U.S. patent number 3,622,839 [Application Number 05/004,004] was granted by the patent office on 1971-11-23 for control system for electrostatic precipitator power supply.
This patent grant is currently assigned to Robicon Corporation. Invention is credited to Harry J. Abrams, Ronald C. Blackmond, Roland W. Roberts.
United States Patent |
3,622,839 |
Abrams , et al. |
November 23, 1971 |
CONTROL SYSTEM FOR ELECTROSTATIC PRECIPITATOR POWER SUPPLY
Abstract
A sparking rate control system for electrostatic precipitators
and the like wherein current supplied through a transformer to a
rectifier connected to the precipitator is compared with a
reference signal to produce an error signal for regulating the
current supplied to the precipitator. In the automatic sparking
rate control mode, the reference signal is integrated such that the
precipitator voltage more or less gradually increases until a spark
occurs. Circuitry is provided for sensing the spark and for
converting it into a pulse which is combined in opposite polarity
relationship with the reference signal fed to the integrator. This
reduces the output of the integrator as well as the current
supplied to the precipitator; whereupon the integrating action
again increases the current until the next successive spark when
the current is again reduced. The effect is to regulate the
precipitator current and voltage near the value which provides a
sparking rate set for optimum precipitator efficiency.
Inventors: |
Abrams; Harry J. (Pittsburgh,
PA), Blackmond; Ronald C. (Allison Park, PA), Roberts;
Roland W. (Pittsburgh, PA) |
Assignee: |
Robicon Corporation
(Pittsburgh, PA)
|
Family
ID: |
21708658 |
Appl.
No.: |
05/004,004 |
Filed: |
January 19, 1970 |
Current U.S.
Class: |
96/21; 96/82 |
Current CPC
Class: |
B03C
3/00 (20130101) |
Current International
Class: |
B03C
3/00 (20060101); B03c 003/68 (); G05f 001/40 ();
H02m 007/20 () |
Field of
Search: |
;317/3 ;55/2,105,139
;307/233 ;321/24 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Beha, Jr.; William H.
Claims
We claim as our invention:
1. In apparatus for controlling the current supplied to an
electrostatic precipitator as a function of the repetition rate of
current impulses between the electrodes of the precipitator, the
combination of means for sensing the magnitude of the current
supplied to said electrodes, means for sensing current spikes in
said supplied current which result upon the occurrence of current
impulses between said electrodes, means for converting said current
spikes to pulses, integrating operational amplifier means having
input terminal means, means for applying said pulses to the input
terminal means of said amplifier means, means for supplying a
reference signal to said input terminal means of the amplifier
means to control the rate of increase in the signal at the output
of the amplifier means, means for comparing the output of said
amplifier means with a signal having a magnitude essentially
proportional to the current supplied to said electrodes whereby an
error signal will be produced when the magnitude of the output of
said amplifier means differs from said signal proportional to
electrode current, and means responsive to said error signal for
controlling the power supplied to said electrodes, said pulses
being supplied to the input terminal means of said amplifier means
in opposed polarity relationship with said reference signal to
reduce the input to said amplifier means each time a current
impulse occurs.
2. The apparatus of claim 1 wherein said pulses are of fixed width
and amplitude.
3. The apparatus of claim 1 including means for producing a direct
current signal proportional to the RMS value of current being
supplied to said precipitator, and means for comparing said direct
current signal with the output of said amplifier means to produce
said error signal for varying the current supplied to the
precipitator.
4. The apparatus of claim 3 wherein current and power are supplied
to said precipitator through phase-controlled semiconductive
controlled rectifiers, a firing circuit for said semiconductor
controlled rectifiers, and means for applying said error signal to
said firing circuit.
5. The apparatus of claim 4 including a transformer for applying
power to said precipitator, and a rectifier interposed between said
transformer and the precipitator.
6. The apparatus of claim 3 wherein the means for producing a
direct current signal includes a current transformer coupled to
power leads supplying power to said precipitator.
Description
BACKGROUND OF THE INVENTION
As is known, an electrostatic precipitator operates by setting up a
very high direct current electrostatic field between collecting
electrodes, usually large plates, and discharge electrodes which
ordinarily comprise wires hanging between the plates. This field
charges the particles with a large negative charge, causing them to
drift to the collecting plates which are grounded with respect to
the charged particles.
Since the collection efficiency is a function mainly of the field
intensity which is proportional to the voltage applied between the
electrodes, it is desirable to maintain the applied voltage as high
as possible. On the other hand, the voltage is limited by the
phenomena of sparking and/or arcing which occurs more and more
frequently as the field strength, temperature of the gas, humidity,
and/or number of particles in the gas is increased. The composition
of the gas and the resistivity of the particles also have a major
effect on the sparking voltage. Each time a spark or arc occurs,
the voltage across the precipitator falls sharply and then, after
the spark or arc is extinguished, recovers to its original value on
a resistance-capacitance charging voltage transient. During an arc
and while the voltage is recovering, the cleaning efficiency is
reduced. As the applied voltage is increased, not only is the
sparking rate increased, but also more and more arcs occur.
It has been found that the average sparking rate is an effective
measure of desirable precipitator performance. That is, when the
spark rate is too low, the precipitator voltage is too low for good
collection efficiency; and when the spark rate is too high, it is
likely that too many arcs will occur, again causing low average
precipitator voltage and low collection efficiency. Beyond this, it
has also been found that the optimum sparking rate varies as a
function of particle and gas resistivity. For high gas resistivity,
the sparking rate should ordinarily be on the order of about 100
sparks per minute or greater; whereas for moderate gas resistivity,
the sparking rate should be about 10 to 100 sparks per minute.
Theoretically, and assuming that all system variables could be held
constant, a set voltage between the precipitator plates would
produce a given sparking rate. The fact of the matter, however, is
that gas temperature, humidity, particle number and the like will
all vary. This means that in order to obtain a given sparking rate,
the current supplied to the precipitator and, consequently, the
voltage must be constantly varied as these various factors
change.
SUMMARY OF THE INVENTION
In accordance with the invention, the sparking rate of an
electrostatic precipitator is regulated to maximize the efficiency
of the precipitator by means including circuitry for converting
current spikes indicative of the existence of sparks into pulses of
fixed amplitude and width. These pulses are summed in opposed
polarity relationship with a current reference signal and fed to
the input of an integrator. Initially, and in the absence of a
spark, the integrator slowly increases its output value to a
predetermined maximum amount. However, upon the occurrence of a
spark, the resulting pulse reduces the output of the integrator
which again increases its value slowly.
In the embodiment of the invention shown herein, the output of the
integrator is compared with a signal which is proportional to the
average RMS value of current supplied to the precipitator. When the
two differ, as upon the occurrence of a spark, an error signal is
produced which reduces the current supplied to the precipitator as
well as the voltage across the precipitator plates. The current
will again increase as the integrator increases its output value to
the aforesaid predetermined maximum amount; whereupon, when the
next spark occurs, the cycle is repeated. In this manner, the
current and voltage supplied to the precipitator are always
maintained at or slightly below the sparking value, and this
regardless of variations in gas density, particle density, and
other factors.
The above and other objects and features of the invention will
become apparent from the following detailed description taken in
connection with the accompanying drawings which form a part of this
specification, and in which:
FIG. 1 is a block schematic diagram of the precipitator control
system of the invention; and
FIG. 2 is a detailed schematic diagram of the current-to-voltage
converter, spark detector and pulse shaper, integrator and
preamplifier of FIG. 1.
With reference now to the drawings, and particularly to FIG. 1, the
system shown includes a pair of input terminals 10 and 12 to which
a single-phase power supply, typically of 480 volts and 60 hertz,
is applied. The terminals 10 and 12 are connected through
switchgear 14 and a pair of semiconductive controlled rectifiers 16
and 18 to the primary winding 20 of a high voltage transformer 22.
The semiconductive controlled rectifiers 16 and 18, in turn, are
connected to a firing circuit 24 in accordance with the usual
practice whereby the rectifiers are caused to conduct after the
lapse of a predetermined portion of each half cycle of the applied
waveform, thereby varying the current and power supplied to the
primary winding 20. That is, the current and power supplied to the
primary winding are controlled in accordance with conventional
phase commutation techniques.
The secondary winding 26 of transformer 22 is connected through
full-wave rectifier 28 to the opposite electrodes 30 and 32 of an
electrostatic precipitator, generally indicated by the reference
numeral 34. The electrode 32 normally comprises large plates; while
the electrode 30 normally comprises wires hanging between the
plates.
In the operation of the precipitator 34, the voltage applied across
the electrodes 30 and 32 will build up until there is an electrical
discharge therebetween. This discharge may be a transient voltage
breakdown of about 0.001 second, comprising a spark, or may be an
arc which may last for many cycles of the applied power frequency.
In either case, when a spark or arc occurs, the voltage across the
electrodes 30 and 32 falls sharply until the spark or arc is
extinguished, whereupon the voltage recovers to its original value
on a resistance-capacitance charging voltage transient. As was
mentioned above, each time a spark or arc occurs, and while the
voltage is recovering, the cleaning efficiency of the precipitator
34 is reduced. Thus, too high a voltage will create an excessive
spark repetition frequency and reduced efficiency; whereas too low
a voltage will result in not enough sparks and also in reduced
efficiency. There is an optimum sparking rate which can be
determined best by experiment for any gas cleaning operation; and
at this sparking rate the efficiency is the greatest. However, if
an attempt is made to adjust the current and voltage at a fixed
value, the sparking rate will not remain constant for the reason
that the density, temperature and other variables of the gas will
vary.
In accordance with the present invention, the sparking rate can be
maintained, as well as the efficiency maximized, regardless of
changes in the characteristics of the gas being cleaned by means
including a current transformer 36 encircling a conductor
connecting the semiconductive controlled rectifiers 16 and 18 to
one end of the primary winding 20. The current transformer 36, in
turn, is connected to a RMS current-to-average voltage converter 38
which converts the current signal induced in the current
transformer 36 into a proportional direct current voltage on lead
40. The circuit 38 also produces a current spike on lead 42 each
time a spark or arc occurs between the electrodes 30 and 32 of the
precipitator 34. That is, when a spark occurs, the current
momentarily increases, thereby producing the aforesaid spike. This
current spike is applied via lead 42 to a spark detector and pulse
shaper circuit 44, hereinafter described in detail, which converts
the current spike into a pulse of fixed width and amplitude. As
will be appreciated, the number of pulses appearing at the output
of circuit 44 will be dependent upon the sparking rate, the higher
the sparking rate the greater the number of pulses of fixed
amplitude and width, and vice versa.
These pulses are applied to the input of an integrator 46 which
also has applied thereto a reference signal derived from a spark
rate reference potentiometer 50 which may be typically calibrated
for a spark rate of 0 to 200 sparks per minute.
ASsuming that no sparks are occurring and that no pulses are
applied to the integrator 46 from spark detector and pulse shaper
44, the output of the integrator 46 will gradually increase when
the reference signal on lead 48 is applied thereto up to a
predetermined maximum value. Again assuming that no sparks are
occurring, this output of the integrator is compared at summing
point 52 with the signal on lead 40 proportional to RMS current. If
the two differ, an error signal is produced on lead 54 which is
applied through a preamplifier 56 to the firing circuit 24 for the
semiconductive controlled rectifiers 16 and 18. Hence, as the
output of the integrator 46 increases, so also will the current and
power supplied through the rectifiers 16 and 18.
Now, if it is assumed that a pulse is fed to the input of the
integrator 46 from circuit 44, its output will be reduced by an
amount proportional to the width and amplitude of the pulse. If the
error signal on lead 54 is zero at this time, reduction in the
output of the integrator 46 causes an unbalance at summing point
52; whereupon the power and current supplied through the rectifiers
16 and 18 is reduced. Thereafter, as the output of integrator 46
again builds up slowly, so also do the power and current supplied
to the primary winding 20 until a succeeding spark occurs,
whereupon the process is repeated. The rate of increase in current
and power supplied to the precipitator during each cycle is
proportional to the setting of the spark rate reference
potentiometer 50. Hence, the number of sparks per unit of time may
be regulated independent of the actual voltage existing between the
electrodes 30 and 32.
It will be noted that the integrator 46 is connected to the pulse
shaper 44 through normally open contacts 58 of relay 60 which may
be energized by closing switch 62. When the switch 62 is not
closed, the contacts 58 remain open. Under these circumstances, the
signal at the output of circuit 38 proportional to RMS current is
compared with a manual current adjust potentiometer 64 which
regulates the desired current and power supplied to the
precipitator 34 independent of sparking rate. In the embodiment of
the invention shown in FIG. 1, the output of the integrator 58 is
superimposed upon the signal supplied by potentiometer 64 in order
to effect spark rate control; however, if desired, the
potentiometer 64 may be disconnected from the summing point 52 when
the output of the integrator is connected thereto.
With reference now to FIG. 2, elements shown therein which
correspond to those of FIG. 1 are identified by like reference
numerals. The circuitry shown in detail includes the RMS
current-to-average voltage converter 38, the spark detector and
pulse shaper 44, the integrator 46 and the preamplifier 56.
The RMS current-to-average voltage converter is connected, as
shown, to the current transformer 36 and includes a full-wave
rectifier 66 having one output terminal connected through diode 68
and resistor 70 to ground. Its other output terminal is connected
to lead 72 on which a voltage proportional to the RMS current
appears. The junction of diode 68 and rectifier 66 is connected
through resistor 74 to lead 72; while the grounded side of resistor
70 is connected to the lead 72 through the parallel combination of
smoothing capacitor 76 and resistor 78.
With the arrangement shown, a large surge in current, resulting
from the existence of a spark, appears across resistor 74 and is
applied through diode 68 and resistor 80 to the base of
NPN-transistor 82 in the spark detector and pulse shaper 44. The
transistor 82 is connected in a flip-flop arrangement with PNP
transistor 84, the feedback from the collector of transistor 84 to
the base of transistor 82 being through resistor 86. When
transistor 82 is turned ON upon the occurrence of a positive spike
at the input to circuit 44, transistor 84 turns ON. This causes the
voltage at the collector of transistor 84 to rise and this rise in
voltage is applied through capacitor 88 and resistor 90 to the base
of a second NPN transistor 92 which turns ON. When transistor 92
turns ON, its collector voltage falls, thereby turning ON
PNP-transistor 94. When transistor 94 conducts, the voltage on its
collector rises; and this rise in voltage is coupled back through
resistor 96 and diode 98 to the emitter of unijunction transistor
100 having its base 1 connected to ground and its base 2 connected
to a source of positive potential through resistor 102.
The emitter of unijunction transistor 100 is also connected through
capacitor 104 and resistor 106 to a source of negative potential
which biases transistor 92 OFF. However, when the voltage on the
collector of transistor 94 rises in the positive direction, the
capacitor 104 will charge through resistor 106 raising the
potential on the base of transistor 92, keeping this transistor ON
until the breakdown potential of the unijunction transistor 100 is
reached, whereupon the capacitor 104 will discharge. This causes
the voltage at point 108 to fall; and this fall in voltage turns
transistors 92 and 94 OFF. Hence, when a spike is applied to
circuit 44, its output appearing at the collector of transistor 94
will rise in voltage and remain at an upper fixed value until the
capacitor 104 charges to the point where the unijunction transistor
100 conducts. In this manner, a pulse of fixed amplitude and width
is produced at the output of circuit 44; and this is applied
through resistor 110 to the input of integrator 46.
The integrator circuit 46 includes an integrating operational
amplifier 112 having a feedback path including an integrating
capacitor 114 in shunt with a diode 116. The positive input
terminal of the differential amplifier 112 is grounded as shown;
while its negative terminal is connected through resistor 118 to
the movable tap on spark rate reference potentiometer 50, also
shown in FIG. 1. Assuming that no pulses are produced at the output
of circuit 44 and that the circuit has just been turned ON, the
output of the integrator 112 will slowly build up. However, when
positive-going pulses from circuit 44 are combined with the
reference signal from potentiometer 50 at summing point 120, the
input to the operational amplifier 112 is more or less
instantaneously reduced, thereby reducing its output. The rate at
which the build up from this reduced valve occurs depends, of
course, upon the setting of potentiometer 50; while the number of
voltage excursions at the output of the operational amplifier
depends upon the number of pulses per unit of time at the output of
circuit 44 and, hence, the number of sparks occurring across the
electrodes 30 and 32 (FIG. 1).
The output of the operational amplifier 112 is applied through
resistor 122 and contacts 58 of relay 60, also shown in FIG. 1, to
the input of the preamplifier 56. Note that relay 60 is provided
with a second pair of normally closed contacts 124 such that when
the relay 60 is deenergized upon opening of switch 62 to change
from automatic spark rate control to manual control, the capacitor
114 in the feedback loop for operational amplifier 112 is
discharged.
The preamplifier 56 includes an operational amplifier 126 having
one of its inputs grounded and its other input connected through
resistor 128 to the output of the integrator 46. Also applied to
the input of operational amplifier 126 via resistor 130 and the
summing point 52 (see also FIG. 1) is the signal on lead 72 from
the RMS current-to-average voltage converter 38. As was explained
above, this signal comprises a voltage proportional to the RMS
current supplied to the precipitator 34. The end of resistor 128
opposite the summing point 52 is connected through diode 132 to the
movable tap on the manual current adjust potentiometer 64, also
shown in FIG. 1.
During manual control, the switch 62 will be open, relay 60
deenergized, and contacts 58 open such that no signal will be
applied to the input of circuit 56 from integrator 46. At this
time, the voltage established by the position of the tap on
potentiometer 64 will be compared with that on lead 72 to manually
control the output of preamplifier 56. Summing point 52 is also
connected through resistor 134 to the movable tap on potentiometer
136 which adjusts the maximum output voltage of the operational
amplifier 126. The output of the operational amplifier 126 is
connected through diode 138 back to the firing circuit 24 for the
semiconductive controlled rectifiers 16 and 18.
Assuming that it is desired to control the system under spark rate
control conditions, the switch 62 is closed, thereby removing the
shunt around capacitor 114. Consequently, the output of operational
amplifier 114 will build up to the point where a spark occurs. When
the spark does occur, the circuit 44 converts the current spike
from converter 38 into a fixed duration, fixed amplitude pulse
which is fed to the integrator 46 in opposite polarity to the spark
rate reference signal from potentiometer 50. The output of the
integrator 46 will then be reduced by a fixed amount, causing the
current supplied to the primary of transformer 22 shown in FIG. 1
to recover to a slightly lower value than existed before the spark,
then slowly increase until another spark occurs. Thus, if the
actual sparking rate is lower than the reference value, the net
effect will be a lengthening of the time between pulses and a net
increase in the current through the primary winding 20 of
transformer 22. Conversely, if the sparking rate is high, the time
between pulses will be shortened, causing a net decrease in
transformer 22 primary current. The total effect, then, is to
regulate precipitator voltage near the value which provides the
sparking rate set for optimum collecting efficiency for the
particular application in question. Note that the control
automatically seeks the optimum level of precipitator voltage, as
determined by the spark rate setting, regardless of changes in
ambient conditions, thus providing automatic regulation to near
optimum collection efficiency.
In starting up the system, the voltage limit potentiometer 136 is
set at 100 percent or at the highest anticipated normal operating
voltage level. Finally the spark rate potentiometer 50 is set to
the desired average sparking rate, assuming that switch 62 is
closed. No further adjustment of controls is necessary; and the
system will operate at an essentially constant spark rate
regardless of variation in the characteristics of the gas being
cleaned.
Although the invention has been shown in connection with a certain
specific embodiment, it will be readily apparent to those skilled
in the art that various changes in form and arrangement of parts
may be made to suit requirements without departing from the spirit
and scope of the invention.
* * * * *