U.S. patent number 3,873,282 [Application Number 05/414,413] was granted by the patent office on 1975-03-25 for automatic voltage control for an electronic precipitator.
This patent grant is currently assigned to General Electric Company. Invention is credited to David C. Finch.
United States Patent |
3,873,282 |
Finch |
March 25, 1975 |
Automatic voltage control for an electronic precipitator
Abstract
In a system for automatically controlling the voltage applied to
an electronic precipitator there is included a lockout circuit for
inhibiting gate pulses to controllable switch means during sparking
states of the precipitator, and a delay circuit for blocking gating
pulses to the controllable switch for a period of time after the
precipitator has sparked. The system further includes a current
limiting reactor for use in limiting current flow through the
controllable switch which reactor comprises a plurality of
series-connected solenoids each having single-layer windings
including spaced-apart turns and a combination air-iron core
magnetic path where the air section of the magnetic path is greater
than that of the iron core section.
Inventors: |
Finch; David C. (Roanoke,
VA) |
Assignee: |
General Electric Company
(Salem, VA)
|
Family
ID: |
26957613 |
Appl.
No.: |
05/414,413 |
Filed: |
November 9, 1973 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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275820 |
Jul 27, 1972 |
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Current U.S.
Class: |
96/22; 336/184;
96/21; 96/82; 323/903 |
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 (); G05f 001/64 () |
Field of
Search: |
;55/105,139 ;307/252T
;321/18,25 ;323/9,17,20,22SC,24,34 ;336/184 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Pellinen; A. D.
Attorney, Agent or Firm: Beusse; James H. Green, Jr.; Harold
H.
Parent Case Text
This is a (X) continuation, of application Ser. No. 275,820, filed
July 27, 1972, now abandoned.
Claims
I claim:
1. A system for automatically controlling the voltage applied to an
electronic precipitator having alternating conducting and
nonconducting states, said system comprising:
a. controllable switch means;
b. means for coupling an AC voltage to said controllable switch
means;
c. a control circuit including gating means responsive to the
nonconducting state of said electronic precipitator to selectively
provide a gating signal to said controllable switch means during
said nonconducting state of said electronic precipitator;
d. said controllable switch means being responsive to said gating
signal to control the application of said AC voltage to said
electronic precipitator; and
e. said control circuit further including means responsive to the
conducting state of said electronic precipitator for varying the
relationship between said AC voltage and said gating signal as a
function of the time duration and intensity of said conducting
state.
2. The system for automatically controlling a voltage applied to an
electronic precipitator as recited in claim 1, wherein said gating
means includes means for generating a control signal responsive to
said conducting and nonconducting states of said electronic
precipitator, said control signal increasing in a first direction
when said precipitator is in a nonconducting state, and said
control signal increasing in an opposite direction from said first
direction when said precipitator is in a conducting state; said
gating means being responsive to said control signal increasing in
said first direction to provide said gating signal at a
progressively decreasing phase retard angle with respect to said AC
voltage.
3. A system as recited in claim 1 and including a current limiting
reactor coupled in circuit with said controllable switch means
including an inductive element having a single-layer winding with a
plurality of electrical turns spaced apart from each other; said
inductive element having a magnetic path which includes an air
section and an iron section, the length of said air section being
greater than the length of said iron section.
4. A system for automatically controlling the voltage applied to an
electronic precipitator having alternating conducting and
nonconducting states, said system comprising:
a. controllable switch means;
b. means for coupling an AC voltage to said controllable switch
means;
c. a control circuit including gating means responsive to the
nonconducting state of said electronic precipitator to provide a
gating signal having a progressively changing phase relationship to
said AC voltage to said controllable switch means during said
nonconducting state of said electronic precipitator;
d. said controllable switch means being responsive to said gating
signal to control the application of said voltage to said
electronic precipitator; and
e. said control circuit further including means responsive to the
conducting state of said electronic precipitator to reinstitute
said gating signal upon termination of said conducting state, said
reinstituted gating signal having a phase relationship to the AC
voltage which differs from said relationship of the previous gating
signal as a function of the duration of said conducting state.
5. A system as recited in claim 4 and including a current limiting
reactor coupled in circuit with said controllable switch means and
including an inductive element having a single-layer winding with a
plurality of electrical turns spaced apart from each other.
6. A system for automatically controlling the voltage applied to an
electronic precipitator having alternating conducting and
nonconducting states, said system comprising:
a. controllable switch means;
b. means for coupling an AC voltage to said controllable switch
means;
c. a control circuit including gating means responsive to the
nonconducting state of said electronic precipitator to provide a
gating signal having a progressively changing phase relationship to
said AC voltage to said controllable switch means during said
nonconducting state of said electronic precipitator to provide a
gating signal having a progressively changing phase relationship to
said AC voltage to said controllable switch means during said
monconducting state of said electronic precipitator;
d. said controllable switch means responsive to said gating means
to control the application of said voltage to said electronic
precipitator;
e. said control circuit further including means responsive to the
conducting state of said electronic precipitator for varying the
relationship between said AC voltage and said gating signal as a
function of the time duration and intensity of said conducting
state; and
f. lockout means responsive to said conducting state of said
electronic precipitator to inhibit said gating signal to said
controllable switch means during said conducting state.
7. the system for automatically controlling a voltage applied to an
electronic precipitator as recited in claim 3 wherein said lockout
means includes switch means.
8. A system as recited in claim 6 and including a current limiting
reactor coupled in circuit with said controllable switch means and
having a magnetic path which includes an air section and an iron
section, the length of said air section being greater than the
length of said iron section.
9. A system for automatically controlling the voltage applied to an
electronic precipitator having alternating conducting and
nonconducting states, said system comprising:
a. controllable switch means;
b. means for coupling an AC voltage to said controllable switch
means;
c. a control circuit including gating means responsive to the
nonconducting state of said electronic precipitator to provide a
gating signal having a progressively changing phase relationship to
said AC voltage to said controllable switch means during said
nonconducting state of said electronic precipitator;
d. said controllable switch means responsive to said gating means
to control the application of said voltage to said electronic
precipitator; and
e. said control circuit further including means responsive to the
conducting state of said electronic precipitator to reinstitute
said gating signal upon termination of said conducting state, said
reinstituted gating signal having a phase relationship to the AC
voltage which differs from said relationship of the previous gating
signal as a function of the duration of the conducting state.
10. A system for automatically controlling a voltage applied to an
electronic precipitator having alternating conducting and
nonconducting states, said system comprising:
a. controllable switch means;
b. means for applying an alternating voltage to said controllable
switch means;
c. a control circuit including gating means responsive to the
nonconducting state of said electronic precipitator for providing a
gating signal at a progressively changing phase retard angle to
said controllable switch means during said nonconducting state of
said electronic precipitator;
d. said controllable switch means responsive to said gating means
to control the application of said voltage to said electronic
precipitator; and
e. said gating means including time delay means responsive to said
conducting state of said electronic precipitator to delay said
gating signal to said controllable switch means for a variable
period of time which varies as a function of the time duration of
said conducting state following an alternation of said electronic
precipitator from a conducting state to a nonconducting state.
11. The system for automatically controlling a voltage applied to
an electronic precipitator as recited in claim 6 wherein said
control circuit includes logic means.
12. The system for automatically controlling a voltage applied to
an electronic precipitator as recited in claim 10 wherein said time
delay means include capacitive means.
13. A system for automatically controlling a voltage applied to an
electronic precipitator having alternating conducting and
nonconducting states, said system comprising:
a. controllable switch means;
b. means for applying an alternating voltage to said controllable
switch means;
c. a control circuit including gating means responsive to the
nonconducting state of said electronic precipitator for providing a
gating signal at a progressively decreasing phase retard angle to
said controllable switch means during said nonconducting state of
said electronic precipitator;
e. said controllable switch means veing responsive to said gating
means to control the application of said voltage to said electronic
precipitator;
e. lockout means responsive to said conducting state of said
electronic precipitator to inhibit said gating signal to said
controllable switch means during said conducting state; and
f. said gating means including time delay means responsive to said
conducting state of said electronic precipitator to delay said
gating signal to said controllable switch means for a period of
time which varies as a function of the duration of said conducting
state following an alternation of said electronic precipitator from
a conducting state to a nonconducting state.
14. A system for automatically controlling the voltage applied to
an electronic precipitator having alternating conducting and
nonconducting states, said system comprising:
a. controllable switch means;
b. means for applying an alternating voltage to said controllable
switch means;
c. a control circuit including gating means responsive to the
nonconducting state of said electronic precipitator to selectively
provide a gating signal to said controllable switch means during
said nonconducting state of said electronic precipitator;
d. said controllable switch means responsive to said gating means
to control the application of said voltage to said electronic
precipitator;
e. lockout means responsive to said conducting state of said
electronic precipitator to inhibit said gating signal to said
controllable switch means during said conducting state;
f. said gating means including time delay means responsive to said
conducting state of said electronic precipitator to delay said
gating signal to said controllable switch means for a variable
period of time following an alternation of said electronic
precipitator from a conducting state to a nonconducting state, said
variable period of time varying as a function of the duration of
said conducting state;
g. a current limiting reactor coupled in circuit with said
controllable switch means and including a plurality of inductive
elements, each of said inductive elements connected in electrical
series with each other; each of said inductive elements including a
single-layer winding having a plurality of electrical turns spaced
apart from each other; and each of said inductive elements having a
magnetic path, said magnetic path including an air section and an
iron section, the length of said air section being greater than the
length of said iron section.
15. A control circuit for an electronic precipitator, said control
circuit comprising:
first means for providing a progressively increasing voltage to
said precipitator;
second means for detecting a conducting state of said precipitator
and responsive thereto to provide an output signal corresponding to
the time duration of said conducting state;
third means responsive to said output signal for inhibiting said
voltage to said precipitator during said conducting state and for
reducing said voltage as a function of the time duration of said
conducting state upon the reapplication thereof.
Description
BACKGROUND OF THE INVENTION
The invention relates to electronic precipitators and more
particularly to a system for automatically controlling the voltage
applied thereto.
Electronic precipitators are well-known in the prior art, most
notably in the industrial field where such devices perform an
important function in removing much of the deliterious particulate
matter present in the gases discharged from some industrial
centers. In recent times the need for electronic precipitators has
increased considerably, largely because of the demand of an
environmental conscious society which has applied increasing
pressure to virtually all industries to clean up the discharged
gases from their plants. This demand has placed a heavy burden on
many of the industries, which in the past, have found the operating
costs of available electronic precipitators prohibitively high. The
precipitators of the prior art, although somewhat adequate,
generally demonstrated an inefficient and somewhat unreliable
operating cycle requiring more or less constant supervision.
The operating principle employed by virtually all electronic
precipitators is to charge the particulate matter suspended in the
exhausted gases by applying a voltage between a system of
discharging and collecting electrodes so as to cause an electrical
current to flow therebetween. When the electrical current begins to
flow the exhausted gases become ionized and the precipitator may be
considered to have advanced from a nonconducting state to a
conducting state. Precipitator operation in general is considered
most favorable when the current flow between the discharging and
collecting electrode systems increases at a mush faster rate than
the voltage being applied thereacross. Such a condition is said to
be a spark discharge and the precipitator is said to be sparking.
in the preferred embodiment when the precipitator is in a
conducting state it will be considered to be sparking.
Thus, as the voltage across the electrode systems is increased so
as to cause sparking, the charged particles migrating through the
exhausted gases are attracted to the collecting electrode system.
The required voltage necessary to cause the sparking is a variable
dependent upon ambient atmospheric conditions, such as humidity,
pressure, temperature, fly ash, and the like. Thus, as the gaseous
effluence passes between the charging and collecting electrodes,
the voltage level is gradually raised until sparking occurs, at
which time the voltage level is then quickly reduced to a lower
voltage level to terminate the sparking. The cleansed gas is
continuously being exhausted, and as the region between the
discharging and collecting electrodes is again filled with
uncleansed gas, the precipitator voltage once again achieves a
level high enough to cause sparking.
The practice, therefore, has been to provide an automatic voltage
control means for gradually increasing the voltage between the
discharging and collecting electrodes until sparking occurs, and
then quickly lowering that voltage to terminate sparking and
prepare for the next gradual voltage build-up.
In the early development of automatic voltage controls for
electronic precipitators, saturable reactors were used as a gating
means for regulating the gradual increase in voltage applied to the
precipitator's electrodes. In recent times, the refinement of
solid-state rectifiers as controllable gating means has provided a
more convenient gating means for accomplishing the same objectives.
these rectifiers are generally called thyristors, the most common
form of thyristor being the silicon controlled rectifier (SCR)
which term, SCR, will be used in the remainder of this
specification for sake of convenience. As is well known in the art,
SCRs are responsive to a control signal to be gated into conduction
at a certain phase angle of a cycle of line voltage. At the next
zero current crossing or end of the half cycle during which the SCR
was gated into conduction, it will cease to conduct until gated
once again. Thus, in order to provide a gradually increasing
voltage to the charging electrodes of the precipitator on each
successive half cycle of operation, means are provided for slightly
increasing the total conduction period of the SCR over the previous
hald cycle. Means are also generally provided such that when the
precipitator sparks, the control voltage providing conduction to
the SCRs is rapidly decreased, thereby reducing the voltage applied
to the precipitator and allowing it to come out of its sparking
state. When sparking does cease, the voltage is once again built up
to a level to cause precipitator sparking, and the cycle is again
repeated.
The automatic voltage control systems of the prior art, however,
have not been without problems. with respect to the precipitator
itself, reignition of the spark and continued arcing can and does
occur due to the fact that although the voltage applied to the
precipitator has been decreased, dust swirls caused by air currents
can change the ambient atmospheric conditions to allow reignition
at an even lower voltage than was required for initial ignition.
Such operation presents an extremely hazardous situation since dust
particles under these circumstances are capable of supporting an
explosion. The improved control circuit according to the invention
introduces a time delay circuit to avoid the reignition
problem.
Another source of problems relates specifically to the SCRs
themselves. Although the voltage applied to the precipitator's
discharging electrodes in response to the sparking of the
precipitator will have been lowered by a control providing gating
signals to the SCRs at a later time or a larger phase angle in a
cycle of line voltage, the gating signals triggering the SCRs will
still continue to be generated. The continuing gating signals to
the SCRs, therefore, may actually feed the spark of the
precipitator if the spark was not completely extinguished.
Prolonged sparking or reignition of the precipitator as mentioned
above would result in excessively high current transients which
could possibly cause saturation of the high voltage transformer
feeding the precipitator. The invention disclosed herein provides
for a lockout circuit which overcomes the foregoing problem by
inhibiting the gating signals to the SCRs whenever the precipitator
is sparking.
Still another problem related specifically to the use of SCRs as
voltage regulating devices exists with the protection of the SCRs
themselves. When the voltage level applied to the precipitator is
sufficient to cause sparking, and in fact sparking does occur,
current rapidly increases in both the precipitator circuit and the
circuit providing the voltage control which includes the SCRs.
Thus, if care were not taken, the sharply increasing current would
quickly destroy the SCRs forming the current controlling means. For
this reason, the prior art has included a current limiting reactor
in series with the SCRs for insuring the protection thereof. These
reactors generally included a single iron core having multi-layered
windings with turns packed closely together. At best the iron core
included a small air gap or gaps in the magnetic path for the
purpose of increasing saturation current. Predictably, these
reactors of the prior art have been less than satisfactory. One
problem encountered has been the reluctance to employ the reactors
in thermally high environments because of their inherent inability
to dissipate heat at a reasonably fast rate. A second problem
relates to the incremental inductance. Since the iron core reactors
have an inherently low incremental inductance, arcing currents
associated with these reactors have been proportionally higher. The
present invention overcomes these limitations of the prior art by
providing a new and improved system for automatically controlling
the voltage applied to an electronic precipitator, which system
includes a new and improved current limiting reactor for protecting
controllable switch means used for selectively applying voltage to
the electronic precipitator.
SUMMARY OF THE INVENTION
The present invention provides an improved means for automatically
controlling the voltage applied to an electronic precipitator. In
general, a gradually increasing voltage is applied to the
precipitator by controlling the conduction angle of a pair of
controllable switching means (SCRs) in series with a power supply
and the precipitator. When the voltage level applid to the
precipitator becomes sufficiently high, a spark discharge or
sparking will occur. At that time, a sudden rush of electrical
current will be caused to flow through the particle-laden gas
passing between the discharging and collecting electrodes of the
precipitator. Means are provided for detecting this sparking of the
precipitator, and in response thereto, lowering the voltage applied
to the electrodes thereof. By this operation the precipitator
voltage is thereby caused to closely track the sparking voltage of
the precipitator, thereby obtaining maximum efficiency.
The invention further provides for a lockout circuit responsive to
the sparking of the precipitator such that when the precipitator
sparks, gating signals to the SCRs are immediately suspended,
thereby insuring against the possibility of the gating signals
prolonging the high voltage applied to the discharging electrodes
and causing prolonged sparking and possible transformer saturation
if the precipitator does not self-extinguish in one half cycle.
Moreover, at that time when the precipitator does cease sparking,
the subject invention also provides for a time delay means which
insures monconduction of the SCRs for at least a preselected period
of time after sparking ceases. The purpose of the time delay is to
prevent reignition of the arc, thus, it is not excessively long and
thereby inefficient, but merely long enough to insure against
restriking the arc.
The subject invention also provides a new and improved current
limiting reactor in series with the SCRs and the primary winding of
the high voltage transformer feeding the precipitator. The winding
structure of the new and improved reactor allows for operation in
thermally higher environments by providing greater heat
dissipation, while the core structure provides a relatively higher
incremental inductance than the prior art reactors without
requiring an excessively large number and size of windings.
It is, therefore, an object of the present invention to provide a
new and improved control for an electronic precipitator.
Another object is to provide a new and improved control for an
electronic precipitator employing solidstate controllable switching
means.
A further object is to provide a new and improved control for an
electronic precipitator wereby a lockout circuit is included for
inhibiting gating signals from being applied to the gates of
controllable switching means during precipitator sparking.
Still another object to provide a new and improved control for an
electronic precipitator which control includes a time delay circuit
for preventing gating signals from being delivered to the gates of
controllable switching means for at least a preselected period of
time after the precipitator has ceased sparking.
A still further object is to provide a new and improved current
limiting reactor for use in an automatic voltage control circuit of
an electronic precipitator.
Yet another object is to provide a current limiting reactor for use
in an automatic voltage control system of an electronic
precipitator, which current limiting reactor is thermally superior
to conventional iron core reactors and which reactor has a higher
incremental inductance.
These and other objects of the subject invention will become
apparent from the following detailed description including the
accompanying drawings forming a part of the specification.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 of the drawing depicts a schematic for an automatic voltage
control circuit for an electronic precipitator embodying the
present invention.
FIG. 2 reveals in more detail a portion of the circuit shown in
FIG. 1.
FIG. 3 discloses a pictorial representation of a section of a
current limiting reactor of the subject invention.
FIG. 4 discloses a cross-sectional view of a portion of the current
limiting reactor of FIG. 3.
FIG. 5 discloses a cross-sectional view of that portion of the
current limiting reactor of FIG. 4.
FIG. 6 reveals a current limiting reactor of the prior art.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring first to FIG. 1 of the drawing, there is shown in
schematic form an electronic precipitator voltage control system
10, including a firing circuit 12 and a control circuit 14. The
firing circuit includes a controllable current conducting circuit
16 comprising a pair of oppositely poled, controllable
unidirectional current conducting devices, or switch means such as
SCRs 18 and 20. The controllable current conducting circuit 16 is
utilized as a switching device to control the amount of energy
supplied from an AC source 22 via lines 23 and 23a to a high
voltage transformer 24, including a primary winding 26 and a
secondary winding 28. The energy from the AC source is supplied to
the transformer primary 26 through normally closed contactors 38a
and 38b of a circuit breaker (not shown), the SCRs 18 and 20 and a
current limiting reactor 39. From the secondary winding 28, the
supplied energy is coupled through a bridge rectifier 40, including
diodes 41, 42, 43 and 44, to a precipitator 45 including a
discharging electrode system 46 and a grounded collecting electrode
system 47 such as are well-known in the art. An ammeter 48 and an
RC network, including a resistor 49 and a capacitor 50, are also
tied in series with the precipitator 45 and the rectifier 40. The
ammeter is provided for monitoring precipitator current while the
RC network functions as a means for developing a control signal
proportional to the precipitator current and delivering that
control signal to a current limit potentiometer 51. A movable arm
of potentiometer 51 is connected to a zener diode 52, and a
resistor 52a included in the control circuit 14 of the system for
the purpose of providing an upper limit on precipitator current
flow.
A second RC network, including a resistor 53 and a capacitor 54,
having properly selected parameters as is well-known in the art, is
electrically connected in parallel with the SCRs 13 and 20 of the
controllable current conducting circuit 16. This second RC network
functions as a bypass circuit for any current spikes associated
with the current line supply, which spikes could cause
unpredictable operation of the SCRs. Also, connected in series with
the controllable current conducting circuit 16 and the primary
winding 26 of the high voltage transformer 24 is the current
limiting reactor 39, functioning to limit current peaks which might
damage the SCRs during precipitaor sparking.
The control circuit 14 is inductively linked from lines 23 and 23a
to the firing circuit 12 through a current transformer 60 and a
potential transformer 61. Current transformer 60 is inductively
coupled to line 23, while a primary winding 62 of potential
transformer 61, having a secondary winding 64, is connected
electrically in parallel with the primary winding 26 of the high
voltage transformer 24, between lines 23 and 23a of the firing
circuit 12. Outputs from the current transformer 60 and the
secondary winding 64 of potential transformer 61 are tied
respectively to the inputs of a first bridge rectifier 68,
including diodes 70, 72, 74 and 76, and a second bridge rectifier
78, including diodes 80, 82, 84 and 86, each located in control
circuit 14. Rectifier 68 developes a DC voltage proportional to the
current flowing through primary winding 26 of the high voltage
transformer 24, and rectifier 78 develops a DC voltage proportional
to the voltage developed across the primary winding 26. The output
from each of the bridge rectifiers 68 and 78 is introduced into
separate input terminals 88 and 90 respectively of a differential
amplifier 92, which amplifier generates either a positive voltage
output or a negative voltage output in response to the sparking or
monsparking states, respectively, of the precipitator 45. A balance
potentiometer 93 is included between the output of bridge rectifier
78 and input terminal 90 of amplifier 92 for balance and alignment
purposes. The output from the differential amplifier 92 is
introduced to a shaper circuit 94, which raises the negative
voltage output to a level of zero volts and adjusts the positive
voltage output to a more useful higher positive level. The output
from shaper circuit 94 is then applied to the anode of a diode 96,
such that when the precipitator sparks, the positive voltage or
output signal appearing at the output of shaper circuit 94 forward
biases diode 96 causing it to conduct and pass an output signal.
The level of zero volts appearing at the output of shaper circuit
94 during periods of precipitator nonsparking will cause diode 96
to be reversed biased, and hence, become nonconductive.
During periods of pecipitator sparking, the positive output signal
taken from the cathode of diode 96 is coupled to an SCR gate pulse
control unit 98 via line 99 and serves as a lockout signal for
preventing the feeding of gating signals to SCRs 18 and 20. The
positive output signal from diode 96 is also fed through a resistor
100 and introduced as a positive flowing, first control current to
an integrating amplifier 102. Amplifier 102, which includes a pair
of feedback components, resistor 103 and capacitor 104, has its
output terminated in an automatic position 105a of an
automatic-manual selector switch 105, a select arm 105b of which is
coupled through to the input of SCR gate pulse control unit 98. A
potentiometer 106, connected between a manual position 105c of the
automaticmanual selector switch 105 and a negative DC supply source
107, serves to control the reference signal fed to SCR gate pulse
control unit 98 when operating in a manual mode. This position is
not normally used for actual circuit operation but is intended
primarily for use during checkout and repair.
A negative flowing, second control current to amplifier 102 is
generated by the voltage developed across sparking rate
potentiometer 108 and fed through resistor 110 to the input of
amplifier 102. The relative size of resistor 100 to resistor 110 is
such as to allow the positive flowing first control current from
conducting diode 96, to be approximately ten times the value of the
negative flowing second control current from sparking rate
potentiometer 108. Thus, when the positive current does flow, for
all practical purposes the negative current, though still flowing,
is insignificant with respect to circuit operation.
A source of positive DC voltage 111 is connected through a normally
open third contactor 38c of the circuit breaker (not shown),
through an RC filter network including resistors 112, 113 and a
capacitor 114, and into the input of amplifier 102 where it serves
to latch amplifier 102 in an OFF condition when contactor 38c is
closed. The OFF condition is that condition which exists when power
is removed from the primary winding 26 of the transformer 24.
Finally, a parallel connected resistor diode combination, including
resistor 120 and diode 122, is tied between the input to the
amplifier 102 and ground, and serves to limit the positive swing of
the input current from diode 96 when that diode is conducting.
Referring now to FIG. 2 of the drawing, it is seen that the
reference control signal from amplifier 102 being fed into SCR gate
pulse control unit 98 is coupled via line 123 and ground, through a
voltage-to-current converter circuit 124, and applied to an emitter
terminal 128 of a unijunction transistor 126, having a first base
terminal 130 tied to a source of positive voltage 130a. A second
base terminal 131 of the unijunction transistor is tied to ground
potential through a base resistor 132, while a timing capacitor
134, which serves as a timing means for coordinating the firing of
SCRs 18 and 20, is connected between the emitter terminal 128 of
the unijunction transistor 126 and ground.
The output from unijunction transistor 126 is taken from the second
base terminal 131 of the unijunction transistor 126 and coupled to
the set terminal of a flip-flop 142, and the input terminal of a
100 microsecond single-shot multivibrator 144. Feeding the clear
terminal of flip-flop 142 is an output from a two-input OR gate
146, one of the inputs of which is the lockout signal from the
cathode of diode 96, the other input being derived from a
zero-crossing detection circuit 148. The zero-crossing detector
circuit, which receives the line voltage as an input signal,
produces an output pulse at the end of each half cycle thereof. The
output from OR gate 146 is also coupled as a first input to an OR
CLAMP circuit 150 which includes as a second input, an output
signal taken from flip-flop 142. The output of OR CLAMP circuit 150
is, in turn, coupled back to the emitter lead 128 of the
unijunction transistor 126.
Flip-flop 142 provides a first input to an AND gate 152, while a
second input to that AND gate is provided from a 15KHz pulse
trigger oscillator 154. When oscillator 154 is gated by the
single-shot multivibrator 144, oscillator 154 is caused to remain
in a steady ON condition for 100 microseconds before the 15KHz
pulse triggers are produced. That is, the single-shot multivibrator
144 insures that the first pulse out of oscillator 154 is at least
100 microseconds long. This feature acts to guarantee conduction of
the SCRs when triggered. Thus, when AND gate 152 receives inputs
from both flip-flop 142 and pulse trigger oscillator 154, a burst
of 15KHz firing pulses including a lead pulse 100 microseconds
long, is passed through a power amplifier 156 and fed to the gate
lead of the proper SCR through the action of a steering diode
circuit 158 responsive to the line voltage. The type of steering
diode circuit used is well-known in the art, one example of which
may be found at page 197 of the "G.E. SCR Manual, 4th Edition."
Referring to FIGS. 3, 4 and 5 of the drawings, a preferred
embodiment of the current limiting reactor of the present invention
is shown. FIG. 6 discloses an example of a reactor of the prior
art. The current limiting reactor 39 of the preferred embodiment,
includes a set of eight, series-connected solenoids 160 (three are
shown), having single-layer windings with spaced-apart turns. Each
of the solenoids are supported in an upright position with the aid
of nonconductive top and bottom members 161 and 161a respectively.
As best seen in FIGS. 4 and 5 of the drawings, each of the
individual solenoids 160 include an iron core 164, conveniently
formed from silicon steel laminations, while the windings
themselves are fashioned from an aluminum conductor and generally
include an insulating material (not shown) between each of the
spaced-apart turns. Each of the solenoids also include a magnetic
path 166, which path, as is shown most clearly in FIG. 5 of the
drawings, includes both an iron core section and an air section.
From that same figure it is clearly seen that the length of the air
section is greater than the length of the iron core section.
The prior art reactor as shown in FIG. 6 does not include
spaced-turn, single-layer windings as does the subject invention,
but instead includes close-turn, multilayered windings as well as a
magnetic path having an iron core section greater in length than
the air gaps separating the split iron core.
Operation of the precipitator system will now be described.
Referring first to FIG. 1 and the firing circuit 12 of the drawing,
activating the circuit breaker (not shown) will close normally open
contacts 38a and 38b (simultaneously opening contactor 38c) and
permit line voltage to be applied to the controllable conducting
circuit 16 including SCRs 18 and 20. Since the SCRs are oppositely
poled, conduction thereof will occur on opposite half cycles of the
line voltage when gating signals are received by the appropriate
SCR from SCR gate pulse control unit 98. The gating signals, which
occur on each successive half cycle of the line voltage at an
increasingly earlier time, permit an increasingly greater period
for conduction of the SCRs, and hence, allow an increasingly larger
voltage to be applied to the primary winding 26 of the high voltage
transformer 24. The voltage developed across the secondary winding
28 of transformer 24 feeds the bridge rectifier 40, the diodes
whereof are poled to develop a large negative potential between the
discharging electrode system 46 and the collecting electrode system
47 tied to ground potential. Eventually, the conduction angle of
the SCRs during one half cycle of the line voltage will be
sufficient to cause ionization of the particle laden gas between
the electrode systems of the precipitator. At that time, the
previously nonconducting precipitator will advance to a conducting
state and the current flow between the electrode systems will
increase rapidly and appreciable. The voltage between the electrode
systems will, at the same time, fall sharply toward zero. The
voltage level at which sparking occurs is a variable depending upon
the ambient atmospheric conditions prevailing between the two
electrode systems. Under those conditions where a relatively low
level of contaminating matter is present in the discharging gas, a
relatively high precipitator voltage would be required to cause the
precipitator to spark. on the other hand, high concentrations of
particulate matter present between the discharging and collecting
electrode systems will cause sparking at a relatively low voltage
applied thereto.
When the precipitator 45 advances from a nondonducting state to a
conducting state, the secondary winding 28, and the primary winding
26, of high voltage transformer 24 are required to carry large
currents, since the current through primary winding 26 increases
rapidly from a relatively low value to a relatively high value.
Similarly, when the precipitator changes from a conducting or
sparking state to a nonconducting or nonsparking state, the voltage
across primary winding 26 and the current therethrough return once
again to their respective high and low states as when before
sparking occurred. The current and voltage changes associated with
the primary winding of the high voltage transformer, as occasioned
by the precipitator changing states, are sensed by current
transformer 60 and the primary winding of potential transformer 61,
and coupled to the control circuit 14 as inputs to the pair of
bridge rectifiers 68 and 78. Rectifier 68, which responds to
changes associated with current transformer 60 provides a first
input to differential amplifier 92 through input terminal 88
thereof, while rectifier 78, which is responsive to potential
transformer 61, provides a second input to differential amplifier
92 through input terminal 90 thereof. During periods when the
precipitator is not sparking, current transformer 60 will be
generating a relatively low voltage thereacross and, hence, the
voltage input to terminal 88 will be practically zero. At the same
time, transformer 61 will be generating a relatively high voltage
which will cause a relatively high positive potential to be
developed at input terminal 90 of the differential amplifier 92.
Similarly, during periods when the precipitator is sparking,
rectifier 68 causes a high positive potential to be developed at
terminal 88 while rectifier 78 causes a larger negative to be
developed at terminal 90. Shaper circuit 94 responds to the voltage
changes of differential amplifier 92 by developing an adjusted
positive voltage out when terminal 88 is positive with respect to
terminal 90 of the differential amplifier, and developing a zero
voltage out when terminal 90 is positive with respect to terminal
88 of the differential amplifier. Thus, shaper circuit 94 produces
a positive or zero voltage signal indicative of whether the
precipitator is in a sparking or a nonsparking state respectively.
It should be noted that the positive voltage signal out of shaper
circuit 94 will persist for the duration of the precipitator spark,
while the zero voltage signal will be present whenever the
precipitator is not sparking.
During those periods of operation when the precipitator is in a
nonsparking state, shaper circuit 94 will be developing a zero
voltage output signal so as to cause diode 96 not to conduct.
Hence, with the circuit breaker actuated and contactor 38c open,
amplifier 102 will be influenced only by the current generated by
negative DC supply source 107. In response to the negative current
from source 107, integrating amplifier 102 generates a negative
going ramp voltage having a slope dependent upon the level of
current being introduced. A higher level of current will cause the
ramp voltage generated to have a steeper slope. Further, since the
level of that current is determined by the level of voltage picked
off by the wiper arm of the spark rate potentiometer 108,
potentiometer 108 determines the slope of the negative going ramp
voltage output developed by the integrating amplifier 102.
As will be later explained in greater detail, the negative going
ramp voltage, which is introduced as a reference signal into SCR
gate pulse control unit 98 through automatic-manual selector switch
105 when in the automatic position, is used in determining the rate
or periodicity at which the precipitator sparks. A reference signal
having a steeper slope will cause the precipitator to spark more
frequently than a reference signal having a more gradual slope.
This result follows because the precipitator will spark whenever
the negative going ramp voltage reaches a definite level determined
by the ambient atmospheric conditions of the precipitator 45. And
since a ramp voltage having a steeper slope will achieve that level
at an earlier time than a ramp voltage having a more gradual slope,
by adjusting the spark rate potentiometer 108, the slope, and
hence, the rate of precipitator sparking may be regulated.
When the precipitator sparks, a positive output signal will be
developed at the output of shaper circuit 94, thereby causing diode
96 to become conductive. The positive output signal from shaper
circuit 94 is fed through resistor 100 to serve as a second input
to integrating amplifier 102. The signal developed by negative
source 107, however, which although still continuing to be applied
to the input of amplifier 102, practically speaking will have no
effect on the output voltage. This is because the relative sizes of
resistors 100 and 110 are chosen such that the current developed by
the positive signal from shaper circuit 94 is approximately ten
times the current developed by the negative signal from negative
voltage source 107. When the precipitator is sparking, therefore,
the much stronger positive signal from shaper 94 will not only
override the weaker negative signal from source 107 to cause
amplifier 102 to generate a positive going ramp voltage, but will
also cause the ramp voltage that is generated to have a much
steeper slope than the negative going ramp voltage developed during
nonsparking conditions. The positive going ramp voltage generated
by amplifier 102 during sparking of the precipitator serves to
quickly drive the reference voltage further away from from the
definite negative level required to cause precipitator sparking.
Thus, if the precipitator sparks for a relatively long period of
time, the reference voltage will be driven much more positive than
if the precipitator had sparked for only a short period of time.
This allows the precipitator a greater time for recovery after
relatively long sparks, while providing for a relatively short
recovery period following what would be considered a short sparking
time. Hence, the voltage applied to the discharge electrode system
46 of the precipitator 45 is caused to more closely track the
sparking thereof, thereby obtaining maximum efficiency.
The reference voltage introduced into the SCR gate pulse control
unit 98 is fed to voltage-to-current converter circuit 124, as
shown in FIG. 2, where the reference voltage signal is converted
into a reference current signal of a reversed polarity.
The reference current signal is applied to capacitor 134, and
during a nonsparking state of the precipitator, the reference
signal would be positive going and would charge the capacitor 134
relatively quickly each half cycle of the line voltage to the
predetermined level necessary to fire the unijunction transistor
126. When that level is reached, the capacitor discharges through
the unijunction transistor 126 causing a pulse to be generated,
which pulse triggers the circuit to cause SCRs 18 and 20 to be
gated on. The particular phase angle of the line voltage at which
the SCRs are gated on, however, is determined by the voltage level
of the reference voltage applied to capacitor 134. This follows
since the rate at which the capacitor charges to the predetermined
level required to fire the unijunction transistor 126 is determined
by the level of the voltage applied to the capacitor. And since on
each successive half cycle of line voltage the reference current
signal from voltage-to-current converter circuit 124 will be
slightly greater because of a continuously increasingly ramp
voltage, capacitor 134 will charge to the predetermined level more
quickly than on the previous half cycle. Consequently, unijunction
transistor 126 will be caused to fire at an earlier phase angle of
a half cycle of the line voltage, causing the SCRs to fire earlier
and hence, a higher voltage will be applied to the
precipitator.
Synchronization of SCR firing with the line voltage is assured
through the operation of zero-crossing detector circuit 148 which
generates a pulse to enable OR gate 146 in an ON condition at the
end of each half cycle of the line voltage. OR gate 146, when gated
ON, causes OR CLAMP circuit 150 to clamp capacitor 134 to ground,
thereby insuring that the capacitor 134 starts charging from zero
volts at the beginning of each half cycle.
When capacitor 134 does develop a charge great enough to fire the
unijunction transistor 126, conduction thereof causes a voltage
spike to be developed across base resistor 132, initializing the
100 microsecond one-shot multivibrator 144 and setting the enable
flip-flop 142. Examining first the circuitry related to the
one-shot multivibrator 144, it is seen that the output pulse from
that circuit is used to inhibit, for 100 microseconds, the 15KHz
signal being fed from oscillator 154 to the input of AND gate 152.
It should be noted, however, that the 100 microsecond inhibit pulse
does not inhibit an output from oscillator 154, but to the
contrary, it insures an output of at least 100 microseconds long.
That is, the inhibit pulse merely inhibits oscillation of the 15KHz
output which, under all other conditions, generates a continuous
output train of pulses approximately 66 microseconds long at the
15KHz frequency. And since the 15KHz frequency pulse train being
continuously generated is used to trigger the SCRs 18 and 20, AND
gate 152 is employed to limit the application of the pulses to the
SCRs by requiring an output pulse from flip-flop 142 before it is
enabled. Flip-flop 142, which had been set by the firing of
unijunction transistor 126, generates the required output pulse at
that time.
As the ramp voltage from integrating amplifier 102 gradually
increases in a negative direction, the reference current signal
from voltage-to-current converter circuit 124 is gradually
increased in a positive direction. Capacitor 134 changes
increasingly sooner to the preselected voltage level required to
fire unijunction transistor 126, thereby increasing the conduction
angle of SCRs 18 and 20. Eventually, depending upon the ambient
atmospheric conditions existing between the discharging and
collecting electrode systems 46 and 47, respectively, of the
precipitator 45, a conduction angle of the SCRs will be reached
that will cause a sufficient voltage to be applied to the
precipitator to cause it to spark. At that time when the
precipitator does spark, th voltage signals generated by the
current and potential transformers 60 and 61, are coupled to shaper
circuit 94 which developes a positive output signal causing diode
96 to become conductive. The positive output signal from diode 96
is introduced via line 99 as a second input into OR gate 146 and
serves as a lockout signal. In response to this lockout signal, OR
gate 146 enables OR CLAMP circuit 150 to clamp one side of timing
capacitor 134 to ground. The positive output signal, and hence, the
lockout signal will persist for as long as the precipitator
continues to spark, thereby preventing the capacitor from
recharging and delivering a firing pulse to the SCRs. The lockout
signal, therefore, avoids the problem of the SCRs trigger pulses
feeding the spark of the precipitator, which could result in
excessively high currents and even saturation of the high voltage
transformer if the precipitator did not self-extinguish in one half
cycle.
And while the positive output signal from shaper circuit 94 is used
as a lockout signal, the positive output signal is also applied to
the input of integrating amplifier 102 causing that amplifier to
integrate in a positive direction at an accelerated rate relative
to that rate at which it was integrating in the negative direction.
Thus, the reference current signal for charging capacitor 134 from
voltage to current converter circuit 124 is made increasingly less
positive for as long as the precipitator is sparking. When sparking
of the precipitator ceases, the positive output signal from shaper
circuit 94 ceases and hence, the lockout signal preventing
capacitor 134 from charging is removed, and once again the
capacitor charges to the new, though less positive, level as
developed by voltage to current converter circuit 124. It should be
noted, however, that at the moment sparking of the precipitator
ceases, control of the firing of the SCRs is not necessarily in
synchronization with the line voltage. That is clear since the
precipitator can stop sparking at any time during a half cycle of
line voltage, and the capacitor will start charging from zero volts
whenever sparking does cease. Thus, a delay is introduced into a
firing circuit. The delay wll be at least equal to the new phase
retard angle as determined by the new ramp voltage, but since the
charging of the capacitor is not synchronized with the line
voltage, the delay may be up to almost twice that length of time.
That is, since the precipitator could cease sparking at any time
during a half cycle of operation, it is entirely possible that the
capacitor would be clamped to ground because of the operation of
the zero-crossing circuit before it has had time to charge to the
preselected level required to fire the unijunction transistor 126.
Conceivably, a zero-crossing could occur immediately prior to the
capacitor charging to the preselected voltage level necessary to
fire the unijunction transistor 126. In that instance, the
capacitor would be discharged at the zero-crossing of the line
voltage and an even longer delay would be introduced. It should
also be noted that at the first zero-crossing the capacitor 134 and
unijunction transistor 126, which had stepped out of
synchronization with the line voltage when the precipitator
sparked, would step back into synchronization and proceed as
described above at that time.
Tlhe result of this time delay, therefore, insures that the high
voltage transformer 24 is not saturated by DC components in the SCR
controlled voltage waveform. The time delay allows for insurance of
extinction of the arc while still avoiding excessive lockout which
would promote inefficiency.
With respect to the current limiting reactor as shown in FIGS. 3, 4
and 5, it has been found that individual, single-layered windings
having spaced-apart turns provide greater heat dissipation, and
hence, allow for operation in thermally higher environments, while
the provision of a magnetic path having an air section of greater
length than an iron section provides for a much higher incremental
inductance than is found in reactors having an air section or gap
that is relatively short compared to the iron section length of the
magnetic path. The increased incremental inductance has the effect
of reducing precipitator arcing currents by an amount proportional
to that of increased incremental inductance. Hence, the greater
incremental inductance and the higher heat dissipating ability are
combined in the reactor of the present invention to provide a final
resultant of an improved, lower cost current limiting reactor.
Thus, by the above described invention a system for automatically
controlling the voltage is provided, which system includes a
lockout circuit for inhibiting the transmission of trigger pulses
to gates of control SCRs during periods of precipitator sparking,
and a time delay circuit which delays the transmission of trigger
pulses to the gates of the control SCRs for at least a
predetermined period of time after the precipitator has ceased
sparking.
The subject invention further discloses a current limiting reactor
comprising a plurality of individual, series connected solenoids,
each having single-layer windings with spaced-apart turns for use
in limiting current flow through the control SCRs during periods of
precipitator sparking.
While there is shown and described a specific embodiment of this
invention, it will be understood that the invention is not limited
to the particular construction shown and described, and it is
intended by the appended claims to cover all modifications within
the spirit and scope of this invention.
* * * * *