 and 506 are connected to terminals 245760 and 30720, respectively. These terminals are the corresponding terminals of Fig. 5 which carry the 245.76 kHz and 30.72 kHz signals, respectively. The outputs of NAND gates 504 and 506 are separately connected to the inputs of NAND gate 508 whose output connects to a normal input of AND gate 94. The inverting input of AND gate 94 is connected to the carry output CO of a counter 80. The output of AND gate 94 is connected to the clock input of counter 80. Preset enable terminal PE of counter 80 is connected to terminal 1 20 which was previously described in Fig. 5.

The inverting preset inputs of counter 80 are connected to the outputs of latch L6 whose inputs are connected to data lines DA which are data output lines from microcomputer COM (Fig. 5). Also the data strobe input Dl for causing latch L6 to store the data on lines DA, is connected to terminal SEL which is another output of microcomputer COM. The carry output CO of counter 80 is also connected to a normal input of AND gate 110 and an inverting input of NOR gate 112. The other normal inputs of gates 110 and 11 2 are connected to previously mentioned terminal 60. The output of AND gate 110 is connected through resistor R32 to the base of NPN transistor Q4 whose emitter is grounded. The output of NOR gate 11 2 is connected through resistor R34 to the base of NPN transistor Q6 whose emitter is also grounded. The collectors of transistors Q4 and Q6, identified herein as terminals P12 and PIO, respectively, may be transformed or coupled to the trigger elec trodes of the thyristors of voltage controller 10 (Fig. 5). This interconnection may be as shown in Fig. 7 of U. S. Patent 4,290,003.

The operation of the apparatus of Fig. 9 can be described as follows: Assume that the nth half cycle has not yet arrived and that one shot 502 produces a zero output voltage which when applied to NAND gate 504 forces its output to remain at a high level. Further more, NAND gate 506 is able to transfer the 30.72 kHz signal through NAND gate 508 to the normal input of AND gate 94.

The digital information supplied by the microcomputer on lines DA correspond to a desired conduction angle for the previously mentioned thyristors. This data can be strobed into latch L6 by a signal on line SEL. Thereafter, the latched data is applied to the presetting inputs of counter 80. Since the pre-setting inputs are complemented, the counter 80 is preset to a complementary number from which it then counts downwardly. Accordingly, by choosing the proper clock frequency and counting range, the counter 80 can count through a range which corresponds to the full 180 conduction angle. If the counter has a count range of 256, a clock repetition rate of 30.72 kHz will allow the counter to count down its full range in about 8 milliseconds which is the nominal duration of a half cycle of a 60 Hz power line.

At the end of the current half cycle, a triggering pulse is applied through preset enable terminal PE of counter 80. In response, the carry output CO of counter 80 goes low (assuming the preset input is not zero). Therefore, AND gate 94 transmits the 30.72 kHz signal from gates 506 and 508 to the clock input CL of counter 80. Consequently, after the expiration of a time period determined by the output of latch L6, counter 80 agains returns to a high state. This high signal is applied to a normal input of AND gate 110 and an inverting input of NOR gate 11 2.

Consequently either of them can then provide a high signal. If the 60 Hz signal on terminal 60 is in a positive phase then a high output is produced from AND gate 110. Otherwise, a high output is produced from NOR gate 11 2.

The foregoing assures proper phasing from one half cycle to the next. The high signal from either gate 110 or 112 causes transistor Q4 or 06, respectively, to become conductive, causing current to be drawn by either terminal P12 or P10, respectively. This conduction causes the firing of either thyristor Q1 or Q2 of controller 10 (Fig. 5).

The foregoing operation proceeds through several half cycles until the nth half cycle is reached. In a preferred embodiment described above n is equal to 6. At this nth half cycle, a positive signal from divider 500 triggers one shot 502 to apply a high input to normal input of NAND gate 504 and to the inverting input of NAND gate 506. Consequently, NAND gate 506 produces a low signal and NAND gate 504 transmits the 245.76 kHz signal on terminal 245760 to an input of NAND gate 508. This 245.76 kHz signal is then applied to the normal input of AND gate 94. Accordingly, when the carry signal CO goes low on the expiration of the last half cycle, AND gate 94, receiving this low signal on its inverting input, is able to convey the 245.76 kHz signal to the clock input CL of counter 80.

This input frequency is eight times the clock frequency previously described. Consequently, any computer-commanded conduction angle is increased by a factor of eight. (Actually, the commanded initial "off" interval is divided by eight.) Therefore the thyristors Q1 and Q2 (Fig.5) will commence conduction earlier than the other half cycles during which divider 500 (Fig. 9) is not influential.

It will be appreciated that the foregoing apparatus of Fig. 9 causes a periodic increase in the high voltage potential in the precipitator once every nth half cycle. This operation is similar to that described for the software control system of Fig. 5 except that the apparatus in Fig. 9 achieves this type of function strictly with hardware. It is to be appreciated that various modifications may be implemented with respect to the above described preferred embodiments. For example, alternate electrical circuits can be employed which also provide AC to AC conversion. While a chopper circuit is illustrated various power oscillators can be employed instead. Furthermore some embodiments may employ a DC motor driving a generator at a variable speed, thereby producing a variable frequency.While full wave rectification is shown in some embodiments, half wave rectification can be employed or a tapped transformer using only two rectifiers to provide full wave rectification. Also while only two precipitator banks are suggested, obviously different numbers may be energized by the power supply disclosed herein. Other illustrated electrical components can be altered depending on the desired speed of operation, power handling capacity, temperature stability, reliabilty, etc. The described apparatus may be constructed in alternate fashions using a different balance of digital and analog circuitry. Moreover, various alternate microprocessor programs may be employed in accordance with the above teachings. For example, certain steps may be reordered, deleted or supplemented in alternate configurations.

Also, the sensitivity of the system to measured parameters may be adjusted to suit the specific precipitator that is being controlled. Furthermore, the number of half cycles which the computer awaits before increasing precipitator voltage can be changed. Similarly the extent to which precipitator voltage is changed for a half cycle can be altered depending upon the precipitator involved. In addition, while an increase in voltage is described for only a portion of a half cycle, in alternate embodiments a lesser time or several half cycles can be subjected to increased voltage. Moreover, it is anticipated that other embodiments will employ circuit components having different values, tolerances and ratings to provide the desired accuracy, power, speed etc.

Obviously, many modifications and variations of the present invention are possible in light of the above teachings. It is, therefore, to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.

Claims (28)

1. A power supply for powering an electrostatic precipitator from a primary power source, comprising: a converter means adapted to be coupled to said primary power source for producing a converted voltage with a different frequency content and; a high voltage means coupled to and driven by said converter means to produce from its converted voltage a high voltage influenced by said different frequency content for performing precipitation, whereby said different frequency content affects the efficiency of precipitation.

2. A power supply according to claim 1 wherein said different frequency content is dimensioned to increase the efficiency of said precipitator over what it would be if the frequency content applied to said high voltage means was the same as that of said power source.

3. A power supply according to claim 2 wherein said power source is an alternating current and wherein said converter means comprises: rectifier means coupled to said power source for producing a unipolar voltage; and a chopper circuit for switching said unipolar voltage to produce said different frequency content.

4. A power supply according to claim 3 wherein said chopper circuit comprises:.

first means for adjusting the switching repetition rate of said chopper circuit.

5. A power supply according to claim 4 wherein said high voltage means comprises: a high voltage transformer for stepping up said converted voltage; and a bridge coupled to and driven by said transformer for producing a high unipolar po potential.

6. A power supply according to claim 1 wherein said content has at least one frequency component at a predetermined low frequency sized to allow efficient precipitation.

7. A power supply according to claim 6 wherein said primary power source is alternating and wherein said converter means is operable to produce said converted voltage with a lower frequency content.

8. A power supply according to claim 6 wherein said converter means is operable to periodically increase the amplitude of said converted voltage at said low frequency.

9. A power supply according to claim 8 wherein said converter means comprises: a controller having a control terminal for producing from said primary power source said converted voltage and for varying it in response to a control signal applied to said control terminal; and a command means for producing and applying the control signal to said control terminal, said command means periodically changing said control signal to increase the amplitude of said converted voltage.

1 0. A power supply according to claim 9 wherein said command means is coupled to said high voltage means and is operable in response to a functional parameter thereof to vary said control signal to regulate the high voltage.

11. A power supply according to claim 10 wherein said primary power source is alternating and wherein said command means is operable after a predetermined number of half cycles of said primary power source to increase the amplitude of said converted voltage for at least a portion of the next half cycle.

1 2. A power supply according to claim 11 wherein said command means further comprises: means for counting the passage of said predetermined number of half cycles and thereafter increasing said control signal by a predetermined amount during the following half cycle.

1 3. A power supply according to claim 12 wherein said predetermined amount corresponds to increasing said control signal by a predetermined factor.

14. A power supply according to claim 8 wherein the predetermined low frequency at which the amplitude of said converted voltage is periodically increased is sized to increase the extent of precipitation.

1 5. A method for powering an electrostatic precipitator from a primary power source and affecting precipitator efficiency, comprising the steps of: converting power from said primary power source into a converted voltage having a different frequency content; stepping up said converted voltage to a high voltage; and applying said high voltage to said precipitator and allowing said different frequency content to affect precipitation efficiency.

16. A method according to claim 15 further comprising the step of: adjusting said different frequency content in a direction to increase the efficiency of said precipitator.

1 7. A method according to claim 1 6 further comprising the step of; rectifying said primary power source into a unipolar voltage; and chopping said unipolar voltage to produce said different frequency content.

1 8. A method according to claim 1 7 further comprising the step of: transforming the chopped voltage into a high voltage; and rectifying said high voltage into a high unipolar potential.

1 9. A method according to claim 1 5 wherein the step of converting power comprises the step of: producing said converted voltage under the influence of said different frequency content, said content having at least one frequency component at a predetermined low frequency sized to allow efficient precipitation.

20. A method according to claim 1 9 wherein said primary power source is alternating and wherein said converting results in a converted voltage having a lower frequency content.

21. A method according to claim 19 wherein said converting includes the step of: periodically increasing the amplitude of said converted voltage at said predetermined low frequency.

22. A method according to claim 21 further comprising the step of: adjusting said converted voltage in response to a functional parameter related to the operation of said precipitator.

23. A method according to claim 20 further comprising the step of: counting the passage of a predetermined number of half cycles of said primary power source; and increasing the amplitude of said converted voltage by a predetermined amount for at least a portion of the next half cycle.

24. A method according to claim 23 wherein said increasing by said predetermined amount corresponds to an increase by a predetermined factor.

25. A method according to claim 21 wherein the predetermined low frequency at which the amplitude of said converted voltage is periodically increased is sized to increase the extent of precipitation.

26. A power supply for an electrostatic precipitator substantially as described herein with reference to the accompanying drawings.

27. A method of powering an electrostatic precipitator suhstantially as described herein with reference to the accompanying drawings.

28. An electrostatic precipitator substantially as described herein with reference to the accompanying drawings.