U.S. patent number 3,745,749 [Application Number 05/161,606] was granted by the patent office on 1973-07-17 for circuits for controlling the power supplied to an electrical precipitator.
This patent grant is currently assigned to Envirotech Corporation. Invention is credited to Peter C. Gelfand.
United States Patent |
3,745,749 |
Gelfand |
July 17, 1973 |
CIRCUITS FOR CONTROLLING THE POWER SUPPLIED TO AN ELECTRICAL
PRECIPITATOR
Abstract
A system for controlling the level of operation of a
precipitator through the use of analog computer circuits programmed
to respond to sparking and short circuits. Reference pulses
representative of instantaneous zero excursions of voltage control
a gate that receives precipitator current pulses. When the
reference pulses coincide with the current pulses, due to sparking,
output signals reduce the power level of the precipitator. A high
sensitivity spark detector circuit also reduces the power level in
response to low intensity precipitator sparks. Following reduction
of the power level, power in the precipitator is rapidly reapplied
to establish a new level of operation slightly lower than the
original level, and this level is then slowly raised until sparking
again occurs. The analog computer circuits are also programmed to
monitor and compare precipitator voltage and current to shut down
the precipitator power supply upon occurrence of a destructive
short circuit.
Inventors: |
Gelfand; Peter C. (Lebanon,
PA) |
Assignee: |
Envirotech Corporation (Menlo
Park, CA)
|
Family
ID: |
22581894 |
Appl.
No.: |
05/161,606 |
Filed: |
July 12, 1971 |
Current U.S.
Class: |
96/21; 323/903;
96/82 |
Current CPC
Class: |
B03C
3/68 (20130101); Y10S 323/903 (20130101) |
Current International
Class: |
B03C
3/66 (20060101); B03C 3/68 (20060101); B03c
003/68 () |
Field of
Search: |
;55/105 ;317/31
;321/18,19 ;323/9,20,22SC,24,34,7,82,89R |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Pellinen; A. D.
Claims
I claim:
1. In a system for controlling AC power supplied to a high voltage
transformer energizing an electrical precipitator through rectifier
means: means for generating reference pulses in response to
instantaneous zero excursions of voltage supplied to the
transformer primary winding, means for generating signals
representative of precipitator current, means coupled to said
signal generating means for suppressing the leading and trailing
edges of said current signals to thereby prevent overlap between
the edges of said signals and said reference pulses during normal
operation of said precipitator, gate means receiving said reference
pulses and said precipitator current signals, said gate means
providing output signals representative of said precipitator
current signals when said reference pulses coincide in time with
said precipitator current signals as a result of sparking or arcing
in the precipitator, and means responsive to the gate output
signals for reducing the power supplied to the precipitator.
2. A system as defined in claim 1, wherein said power reducing
means includes means for double integrating the gate output
signals.
3. A system as defined in claim 1, in which means are provided for
increasing the power supplied to the precipitator following said
power reduction.
4. A system as defined in claim 3, in which said power increasing
means restores power to the precipitator rapidly during an initial
period immediately subsequent to the sparking that resulted in the
reduction of power and then more slowly following said initial
period.
5. A system as defined in claim 4, in which said power increasing
means includes a capacitor, and means for charging said capacitor
in response to the gate output signals.
6. A system as defined in claim 5, in which said means for charging
the capacitor comprises a one shot multivibrator responsive to the
gate output signals.
7. A system as defined in claim 1, in which is provided a high
sensitivity spark detector means for generating output signals
responsive to the precipitator current signals when there is
lighter sparking in the precipitator than would cause said zero
excursions of the primary voltage, and said power reducing means is
responsive to said spark detector output signals independently of
said gate output signals, for reducing the power supplied to said
precipitator.
8. A system as defined in claim 7, in which means are provided for
increasing the power supplied to the precipitator following
operation of said power reducing means.
9. A system as defined in claim 8, in which said power increasing
means includes a capacitor, and a one shot multivibrator responsive
to the spark detector output signals for providing a pulse for
charging the capacitor.
10. A system as defined in claim 1 wherein said power reducing
means includes a saturable reactor interposed before the primary
winding of the high voltage transformer to receive the gate output
signals to reduce the power supplied to the precipitator.
11. In a system for controlling AC power supplied to a high voltage
transformer energizing an electrical precipitator through rectifier
means: thyristors for supplying power to the transformer, a trigger
unit for firing the thyristors at selected phase angles to control
the power supplied to the precipitator, means for supplying control
current to the trigger unit to advance and retard the phase angles
at which the thyristors are fired to increase and decrease the
power supplied to the precipitator means for supplying control
current to the trigger unit to advance and retard the phase angles
at which the thyristors are fired to increase and decrease the
power supplied to the precipitator, means for generating reference
pulses in response to instantaneous zero excursions of voltage
supplied to the high voltage transformer primary winding, means for
generating signals representative of precipitator current, means
coupled to said signal generating means for suppressing the leading
and trailing edges of said current signals to thereby prevent
overlap between the edges of said signals and said reference pulses
during normal operation of said precipitator, gate means receiving
said reference pulses and said precipitator current signals, said
gate means providing output signals representative of said
precipitator current signals when said reference pulses coincide
with said precipitator current signals as a result of sparking or
arcing in the precipitator, and means responsive to the gate output
signals for changing the control current, the trigger unit
responding to the changed control current for retarding the firing
phase angles of the thyristors to reduce the power supplied to the
precipitator.
12. A system as defined in claim 11, wherein said control current
changing means includes means for double integrating the gate
output signals.
13. A system as defined in claim 11, in which means are provided
for changing the control current supplied to the trigger unit to
advance the phase angles at which the thyristors are fired to
increase the power supplied to the precipitator following said
power reduction.
14. A system as defined in claim 13, in which said control current
changing means advances the phase angles rapidly during an initial
period immediately subsequent to the sparking that resulted in the
reduction of power and then more slowly following said initial
period.
15. A system as defined in claim 14, in which said control current
changing means includes a capacitor, and means for charging said
capacitor in response to the gate output signals.
16. A system as defined in claim 15, in which said means for
charging the capacitor comprises a one shot multivibrator
responsive to the gate output signals.
17. A system as defined in claim 11, in which is provided a high
sensitivity spark detector means for generating output signals
responsive to the precipitator current signals when there is
lighter sparking in the precipitator than would cause said zero
excursions of the primary voltage, and said control current
changing means responsive to said spark detector output signals
independently of gate output signals for retarding the the firing
phase angles of the thyristors to reduce the power supplied to the
precipitator.
18. A system as defined in claim 17, in which means are provided
for changing the control current supplied to the trigger unit to
advance the phase angles at which the thyristors are fired to
increase the power supplied to the precipitator following said
power reduction.
19. A system as defined in claim 18, in which said control current
changing means includes a capacitor, and a one shot multivibrator
responsive to the spark detector output signals for providing
pulses for charging the capacitor.
20. A system as defined in claim 11, which includes voltage
monitoring circuit means providing an output when the level of
voltage supplied to the primary winding of the high voltage
transformer drops below a selected value, current monitoring
circuit means providing an output when the level of precipitator
current rises above a selected value, and means responsive to the
outputs of said voltage circuit means and said current circuit
means to interrupt the power supply to the transformer upon
occurrence of a short circuit resulting in trans-former voltage
below said selected value and precipitator current above said
selected value.
21. In a system for controlling AC power supplied to a high voltage
transformer energizing an electrical precipitator through rectifier
means: voltage monitoring circuit means providing an output when
the level of voltage supplied to the primary winding of the high
voltage transformer drops below a selected value, current
monitoring circuit means providing an output when the level of
precipitator current rises above a selected value, means for
simulating a precipitator current level above said selected value
when a short circuit occurs in the precipitator at a relatively low
power level, and means responsive to the outputs of said voltage
circuit means, said current circuit means and said simulating
current means to interrupt the power supply to the transformer upon
occurrence of a short circuit resulting in transformer voltage
below said selected value and at least one of the precipitator
current and the simulated precipitator current above said selected
value.
22. A system as defined in claim 21, wherein the precipitator
current simulating means includes means for generating pulses in
response to short circuit conditions in the precipitator that cause
the precipitator current to lag the voltage across the high voltage
transformer by 90.degree., and circuit means responding to said
pulses to provide output signals simulating precipitator
current.
23. In a system for controlling AC power supplied to a high voltage
transformer energizing an electrical precipitator through rectifier
means: voltage monitoring circuit means providing an output when
the level of voltage supplied to the primary winding of the high
voltage transformer drops below a selected value, current
monitoring circuit means providing an output when the level of
precipitator current rises above a selected value, means for
simulating a precipitator current level above said selected value
when a short circuit occurs in the precipitator at a relatively low
power level, and means responsive to the outputs of said voltage
circuit means, said current circuit means and said simulating
current means to operate an alarm circuit upon occurrence of a
short circuit resulting in transformer voltage below said selected
value and at least one of the precipitator current and the
simulated precipitator current above said selected value.
24. A system as defined in claim 23, wherein the precipitator
current simulating means includes means for generating pulses in
response to short circuit conditions in the precipitator that cause
the precipitator current to lag the voltage across the high voltage
transformer by 90.degree., and circuit means responding to said
pulses to provide output signals simulating precipitator
current.
25. In a system for controlling AC power supplied to a high voltage
transformer energizing an electrical precipitator through rectifier
means: thyristors for supplying power to the transformer, a
magnetic amplifier trigger unit for firing the thyristors at
selected phase angles to control the power supplied to the
precipitator, means for supplying control current to a first
winding on the magnetic amplifier in the trigger unit to advance
and retard the phase angles at which the thyristors are fired to
increase and decrease the power supplied to the precipitator, means
for generating reference pulses in response to instantaneous zero
excursions of voltage supplied to the primary winding of the high
voltage transformer, means for generating signals representative of
precipitator current, means coupled to said signal generating means
for suppressing the leading and trailing edges of said current
signals to thereby prevent overlap between the edges of said
signals and said reference pulses during normal operation of said
precipitator, gate means receiving said reference pulses and said
precipitator current signals, said gate means providing output
signals representative of said precipitator current signals when
said reference pulses coincide with said precipitator current
signals as a result of sparking or arcing in the precipitator, and
means responsive to the gate output signals for changing the
control current, the trigger unit responding to the changed control
current for retarding the firing phase angles of the thyristors to
reduce the power supplied to the precipitator.
26. A system as defined in claim 25, in which is provided a second
control winding on the magnetic amplifier, a circuit responsive to
gate output for temporarily supplying high level currents to said
second winding on the magnetic amplifier to rapidly overdrive the
amplifier and retard the firing phase angles of the thyristors.
27. A system as defined in claim 26, wherein said control current
changing means includes means for double integrating the gate
output signals.
28. A system as defined in claim 26, in which means are provided
for changing the control current supplied to the first control
winding to advance the firing phase angles to increase the power
supplied to the precipitator following said power reduction.
29. A system as defined in claim 26, in which said control current
changing means advances the phase angles rapidly during an initial
period immediately subsequent to the sparking that resulted in the
reduction of power and then more slowly following said initial
period.
30. A system as defined in claim 29, in which said control current
changing means includes a capacitor, and means for charging said
capacitor in response to the gate output signals.
31. A system as defined in claim 30, in which said means for
charging the capacitor comprises a one shot multivibrator
responsive to the gate output signals.
32. A system as defined in claim 26, in which is provided a high
sensitivity spark detector response to the precipitator current
signals representative of sparking for generating output signals,
and said control current changing means responsive to said spark
detector output signals for retarding the firing phase angles of
the thyristors to reduce the power supplied to said
precipitator.
33. In a system for controlling AC power supplied to a high voltage
transformer energizing an electrical precipitator through rectifier
means:
A.
a. thyristors for supplying power to the transformer;
b. a trigger unit for firing the thyristors at selected phase
angles to control the power supplied to the precipitator;
c. means for supplying control current to the trigger unit to
advance and retard the phase angles at which the thyristors are
fired to increase and decrease the power supplied to the
precipitator;
d. means for generating reference pulses in response to
instantaneous zero excursions of voltage supplied to the primary
winding of the high voltage transformer;
e. means for generating signals representative of precipitator
current;
f. gate means receiving said reference pulses and said precipitator
current signals, said gate means providing output signals
representative of said precipitator current signals when said
reference pulses coincide in time with said precipitator current
signals as a result of sparking or arcing in the precipitator;
g. means responsive to the gate output signals for changing the
control current, the trigger unit responding to the changed control
current for retarding the firing phase angles of the thyristors to
reduce the power supplied to the precipitator; and
B.
a. voltage monitoring circuit means providing an output when the
level of voltage supplied to the high voltage transformer drops
below a selected value;
b. current monitoring circuit means providing an output when the
level of precipitator current rises above a selected value;
c. precipitator current simulating means for generating pulses in
response to short circuit conditions in the precipitator that cause
the precipitator current to lag the voltage across the high voltage
transformer by 90.degree.;
d. circuit means responding to said pulses to provide output
signals simulating precipitator current; and
e. means responsive to the outputs of said voltage circuit means,
said current circuit means and said current simulating means to
interrupt the power supply to the trans-former upon occurrence of a
short circuit resulting in transformer voltage below said selected
value and at least one of the precipitator current and the
simulated precipitator current above said selected value.
34. In a system for controlling AC power supplied to a high voltage
transformer energizing an electrical precipitator through rectifier
means:
A.
a. thyristors for supplying power to the transformer;
b. a magnetic amplifier trigger unit for firing the thyristors at
selected phase angles to control the power supplied to the
precipitator;
c. means for supplying control current to a first winding on the
magnetic amplifier in the trigger unit to advance and retard the
phase angles at which the thyristors are fired to increase and
decrease the power supplied to the precipitator;
d. means for generating reference pulses in response to
instantaneous zero excursions of voltage supplied to the primary
winding of the high voltage transformer;
e. means for generating signals representative of precipitator
current;
f. gate means receiving said reference pulses and said precipitator
current signals, said gate means providing output signals
representative of said precipitator current signals when said
reference pulses coincide with said precipitator current signals as
a result of sparking or arcing in the precipitator; and
g. means responsive to the gate output signals for changing the
control current, the trig-ger unit responding to the changed
control current for retarding the firing phase angles of the
thyristors to reduce the power supplied to the precipitator;
and
B.
a. voltage monitoring circuit means providing an output when the
level of voltage supplied to the high voltage transformer drops
below a selected value;
b. current monitoring circuit means providing an output when the
level of precipitator current rises above a selected value;
c. means for simulating a precipitator current level above said
selected value when a short circuit occurs in the precipitator at a
relatively low power level; and
d. means responsive to the outputs of said voltage circuit means,
said current circuit means and said simulating current means to
interrupt the power supply to the transformer upon occurrence of a
short circuit resulting in transformer voltage below said selected
value and at least one of the precipitator current and the
simulated precipitator current above said selected value.
35. A system as defined in claim 34, wherein the precipitator
current simulating means includes means for generating pulses in
response to short circuit conditions in the precipitator that cause
the precipitator current to lag the voltage across the high voltage
transformer by 90.degree., and circuit means responding to said
pulses to provide output signals simulating precipitator
current.
36. In a system for controlling AC power supplied to a high voltage
transformer energizing an electrical precipitator through rectifier
means:
A.
a. thyristors for supplying power to the transformer;
b. a magnetic amplifier trigger unit for firing the thyristors at
selected phase angles to control the power supplied to the
precipitator;
c. means for supplying control current to a first winding on the
magnetic amplifier in the trigger unit to advance and retard the
phase angles at which the thyristors are fired to increase and
decrease the power supplied to the precipitator;
d. means for generating reference pulses in response to
instantaneous zero excursions of voltage supplied to the primary
winding of the high voltage transformer;
e. means for generating signals representative of precipitator
current;
f. gate means receiving said reference pulses and said precipitator
current signals, said gate means providing output signals
representative of said precipitator current signals when said
reference pulses coincide with said precipitator current signals as
a result of sparking or arcing in the precipitator; and
g. means responsive to the gate output signals for changing the
control current, the trigger unit responding to the changed control
current for retarding the firing phase angles of the thyristors to
reduce the power supplied to the precipitator; and
B.
a. voltage monitoring circuit means providing an output when the
level of voltage supplied to the high voltage transformer drops
below a selected value;
b. current monitoring circuit means providing an output when the
level of precipitator current rises above a selected value;
c. means for simulating a precipitator current level above said
selected value when a short circuit occurs in the precipitator at a
relatively low power level; and
d. means responsive to the outputs of said voltage circuit means,
said current circuit means and said simulating current means to
operate an alarm circuit upon occurrence of a short circuit
resulting in transformer voltage below said selected value and at
least one of the precipitator current and the simulated
precipitator current above said selected value.
37. A system as defined in claim 36, wherein the precipitator
current simulating means includes:
A.
a. means for generating pulses in response to short circuit
conditions in the precipitator that cause the precipitator current
to lag the voltage across the high voltage transformer by
90.degree.; and
b. circuit means responding to said pulses to provide output
signals simulating precipitator current.
Description
BACKGROUND OF THE INVENTION
This invention relates to power control systems for electrical
precipitators and, in particular, to such control systems that
provide for operation of precipitators at maximum efficiency.
Conventional electrostatic precipitators, used to remove suspended
particles from gas streams, operate with relatively high DC
voltages applied across their collecting electrodes. Increasing the
power level at which a precipitator operates to obtain maximum
operating efficiency leads first to sparking and then to arcing
between the electrodes. Precipitator efficiency is at its maximum
during sparking and falls off sharply during arcing.
To maintain a power operating level of a precipitator to obtain
optimum sparking but not arcing, the practice has been to sense the
sparking and to integrate two times a signal representative of the
sparking to produce a control signal used to vary the power level
of the precipitator, such as shown in my U.S. Pat. No 3,173,772. It
has been found in practice that such an integrated control signal
can lead to higher precipitator operating levels and greater
precipitator efficiency.
While using control signals derived by doubly integrating the spark
pulses has afforded improved precipitator efficiency, there has
remained a need for a control system for precipitators having
greater flexibility and providing for immediate response in the
event of sparking or arcing, and shut down of the system in the
event of certain types of short circuits.
SUMMARY OF THE INVENTION
The present invention provides a system for controlling the power
level of a precipitator through use of analog computer circuits
programmed to respond to sparking and short circuits in a manner
that affords optimum efficiency of the precipitator, and yet
safeguards the precipitator from destructive arcs and short
circuits. To this end, circuits are provided that produce reference
pulses representative of instantaneous zero excursions of voltage
supplied to the primary of the precipitator's high voltage
transformer, and that provide an output for reducing the power
level of the precipitator when precipitator current sparking pulses
coincide with the reference pulses. High sensitivity spark detector
circuits are also provided to reduce the power level in response to
low intensity sparks. Power is then increased rapidly to a new
level slightly lower than the original level, and then slowly
raised until sparking again occurs.
To insure interruption of power to the precipitator upon the
occurrence of destructive short circuits, the analog computer
circuits are programmed to monitor and compare precipitator voltage
and precipitator current and to shut down the precipitator power
supply upon occurrence of a short circuit to safeguard the
precipitator. At the same time, the user of the precipitator is
advised of the short circuit.
DESCRIPTION OF THE DRAWINGS
The invention and its advantages will be better understood when the
following detailed description is read in conjunction with the
accompanying drawings, in which:
FIG. 1 is a block and schematic circuit diagram of an illustrative
precipitator circuit embodying the principals of the present
invention;
FIG. 2 illustrates a block and schematic circuit diagram of the
analog computer circuits 29, shown in FIG. 1, which are programmed
to control the power supplied to a precipitator in response to
sparking and arcing, and to remove power from the precipitator and
at the same time energize an alarm circuit in response to short
circuits;
FIG. 3 is a block diagram of the analog computer circuits shown in
the schematic diagram of FIG. 2;
FIG. 4 illustrates a power supply for the analog computer
circuits;
FIG. 5 is a schematic diagram of the SCR power controller 14 shown
in FIG. 1;
FIG. 6 is a circuit diagram showing substitution of a saturable
reactor for thyristors used in the circuit of FIG. 1; and
FIGS. 7, 8 and 9 illustrate waveforms of signals at various points
in the circuits of FIG. 2.
DESCRIPTION OF THE PREFERRED EMBODIMENT
THE PRECIPITATOR POWER CONTROL SYSTEM
Referring now to an illustrative embodiment of the invention with
particular reference to FIG. 1, a power source 10 furnishing, for
example, 440 volts at 60 hertz, supplies power to lines 11 and 12.
Relay contacts MR1 and MR2 couple the power supply to the primary
of a high voltage transformer 13 through an overload relay OLR and
line 16 connected to its upper terminal, and through an SCR power
controller 14 and an inductive reactor 15. The secondary of the
high voltage transformer is connected by lines 17a and 17b across a
bridge rectifier 18 having its output connected by conductor 19 to
a precipitator 20, and by a conductor 21 to ground through a
milliammeter 21a and a load resistor 22. A conductor 23 couples a
voltage signal representative of precipitator current from the
resistor 22.
Circuits to control the power supplied to the precipitator 20 are
powered in part by a step-down transformer 24 having its primary
coupled by conductors 25a and 25b to lines 11 and 12. The upper
terminal of the secondary winding of the transformer 24 is coupled
by a line 26 to a ground bus 27 and is also connected by a line 28
to analog computer circuits 29. The lower terminal of the
transformer's secondary is joined by a conductor 30 to a series
circuit including normally closed relay contacts 1CR1, a normally
closed stop push button switch 31, a parallel combination of a
normally open start push button switch 32 and relay contacts MR3,
and a time delay relay TDR which is connected to the ground bus 27.
Conductor 33 supplies power from the transformer 24 to the analog
computer circuits 29.
A conductor 35, coupled to the transformer 24 through line 30, is
connected to a series circuit including normally open overload
relay contacts OLR 1, a normally closed push button switch 36 and a
first control relay 1CR joined to the ground bus 27. Also coupled
between the conductor 35 and the ground bus 27 is another series
circuit including normally open control relay contacts 1CR2 and an
alarm indicator 37. A conductor 38 joins the two series circuits
and is connected to one side of relay contacts YR1, shown closed
but normally open when the control circuits are energized, that are
included in the analog computer circuits 29 as indicated by the
broken line surrounding the contacts. The other side of the
contacts YR1 is joined to a line 39 connected by contacts TDR1,
normally closed a short time after the circuits are energized, to
conductor 35.
Another series circuit between the conductor 39 and the ground bus
27 includes relay contacts TDR2, normally open a short time after
the circuits are energized, first control relay contacts 1CR3 and a
second control relay 2CR. Conductors 40 and 41 are connected to
second control relay contacts 2CR1 and conductors 41 and 42 to
second control relay contacts 2CR2, the conductors 40, 41 and 42
leading to the user's alarm, as indicated in the legend on the
drawing.
To supply information as to instantaneous zero excursions of the
primary voltage of the transformer 13, caused by sparking or arcing
in the precipitator 20, conductors 42 and 43 are coupled to the
analog computer circuits 29.
To control the SCR power controller by outputs from the analog
computer circuits, two pair of conductors 44a, 44b and 45a, 45b
lead from the analog computer circuits 29 to the power controller
14. Another conductor 46 couples power from the conductor 33 to the
power controller 14, grounded through line 46a.
In order to select manual or automatic operation of the
precipitator control circuits, a selector switch 125 that controls
circuits in the analog computer circuits 29 is operated. If manual
control is selected, a user can suitably adjust the circuits by
varying potentiometer 50. Lines 51, 52 and 53 connect the
potentiometer to the analog computer circuits.
In operation, the start push button 32 is actuated to energize the
main relay MR and the time delay relay TDR. The main relay contacts
MR1, MR2 and MR3 close immediately to couple the main AC power
supply 10 to the SCR power controller 14 and the high voltage
transformer 13. After a time delay, for example 13 seconds, the
time delay relay contacts TDR1 close and the time delay contacts
TDR2 open. The relay contacts MR3 act to hold the main relay and
the time delay relay closed.
During the 13 second time delay, the SCR power controller 14 has
been turned on by the analog computer circuits 29 to furnish power
to the high voltage transformer 13. This results in the power
supplied to the precipitator increasing to a value determined, on
the one hand, by the manual control 50, if the selector switch 125
is in manual position, and on the other hand, by the automatic
operation of circuits to be explained in detail hereinafter. Note
also that the relay contacts YR1 open during the 13 second
period.
Assuming a spark or arc in the precipitator 20, the analog computer
circuits 29 will receive signals representative of precipitator
current on the conductor 23from the resistor 22, and signals from
the conductors 42 and 43. Assuming normal sparking, the analog
computer circuits will respond to sparking signals by generating
control signals that will cause the SCR power controller 14 to
reduce temporarily the power supplied to the transformer 13, as
will be described in detail hereinafter.
If a short circuit exists in the precipitator 20, the signals on
the lines 23 and 42, 43 will satisfy certain conditions programmed
into the analog computer circuits, as explained hereafter, and the
relay contacts YR1 will close, thereby energizing the alarm
indicator 37 and the first control relay 1CR. The normally closed
1CR1 relay contacts will open to drop out the main relay MR and the
time delay relay TDR. The first control contacts 1CR2 will also
close to hold the first control relay energized until actuation of
the push button 36. Closure of the first control relay control
contacts 1CR3 will energize the second control relay to open the
second control relay contacts 2CR1 and close the second control
relay contacts 2CR2 to indicate to the user the short circuit
condition in the precipitator.
Further operation of the power control circuits for the
precipitator will be better understood after reading a description
of the analog computer circuits 29 in connection with the
drawings.
ANALOG COMPUTER CIRCUITS
In describing the analog computer circuits 29, reference will be
made to FIG. 2 which shows a schematic circuit diagram of the
analog computer circuits 29, to FIG. 3, a block diagram of the
analog computer circuits 29 which facilitates an understanding of
the computer arrangement, and to FIG. 4 which illustrates the power
supply for the analog computer circuits.
POWER SUPPLY
A power supply 200 is energized by the conductors 28 and 33. The
power supply provides +15 volts DC on conductor 201, -15 volts DC
on conductor 202, and both voltages are with respect to ground line
203. It should be understood that the several + and - voltage
points in the schematic circuit diagram of FIG. 2 are connected to
lines 201 and 202 and thus are energized upon energization of the
conductors 28 and 33, shown in FIGS. 1 and 4.
INSTANTANEOUS ZERO VOLTAGE DETECTOR
Conductors 42 and 43, which are at power line potential, lead to
step down transformer 60. Note that the precipitator voltage
V.sub.P across the lines 42 and 43 is the voltage across the
primary of the power transformer 13. The transformer 60 provides
isolation from the primary of the high voltage transformer 13.
Conductors 61 and 62 leading from the transformer 60 are
coupledthrough a capacitor 63 that suppresses harmful high
frequency component voltages. The input of a full wave bridge
rectifier 64 is joined to the conductors 61 and 62.
Coupled to the output of rectifier 64 is an instantaneous zero
voltage detector which functions to provide negative 1 millisecond
output pulse on line 65 whenever the voltage across the primary of
the transformer 13, and hence across the bridge rectifier 64, makes
a zero excursion. With respect to the precipitator voltage wave,
note that this will provide reference or marker output pulses at
the beginning, middle and end of a cycle of the voltage V.sub.P.
Reference pulses will also be generated when a sparking occurs in
the precipitator and causes zero excursions of the voltage
V.sub.P.
Resistors 65 and 66 function as a voltage divider with the resistor
65 dropping most of the voltage present at the bridge output when
the magnitude of the bridge voltage is sufficient to permit current
to flow through zener diode 67. Resistor 68 provides a discharge
path for the voltage commutating capacitor 69 and also provides
input impedance for the voltage applied to operational amplifier
70. Resistor 71 functions as the amplifier feedback resistance when
its output is negative in polarity. However, the feedback
resistance in effect equals 0 and therefore provides a gain of zero
when the amplifier's output attempts to swing positive due to the
action of diode 71a.
A one millisecond negative pulse appears at the output of the
amplifier 70 on line 65 whenever an excursion to zero occurs in the
precipitator primary voltage V.sub.P. This is accomplished by
charging the capacitor 69 whenever the primary voltage V.sub.P is
above zero. When V.sub.P drops to zero, due to normal zero crossing
at zero, one half cycle or full cycle, or due to a spark or arc
within the precipitator 20, the negatively charged plate of the
capacitor 69 is effectively connected to ground through the
parallel combination of the resistors 65 and 66. The net input
voltage is therefore positive and the amplifier output will produce
a negative pulse on the conductor 65. The voltage divider formed by
resistors 72 and 73, combined with amplifier input resistance 74,
provides a fixed negative reference voltage to eliminate the
possibility of the amplifier 70 producing false output signals.
INVERTER AND GATE
An inverter 75 functions to change the negative polarity 1
millisecond pulses provided on the conductor 65 to positive 1
millisecond pulses on an output conductor 76. A gate 77 is placed
in a pass or open condition by each pulse for 1 millisecond. Note
that when the gate 77 is in pass condition, all signals passed are
inverted in polarity.
CLAMPING AND FILTERING CIRCUIT
Precipitator current flows through conductor 21, milliammeter 21a
and the load resistor 22, and these components are shown in dashed
outline to facilitate explanation of the analog computer circuits.
The conductor 23 leads from resistor 22 to a clamping and filtering
circuit 80. The output of the clamping circuit is supplied on line
81 to the gate 77 and appears on gate output line 82 when the gate
is in condition to pass pulses; i.e., when it is opened by a 1
millisecond positive pulse from the inverter 75.
To illustrate the function of the clamping and filtering circuits
80, reference is made to FIG. 7 which illustrates signals at two
points in the circuit. Waveform A shows a signal representative of
precipitator current on the conductor 23 at the input to the
clamping and filtering circuit 80. Waveform B shows the output of
the clamping and filtering circuit 80 on the line 81. Note that
this is the upper portion of waveform A and that the leading and
trailing edges of the pulse B are well removed from zero and one
half cycle zero. This prevents false signals at the output of the
gate by insuring that the signal representative of precipitator
current does not overlap the reference pulses from the inverter
during normal non-sparking operation of the circuit.
HIGH SPEED OVERDRIVE
The output of the gate 77 is coupled by the line 82 to a high speed
overdrive circuit 83 joined by conductors 45a and 45b to an SCR
trigger unit winding 84 located in the SCR power controller 14. The
high speed overdrive circuit functions to switch an instantaneous
high level control current into the SCR trigger unit when a gate
output pulse appears on the line 82. Thus a transistor switch can
be used to cut in a strong current to the winding 84. If the gate
output at the input to the high speed overdrive results from strong
sparking that causes ringing, the transistor switch will commutate
current into the winding 84 to cause full phasing back of the SCR's
by the trigger unit in one half cycle. If the sparking is
relatively light, the current supplied to the winding 84 by the
high speed overdrive in response to gate output will only partially
phase back the SCR's.
The high speed overdrive 83 is required to compensate for the time
lag inherent in magnetic amplifiers used to trigger the SCR's. The
use of a forcing heavy current on the magnetic amplifier winding 84
overcomes such lag. If a trigger circuit is used that responds
instantaneously to control currents, for example a transistorized
SCR trigger unit, the high speed overdrive 83 may be omitted.
IMMEDIATE RESPONSE NETWORK
Gate output pulses on the line 82 are also coupled by lines 86 and
87 to an immediate response network formed by circuits providing
double integration of the gate output pulses. As explained in my
U.S. Pat. No. 3,173,772 for "Apparatus for Controlling an
Electrical Precipitator," integrating a sparking signal twice
provides a signal representative of the energy of the sparking
pulses, their rate of rise, and their magnitudes. This feature
enables derivation of a control signal that facilitates operation
of precipitators at high power levels with greater efficiency.
The immediate response network integrates a negative polarity
output signal from the gate output twice and applies a signal to
the input of a driver amplifier 88. The effect on the driver
amplifier output is an instantaneous excursion to a positive
polarity and a return on a ramp of 100 milliseconds to a negative
polarity equal, if the effect of other inputs to be described
hereinafter are neglected, to the level prior to the negative gate
signal.
Examining the immediate response network in greater detail,
resistor 89 is a load resistor providing a portion of the discharge
path for coupling capacitor 90. Resistor 91 limits pulse current
and works with diode 92 to complete the discharge path for
capacitor 90. The resistor 91 and capacitor 93 functions to provide
the first integration of the gate output pulse. Diode 94 and
capacitor 95 function to perform the second integration. Diodes 96
and 97 are steering diodes and resistor 98 is the output resistance
for the double integration circuit.
To illustrate the functioning of the immediate response network,
reference is made to FIG. 8 which shows curve C, a waveform of a
gate output pulse on line 87 applied to the double integration
network. Waveform D indicates the signal supplied to the input of
the driver amplifier 88 in response to the input signal generated
by the double integration circuit from the pulse C. Note the
immediate negative excursion and then the gradual ramp rise. This
action provides a momentary and immediate reduction of the
precipitator output level by reducing the current supplied to
trigger unit control winding 99 located in the SCR power controller
14. Power is then reapplied to the precipitator on a rapid ramp as
shown in FIG. 8, and this action will be described in greater
detail below.
LONGTIME ONE SHOT MULTIVIBRATOR AND AUTOMATIC POWER SET POINT
SIGNAL
In operation of a precipitator, optimum efficiency is obtained by
applying a voltage across the precipitator plates just below the
point at which sparking takes place; i.e., the threshold voltage.
Thus it is desirable after sparking to adjust the precipitator
operating voltage to a level slightly lower than that voltage which
initiated the sparking or arcing. In the inventive system such
threshold voltage or set point can be achieved automatically
through functioning of a longtime monostable or one shot
multivibrator 110.
The gate output pulses on the line 86 are supplied to the input of
the one shot multivibrator 110 which, when actuated, generates a
170 millisecond negative polarity pulse (see FIG. 8) on output line
111. A line 112, resistor 113 and diode 114 function to couple the
negative polarity output pulse from the one shot multivibrator 110
to storage capacitor 115. Zener diode 116 sets an upper limit to
the voltage which may appear across the capacitor 115 and also
provides, with resistor 117, some discharge impedance for the
capacitor 115. The voltage across the capacitor 115 appears across
resistor 118 and potentiometer 119. The voltage from potentiometer
tap 120 is coupled by conductor 121 to a control signal mixer
122a.
RAMP START OPERATION
To initiate operation of the analog computer circuits, a capacitor
122 is charged through diode 123 to provide a ramp voltage coupled
by conductor 124, switch 125, when in automatic position, conductor
126, potentiometer 127, which facilitates adjustment of the maximum
set point level, and conductor 128 to the control signal mixer
122a. Zener diode 129 limits the amount of voltage that can appear
across potentiometer 127 to prevent excessive signals from being
supplied to the control signal mixer 122a.
MANUAL POWER SET POINT SIGNAL
If it is desired to set the threshold or set point manually, the
switch 125 is operated to manual position. This has the effect of
connecting the conductor 126 through the conductor 52 to the
potentiometer 50 shown in FIG. 1. The conductor 51 couples the
potentiometer 50 through a resistor 130 to -15 volts DC. The set
point may then be manually adjusted by operation of the
potentiometer 50.
Note that when the switch 125 is moved to manual position, the
capacitor 115 is grounded through resistor 131 and the ramp start
capacitor 122 is grounded through resistor 132.
CONTROL SIGNAL MIXER
A line 140 couples the precipitator current signal to the control
signal mixer 122a which functions to add algebraically the three
inputs and provide a resultant output signal on the line 141
coupled to the input of the driver amplifier 88. The precipitator
current signal acts as a feedback signal and, together with the
signal from the capacitor 115, tends to reduce the output on line
141 that would otherwise be caused by the signal from capacitor 122
or the potentiometer 50. In other words, the control milliamperes
produced by the driver amplifier 88 are reduced by turn off
polarity signals from the capacitor 115 and the conductor 140 which
oppose the effect of signals provided by the ramp start capacitor
122 or by the manual potentiometer 50.
HIGH SENSITIVITY SPARK DETECTOR
With only slight sparking in the precipitator 20, zero excursions
of the voltate V.sub.P may not be sufficient to produce pulses that
open the gate 77. Yet it is important to detect such light sparking
and lower the voltage on the precipitator plates by decreasing the
power supplied to the precipitator. To accomplish this function, a
high sensitivity spark detector includes a differentiator 145 that
receives signals representative of precipitator current from the
conductor 23 on a line 146. When the current pulses flowing through
the precipitator are stable, such as shown during time period 1,
shown in composite waveform R in FIG. 8, the differentiator 145
does not provide a pulse output on line 147. However, a spark in
the precipitator, which by its nature causes a rapid rise current
pulse, produces an output pulse from the differentiator 145 which
actuates a one shot multivibrator 148. The resulting negative
output pulse, 16 milliseconds long, on the output line 149 is
coupled by a diode 150 and resistor 151 to the input of the driver
amplifier 88, thereby reducing the negative current supplied to the
control winding 99 for one cycle. This phases back the SCR's to
reduce the power supplied to the precipitator.
The negative output pulse on the line 149 is also supplied via
conductor 152 to the longtime one shot multivibrator 110. The
resulting 170 millisecond negative pulse is coupled by the
conductor 112 to the capacitor 115 which has the effect of lowering
the threshold or set point level of the precipitator 20 as
heretofore described.
SHORT CIRCUIT ALARM
A short circuit alarm system functions to remove power from the
precipitator 20 during a short circuit condition between the high
voltage elements within the precipitator and ground. A short
circuit condition is detected by monitoring and evaluating the
magnitude of the primary voltage V.sub.P of the high voltage
transformer and the magnitude of the precipitator current. A short
circuit condition programmed into these analog circuits means that
the primary voltage V.sub.P will be low, for example two to forty
volts RMS, and the precipitator current will be greater than or
equal to 5 percent of the maximum precipitator current unless the
one shot multivibrator 110 has been actuated. The functioning of
the circuit will be more readily understood in connection with the
following detailed description.
A transformer 155 energized by conductors 42 and 43 steps down the
high voltage and isolates the alarm circuits from the power
circuit. A bridge rectifier 156 is connected across the secondary
of the transformer 155 and a capacitor 157 functions to bypass high
frequencies and protect the bridge rectifier. Resistor 158
functions to limit current while capacitor 159 acts as a filter.
The resistors 160 and 161 serve as a bleeder resistance and voltage
divider. The maximum voltage which may exist across the resistor
161 is limited to 10 volts by zener diode 162. Resistor 163 is an
input resistance for an amplifier 164. A voltage divider formed by
resistor 165, connected to a +15 volts, and potentiometer 166
together with input resistor 163 form an offset voltage source. A
feedback capacitor 167 provides infinite gain characteristics for
the amplifier 164 together with smooth transition between one
saturated state and the other state. Load resistance is provided by
resistor 168.
A current monitoring circuit is similar to the just described
voltage monitoring circuit. An input resistor 170, connected to the
resistor 22 by conductors 171 and 140, is subjected to a voltage
signal representative to the magnitude of precipitator current. A
resistor 172, connected to -15 volts, potentiometer 173 and
resistor 174 function to provide an offset voltage source. The
feedback capacitor 175 provides infinite gain characteristics to
amplifier 176 with smooth transition between one saturated state
and the other. Resistor 177 acts as a load resistance for the
amplifier 176.
Resistors 178, 179 and a diode 180 transfer a portion of the 170
millisecond negative pulse on the output line 111 of the longtime
one shot multivibrator 110 to a differentiating network formed by
capacitor 181 and resistor 182. Supplying this signal to the
amplifier 176 simulates a precipitator current greater than 5
percent of maximum precipitator current. This simulation occurs
when a 170 millisecond multivibrator pulse is produced due to a
precipitator short circuit at very low values of precipitator
current. Resistor 183 functions as an input resistance for the
amplifier 176, for simulation of precipitator currents greater than
5 percent.
The voltage monitoring circuit and current monitoring circuit
together perform a voltage and current comparison function. The
outputs of the amplifier 176 and 164 respectively feed into diodes
184 and 185 and deenergize relay YR when both inputs to the diodes
are negative in polarity. Diode 186 acts as a free wheeling diode
for the relay YR when the signal commutates between the amplifier
164 and the amplifier 176.
The relay YR is normally energized when the precipitator is
operating to open relay contacts YR1 (see also FIG. 1). Upon
deenergization of the relay YR, relay contacts YR1 close to permit
current flow between lines 38 and 39 which, as will be fully
understood from FIG. 1, results in energization of the first
control relay 1CR and the main relay MR, thus dropping out relay
contacts MR1 and MR2 to remove power from the precipitator 20.
SCR POWER CONTROLLER
Examining in greater detail the SCR power controller 14, FIG. 5
illustrates a typical network that may be used in the precipitator
power control system disclosed herein. Connected in parallel in
line 12 are a pair of inversely related thyristors 200 and 201,
commonly referred to as silicon controlled rectifiers or SCR's.
These thyristors may be, for example, Westinghouse Type 260ED to
meet the necessary requirements of withstanding peak forward and
reverse voltages greater than 1,200 volts, average currents on the
order of 175 amperes, and holding currents on the order of 30
milliamperes. A holding current resistor 202 is connected across
the lines 12 and 16 to provide a current through the SCR's slightly
greater than their holding current.
The inductive reactor 15, provided in the line 12 between the SCR's
200 and 201 and the primary of transformer 13, functions in a
normal manner to establish a minimum circuit impedance during
sparking or short circuits in the precipitator. The inductive
reactor also provides a smoothing effect on the voltage and current
waveform applied to the primary of the high voltage transformer 13,
and, in addition, imposes a safe limitation to di/dt.
An SCR protection network 203 is connected across the thyristors
200 and 201. It includes a thyrector 204 to establish an absolute
maximum voltage which may appear across the SCR's. The thyrector
204, a nonpolarized device, functions to conduct current from the
power source 10 through the inductive reactor 15 if the voltage
across the SCR power controller becomes too great.
Also included in the protection network is resistor 205 for
charging capacitor 206 through diodes 209 and 210, and resistor 207
for charging capacitor 208 through diodes 211 and 212. Resistors
213 and 214 provide discharge paths for the capacitors 206 and 208.
Resistors 215 and 216 provide equal voltage distribution across the
diodes 209 and 210, and resistors 217 and 218 provide equal voltage
distribution across the diodes 211 and 212. High frequency bypass
capacitors 219, 220, 221 and 222 protect the diodes 209, 210, 211
and 212 from high frequency voltages. Note that the values of the
components in the SCR protection network are selected on the basis
of providing a high resonant frequency with the ranges of
inductances used and the lowest possible Q to insure that no
oscillations occur. Q's range between 5 and 10, and resonant
frequencies between 4.5 KC to 2 KC.
An SCR trigger unit 225 furnishes properly phased gating pulses to
the SCR 200 on line 226 and the SCR 201 on line 227. The SCR
trigger unit used with the inventive precipitator power system may
be of a conventional design that incorporates a magnetic amplifier
and associated circuits to supply fast rise gate pulses phased to
afford SCR conduction angles of 0.degree. to 180.degree. with
control currents from 0 to 5 milliamperes, for example, on lines
44a and 44b leading to control winding 99 (see FIG. 5). The
conductors 46 and 46a supply power to the SCR trigger unit 225.
Lines 45a and 45b from the high speed overdrive 83 supply a forcing
high level control current into the winding 84 on the magnetic
amplifier in the SCR trigger unit to turn the SCR's off
rapidly.
To provide gain control for the SCR circuits, a step down
transformer 230 directly samples the output voltage to derive gain
feedback ampere turns for the SCR trigger unit. A bridge rectifier
231 is coupled to the secondary of the transformer 230 by a current
limiting resistor 232. The transformer 230 is connected by
conductor 14a to power line conductor 16 at one side and by
conductor 14b to power line conductor 12 at the other side. A
current divider resistor 233 connected across the output of the
bridge 231 is in parallel with a potentiometer 234. Adjustment of
the potentiometer varies gain by regulating the amount of feedback
to the SCR trigger unit through conductor 235 and a resistor 236. A
minimum resistance across the gain winding is set by the value of
the resistor 236.
In providing conduction of the SCR's 200 and 201, the SCR trigger
unit 225 responds to the summation of control current (control
ampere turns), bias current provided by an internal bias source
(bias ampere turns) and gain current (gain ampere turns). The
control ampere turns are derived from a 0 to 5 milliamper DC
current supplied to the lines 44a and 44b by the driver amplifier
88 (FIG. 2). As has been pointed out, the control current is varied
to adjust the level of operation of the precipitator.
Note that gain ampere turns are a direct function of the SCR power
controller output voltage. By adjustment of the potentiometer 234
to adjust the magnitude of the substractive ampere turns, a variety
of gain curves can be obtained.
A conventional SCR trigger unit may be used in this circuit, for
example an SCR trigger unit available from Magnetic Specialties,
SCR trigger unit Part No. 1395. Another SCR trigger unit that has
been used is available from Firing Circuits Inc., Norwalk,
Connecticut, Model No. 233F372. Note that the latter SCR trigger
unit has an external bias connection which can be supplied with a
suitable DC source.
SYSTEM OPERATION
To understand more clearly the analog computer circuits and the
entire precipitator power control system, reference will be made to
FIGS. 2 and 3 and the waveforms of FIGS. 8 and 9 in describing the
operation of the system. To initiate operation of the precipitator
20, the start push button 32 is depressed and, as explained
heretofore in connection with FIG. 1, main relay MR is energized
and power is supplied to the analog computer circuits 29 and to the
SCR power controlled 14. With the switch 125 in automatic position
as shown, the power applied to the precipitator will automatically
increase due to voltage build up on the ramp start capacitor 122.
This ramp start voltage is fed into the control signal mixer 122a
where it is algebraically added to the precipitator current
feedback signal on the line 140 (the voltage across the resistor
22). The resultant signal at the output of the control signal mixer
is coupled by line 141 to the driver amplifier 88 to cause control
current to flow through winding 99 in the SCR trigger circuit.
Depending on the power level at which the precipitator 20 operates
most efficiently, which will be achieved in a manner to be
described, a control current, for example up to 5 milliamps, phases
the SCR's 200 and 201 on for a predetermined number of degrees to
correspond with the correct amount of power to be transferred from
the AC power source 10 to the precipitator 20.
In the event manual operation is desired, the switch 125 is moved
to its manual position and the potentiometer 50 gradually adjusted
to increase the voltage supplied to the control signal mixer. The
remaining circuits operate in the same fashion as described in
connection with automatic control.
During steady state operation of the precipitator power control
system as shown in time period 1 composite waveform R in FIG. 8,
the instantaneous zero voltage detector, the gate 77, the high
sensitivity spark detector, the one shot multivibrator 110 and the
short circuit alarm have no influence on the SCR power
controller.
As the power supplied to the precipitator 20 increases and the
voltage on the precipitator electrodes rises, a point is reached
when a spark occurs within the precipitator. Referring to the
waveforms shown in FIG. 9 to better understand the action of the
system upon occurrence of a spark, precipitator current pulses E,
which are represented by the voltage across the resistor 22, occur
regularly until a spark F occurs in the precipitator. The control
function of the system then causes the precipitator current to
decrease to a value indicated by the pulse G.
The primary voltage V.sub.P across the transformer 13 is shown as
waveform H. Note the regular zero crossings which, following
rectification by the bridge 64, provide zero excursions of the
pulsating voltage. Upon each zero excursion, the instantaneous zero
voltage detector generates a one millisecond reference pulse of
negative polarity which is changed to positive polarity by the
inverter 75 to provide positive pulses K. A spark in the
precipitator also results in a zero excursion J of the primary
voltage V.sub.P and the instantaneous zero voltage detector
responds by generating another pulse L. At the same time the pulses
K and L are fed to the gate 77 to place it in a pass or open
condition, stable state pulses M, derived from the voltage across
the precipitator current resistor 22 by the clamping and filtering
circuit 80, are supplied to the gate input 81. Normally the pulses
K and M are fed to the gate 77 in out-of-phase relationship and
consequently there is no gate output. Upon occurrence of a spark, a
pulse L is generated, as explained above, and a signal N
representative of precipitator current flow coincides with the
pulse L and results in the coupling of at least a portion of the
pulse N through the gate 77. The result is a gate output pulse O
having a magnitude and shape dependent on the characteristics of
the precipitator current flowing as a result of the spark or arc.
For example, a high current spark often results in a disturbance
and ringing which causes several zero excursions and greater gate
output. The high speed overdrive 83 responds to the gate output
pulse O by switching a high level turnoff current into the winding
84 in the SCR trigger circuit to phase back the SCR's and
immediately reduce current flow to the precipitator, as shown by
composite curve R in FIG. 8.
In the event of a short circuit, such as a solid bus fault, which
causes the precipitator current to lag the precipitator voltage by
90.degree., there will be a coincidence between the reference
pulses K and the precipitator current M (FIG. 9), thereby producing
gate output signals.
The sparking signal F (FIG. 9) will also be supplied to the high
sensitivity spark detector and differentiated by the differentiator
145 to supply a turn on pulse to the one shot multivibrator 148.
The resultant 16 millisecond negative pulse P (FIG. 9) is coupled
by the line 149 to the driver amplifier 88 to reduce current flow
in the control winding 99. Thus the occurrence of a spark results
in a reduction of power to the precipitator 20 even if the spark is
so light that it failed to cause a detectable zero excursion of the
primary voltage V.sub.P and an output from the gate 77.
The gate output pulse C (FIG. 8) is also supplied on the lines 86
and 87 to the immediate response network formed by the double
integrator. The parameters of the sparking pulse determine the
output D, shown in FIG. 8, of the double integrator which is
supplied to the driver amplifier. Note that this causes a rapid
reduction of current flowing in the control winding 99, thereby
reducing the precipitator current as shown in waveform R. However,
a rapid but not instantaneous reapplication of power to the
precipitator by flow of current through the winding 99 takes place
if the spark has been not serious, such as a power arc, which, as
will be explained hereinafter, shuts off the system. Thus, as part
of the reaction of the double integration network, power is rapidly
reapplied to the precipitator on a 100 millisecond ramp as best
shown in FIG. 8.
Examining the waveforms in further detail, note that the
precipitator current is quickly reduced to zero during time period
2 following a sparking pulse F which terminates time period 1 --
steady state operation. During time period 3 the double integration
circuit provides a rapid ramp reapplication of power. The maximum
power level achieved at the end of the rapid return ramp is equal
to that operating level previous to the sparking minus an
incremental reduction caused by a charge stored on the capacitor
115 by reason of the operation of the one shot multivibrator 110.
As explained heretofore, the 170 millisecond negative pulse is
coupled to the capacitor 115 through line 112.
During time period 4, as shown in FIG. 8, precipitator power is
gradually raised due to discharge of the capacitor 115. This
results in an output from the control signal mixer 122a that causes
the driver amplifier 88 gradually to increase the current to the
control winding 99 on a slow ramp. This slow rise in the power
level of the precipitator continues until a new sparking level is
reached at which time the entire process will be repeated.
As heretofore explained, in certain instances sparking is so light
in the precipitator that zero excursions in the primary voltage
V.sub.P are not sufficient to result in pulses from the
instantaneous zero voltage detector that will open the gate 77 to
provide output pulses to the immediate response network and high
speed overdrive 83. In those instances, the high sensitivity spark
detector functions to provide a 16 millisecond negative pulse
directly to the driver amplifier 88 to reduce the level of current
in the control winding 99, thereby reducing power supplied to the
precipitator. In addition, the 16 millisecond negative pulse will
fire the one shot multivibrator 110 to charge the capacitor 115 for
the purpose explained above.
When a short circuit occurs in the precipitator, it is desirable to
remove power completely from the system by energizing the main
relay MR and opening the main relay contacts MR1 and MR2 (FIG. 1).
To accomplish this function, the short circuit alarm monitors and
evaluates the magnitude of the voltage across the primary of the
high voltage transformer 13 and the precipitator current. In
summary, during a short circuit condition the primary voltage will
be quite low, for example between 2 and 40 volts RMS, and the
magnitude of the precipitator current will be greater than or equal
to 5 percent of the maximum precipitator current unless the one
shot multivibrator 110 has been operated.
Detection of a short circuit is provided by the analog computer
circuits by defining levels of voltage and current combinations
which would never represent a short circuit, programming these
values into the short circuit alarm circuits and defining all other
combinations of primary voltage V.sub.P and precipitator current as
representing a short circuit. A table of the logic function of the
circuits follows, the table showing the conditions under which the
alarm is activated.
TABLE 1
170 msec.Monostable V.sub.P I Precipitator Multivibrator pulse
Alarm < 100 < 5 % No No < 100 < 5 % Yes Yes < 100
> 5 % No Yes < 100 > 5 % Yes No > 100 < 5 % No No
> 100 < 5 % Yes No > 100 > 5 % No No > 100 > 5 %
Yes No
It will be apparent from the description of the short circuit alarm
that it functions in accordance with Table 1. Thus, when the input
to the diodes 184 and 185 from the amplifiers 176 and 164 are both
negative, the relay YR is deenergized and drops out, and the
contacts YR1 close. As explained above, this results in
energization of the main relay MR, thereby opening the main relay
contacts MR1 and MR2.
In connection with the operation of the short circuit alarm system,
it should be remembered that the 170 millisecond negative pulse
from the one shot multivibrator 110 simulates a precipitator
current greater than 5 percent of maximum in the event of a short
circuit at a very low value of precipitator current. Such a low
value would not be sufficient to operate the amplifier 176 through
line 140, 171 and resistor 170. However, the high sensitivity spark
detector may detect such short circuits, even at low precipitator
currents, to cause actuation of the longtime one shot multivibrator
110. Note also that a solid bus fault, even at very low values of
precipitator current less than 5 percent of maximum, will cause the
precipitator current to lag the precipitator voltage by 90.degree..
As will be understood from a consideration of the waveforms of FIG.
9, precipitator current lagging the primary voltage by 90.degree.,
no matter how small the current, will result in a coincidence of
reference pulses K and current pulses M, thereby resulting in gate
output which actuates the longtime multivibrator 110 to simulate
precipitator current greater than 5 percent of maximum.
While the inventive system has been described with an SCR power
controller regulating the power supplied to the precipitator 20,
the analog computer circuits and remaining circuits may also be
used with a saturable reactor power control. Referring to FIG. 6,
in which elements similar to those in FIG. 1 have been given like
reference numerals, a saturable reactor 300 is interposed in the
power line 12 between the AC power source 10 and the high voltage
transformer 13. A control winding 301 in the saturable reactor 300
is supplied with current by a DC to AC power amplifier 302. The
control leads 44a and 44b from the driver amplifier 88 in the
analog computer circuits 29 supply 0 to 5 milliamps DC current to
the power amplifier. AC power is supplied to the power amplifier on
the lines 303 and 304, respectively connected to lines 28 and
33.
In operation, the analog computer circuits function in the same
manner as described in connection with the SCR controller except
that the high speed over-drive 83 may be omitted. Note that the
short circuit alarm functions equally well with the saturable
reactor control circuit.
While the invention has been described with reference to particular
embodiments, it will be understood that modifications thereof will
be obvious to those skilled in the art. Therefore the invention is
not limited to the particular embodiments described herein but is
defined by the appended claims.
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