U.S. patent number 3,877,896 [Application Number 05/388,084] was granted by the patent office on 1975-04-15 for solid state voltage control system for electrostatic precipitators.
This patent grant is currently assigned to Vectrol Inc.. Invention is credited to Nicholas G. Muskovac.
United States Patent |
3,877,896 |
Muskovac |
April 15, 1975 |
Solid state voltage control system for electrostatic
precipitators
Abstract
A system is provided for automatically controlling power
delivery to an electrostatic precipitator. This system includes an
SCR power controller which drives a transformer-rectifier set. The
output of the transformer-rectifier set is connected to the
precipitator. A current transformer monitors the current flow in
the transformer and when a certain current level is exceeded, a
solid state current limiter responds to limit operation of the gate
circuit until a proper level is achieved. Circuitry is also
provided for detecting the occurrence of sparking in the
precipitator and generating pulse signals as a result thereof. The
pulse signals are fed to a solid state logic circuit that quickly
shuts off the SCR gate circuit, then subsequently causes soft start
to full power.
Inventors: |
Muskovac; Nicholas G.
(Rockville, MD) |
Assignee: |
Vectrol Inc. (Rockville,
MD)
|
Family
ID: |
23532602 |
Appl.
No.: |
05/388,084 |
Filed: |
August 14, 1973 |
Current U.S.
Class: |
96/22; 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 ;321/18,19
;323/4,7,9,17,20,22SC,24,34 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Pellinen; A. D.
Attorney, Agent or Firm: Liss; Morris
Claims
What is claimed is:
1. In a voltage control system for electrostatic precipitators
comprising:
power input means for providing power to the system from power
lines;
a transformer;
a primary of the transformer connected to the input means for
developing an AC signal thereacross;
solid state gating means connected between the input means and the
transformer primary for controlling power delivery to the
primary;
first current sensing means coupled to the primary for developing a
signal corresponding to current flow through the primary;
first switching means connected at the input thereof to the output
of the sensing means for switching to a conductive state when a
preselected threshold of the sensing means signal is exceeded
indicating excessive current in the primary;
gate drive means connected at inputs thereof to the power input
means and said first switching means, the output of the drive means
connected to the gate means for triggering the gating means into
conduction when said first switching means is in a non-conductive
state;
the secondary of the transformer transforming the AC signal to a
rectifier for rectification of the AC signal;
means connecting the rectifier to the precipitators for developing
electrostatic charges thereon which attract particulate matter
passing across the precipitators; and
resistor means connected to said rectifier for developing voltage
pulses thereacross when arcing occurs between said
precipitators;
logic means connected to receive said voltage pulses from said
resistor means and for producing first and second control voltages
in response thereto; and
soft start analog circuit means including second switching means
responsive to said first and second control voltages for
momentarily disabling said gate drive means and subsequently
causing gradual increased power delivery from the power lines to
the gating means.
2. The subject matter as defined in claim 1 wherein the gating
means comprises at least one pair of reverse positioned parallel
connected SCRs, the anode and cathode of each SCR conducting line
current therethrough in response to trigger signals applied to gate
terminals of the SCRs by the drive means.
3. The subject matter set forth in claim 1 together with an
inductor connected in series with the transformer primary for
inhibiting momentary increases in primary current.
4. A voltage control system for electrostatic precipitators
comprising:
power input means for providing power to the system from power
lines;
a transformer;
the primary of the transformer connected to the input means for
developing an AC signal thereacross;
an inductor connected in series with the transformer primary for
inhibiting momentary increases in primary current;
solid state gating means connected between the input means and the
transformer primary for controlling power delivery to the primary,
the gating means including a pair of parallel connected, reverse
positioned SCRs, the anode and cathode of each SCR conducting line
current therethrough in response to trigger signals applied to the
gate terminals of the SCRs;
first current sensing means coupled to the primary for developing a
signal corresponding to current flow through the primary;
switching means including a transformer/rectifier set connected at
the input thereof to the output of said first current sensing
means, said switching means including means connected to the output
of said transformer/rectifier set for establishing a threshold
reference voltage and a first switching device responsive to the
output of said establishing means for switching to a conductive
state when a preselected threshold of the sensing means signal is
exceeded, indicating excessive current in the primary;
gate drive means connected at inputs thereof to the power input
means and the switching means, the output of the drive means
connected to the gate terminals of the SCRs for generating the
trigger signals when the switching means is in a non-conductive
state;
the secondary of the transformer transforming the AC signal to a
rectifier for rectification of the AC signal;
means connecting the rectifier to the precipitators for developing
electrostatic charges thereon which attract particulate matter
passing across the precipitators;
second current sensing means coupled to the primary for developing
a signal corresponding to current flow through the primary;
circuit means connected to the output of the second sensing means
for generating a pulse voltage signal from the second sensing means
output in response to arcing at the precipitator; means connecting
the circuit means to logic means for detecting the occurrence of
the pulse voltage signal;
and soft start means connected between the logic means and the gate
drive means, the soft start means being responsive when the logic
means detects the pulse voltage for momentarily disabling the gate
drive means and subsequently causing gradual increased power
delivery from the power lines to the gating means;
the logic means comprising first and second circuit paths;
the first path including a one-shot for generating disabling pulses
of predetermined duration in response to said pulse voltage
signal;
a second switching device connected at its input to the one-shot
output, the second switching device changing state for the duration
of each disabling pulse, and means connecting the output of the
switching means to the soft start means which momentarily disables
the gate drive means in response to the change of state of the
second switching means;
the second current path including a second one-shot having its
input connected to the input of the first one-shot;
the second one-shot generating a squarewave pulse train, the pulses
of the train being of constant width and amplitude but having a
repetition rate equal to that of the pulses from the pulse voltage
signal;
the output from the second one-shot connected to an operational
amplifier for integrating the pulse train and forming a DC level
therefrom, the DC level being an input to the soft start means for
achieving the gradual increase of power delivered from the power
lines to the gating means after the momentary disabling of the gate
drive means.
5. The subject matter as described in claim 4 wherein the soft
start means comprises:
a transformer having a primary and secondary;
means connecting the primary to the power input means for
impressing an AC signal thereacross;
means connecting the secondary to a rectifier for rectifying the AC
signal;
means connected to the output of the rectifier for filtering the
rectified signal;
a charging circuit connected to the output of the filtering means
for charging at a predetermined relatively slow rate when power is
restored after an initial power interruption;
diode means connected to the charging circuit for effecting rapid
discharge thereof in the event of power interruption; and
a buffer transistor connected at its base to the charging circuit
for disabling the gate drive means via the transistor emitter, in
response to power interruption, the gate drive means subsequently
achieving gradual increase of power delivered from the power lines
to the gating means after disabling of the gate drive means;
the collector of the transistor connected to the output of the
operational amplifier for conducting the DC level therefrom,
through the transistor to the gate drive means;
the base of the buffer transistor being connected to the switching
device of the logic means for causing momentary disabling of the
gate drive means in response to arcing at the precipitators, the
charging circuit achieving gradual increase of power delivered from
the power lines to the gating means after the momentary
disabling.
6. The subject matter defined in claim 5 wherein the current
sensing means comprises a current transformer.
7. The subject matter of claim 6 wherein the first and second
switching devices are transistors.
Description
FIELD OF THE INVENTION
The following invention relates to voltage control systems, and
more particularly to a solid state voltage control system for
powering electrostatic precipitators.
BRIEF DESCRIPTION OF THE PRIOR ART
In many industrial processes, gas or smoke is produced having
particulate matter that must be removed to achieve air pollution
abatement. Electrostatic precipitators have been widely used in
this capacity. Precipitators may be mounted in smokestacks or other
gas and smoke conduits. By way of example, one manufacturer of
these electrostatic precipitators is the Koppers Corporation. The
main scientific principle behind these precipitators is the
generation of high voltages on metallic precipitator plates.
Adjacent plates are oppositely charged, electrostatically, to
attract particulate matter from a smoke or gas stream.
Occassionally, the plates are mechanically vibrated so that the
accumulated particulate matter is shaken from the plates and
collected. A primary problem in the use of electrostatic
precipitators relates to power surges in the voltage control system
as particulate matter accumulates on the adjacent precipitator
plates resulting in arcing. As will be appreciated, the system must
respond very quickly to this arcing situation to prevent power
surges that might destroy the system. Other factors that are
important to consider relate to achieving high component
reliability and system stability over a wide range of operating
voltages.
U.S. Pat. No. 3,507,096 relates to an improvement in the prior art,
whereby this patent recognizes the desirability of utilizing SCR
components to provide controlled power to a transformer-rectifier
set that energizes the electrostatic precipitators. This patent
further employs a magnetic device in the form of a reactor to limit
current in the circuitry so that the circuitry is protected from
burn out.
Although the patented subject matter is designed to operate
satisfactorily, there are several disadvantages to the use of the
reactor. Initially, greater cost is a factor when comparing
utilization of the magnetic device as opposed to solid state
non-magnetic circuits. An additional problem is the response time
of the circuitry. Response for these devices is somewhat slower
than for solid state devices, due to the hysteresis effect.
Otherwise stated, rapid response by the reactor is slowed due to
magnetic reluctance.
BRIEF DESCRIPTION OF THE PRESENT INVENTION
The present invention is an improvement of voltage control systems
for electrostatic precipitators, as described in the previously
mentioned U.S. Pat. No. 3,507,096. The present invention does not
employ a magnetic device, in the form of a reactor to limit current
in the system. Rather, solid state switching devices are used.
These obviate the disadvantages of the prior art. As a result, a
system having maximum performance is made available.
BRIEF DESCRIPTION OF THE FIGURES
FIG. 1 is an electrical schematic diagram illustrating the total
system for voltage control of electrostatic precipitators. Several
of the control circuits are indicated in blocks.
FIG. 2 is a plot of a signal appearing at the output of the system
when a spark occurs.
FIG. 3 is a plot of the signal at the input to a
transformer-rectifier set, the plot particularly demonstrating a
soft start condition.
FIG. 4 is a block diagram of the control portion of the system.
FIG. 5 is a plot of a pulse train generated from the logic
circuitry of FIG. 4, the pulses of the train being initiated when a
spark at the precipitator is developed.
FIG. 6 is an electrical schematic diagram of a soft start circuit,
as shown in FIG. 4.
FIG. 7 is a logic diagram of a logic circuit which controls the
soft start circuitry.
FIG. 8 is an electrical schematic diagram of a current limiting
circuit which is responsive to current flow in the primary of a
transformer-rectifier set.
FIG. 9 is a plot showing the phase relationship between the output
of a gate drive and the AC line signal existing in the system.
FIG. 10 shows alternate circuitry for spark detection.
DETAILED DESCRIPTION OF THE INVENTION
Referring to the figures and more particularly FIG. 1 thereof,
reference numerals 10 and 12 denote adjacent electrostatic plates
that become electrostatically charged with opposite polarities to
attract particulate matter from smoke or gas 14 that flows between
the precipitator plates. Usually, the precipitator plates are
positioned in adjacent parallel relationship and in a typical
installation, many such pairs of parallel plates exist.
Occassionally, the plates are vibrated or otherwise mechanically
moved to shake the accumulated particulate matter therefrom. Then,
this particulate matter is collected for proper disposal. It is to
be clearly understood that the mechanical portion of the system,
namely the precipitator plate assembly and vibrating mechanisms do
not form a part of the present invention. This type of equipment is
available from several manufacturers, including the Koppers
Corporation. The present invention is more particularly directed to
the voltage control system for the electrostatic plates.
Power leads 16 and 18 provide electrical energization for the
system shown in FIG. 1. Typically, 440 volts are supplied to such a
system. A main power control unit 20 furnishes controlled power to
the primary 22 of a transformer 24. The secondary of the
transformer 26 is fed to a rectifier, such as the diode rectifier
28. The rectifier performs a full wave rectification of the
transformed line signal. The transformer 24 and the rectifier 28
are referred to as a transformer-rectifier set. The output from the
rectifier is shown at lines 30 and 52. These lines are directly
connected to the plates of the precipitator to cause electrostatic
charging of the plates, with opposite polarities.
An inductor 34 is added in series with the primary 22 of
transformer 24. This inductor adds impedance to the primary circuit
so that rapid changes in current to a value exceeding a
predetermined threshhold is restrained until the SCR gate 36 can
become operative.
A resistor 38 is connected between point 44 in the rectifier 28 and
ground. A voltage develops across the resistor in accordance with
the current flowing through the rectifier. This voltage is an
indication of electrical parameters existing at the precipitator
plates. For example, in the event particulate matter builds up
across the plates to effect a very narrow gap, sparking may occur.
A typical resulting signal across the resistor is shown and
indicated by reference numeral 48. This signal represents a voltage
pulse between points 44 and ground 42, the two points defining the
ends of resistor 38. Leads from these points are connected to logic
circuitry generally indicated by 46, which detect the occurrence of
this pulse, and upon such detection, the system is prepared to
limit the operation of SCR gate 36 until there is no further
arcing.
FIG. 2 illustrates the signal at point 30. The plot shown in FIG. 2
illustrates the occurrence of a pulse signal 48 as just described.
When the threshhold is exceeded, the logic circuitry 46 (FIG. 1)
ensures a momentary disabling of the SCR gate 36, after which time
there is a soft start condition. This means that the energization
to the SCR gate 36 builds up gradually as shown. FIGS. 2 & 3
show the time relationship in a soft start condition, between the
signals at points 30 and point 22.
FIG. 4 shows, in greater detail, the circuitry of the control unit
20 (FIG. 1). As indicated, power lines 1 and 2 drive the unit. In
FIG. 4, these lines are referred to by 16' and 18'. These lines
have respective circuit breakers 54 and 52 serially connected
therein. After the breakers are switched on, the lines are
energized and supply power to the control unit 20. The power lines
are connected to a power supply 56 that converts the 440 volts AC
to lower AC and DC levels. One of the AC tap off points from supply
56 is connected to the soft start circuit 58. The purpose of this
circuit is to provide slowly increasing power levels to the gate
drive 60 after the breakers 52 and 54 have been closed.
A conventional current transformer 64 is positioned over line
section 63, in series with the primary 22 of transformer 24.
Through inductive coupling, excessive current flow through the
primary will cause a response through the current transformer 64
along line 66. This line provides an input to the current limiter
62. The current limiter 62 responds to this excessive current
condition by bleeding current from the soft start circuit 58. As a
result, the gate drive 60 will not be energized, or will be greatly
reduced, therefore preventing, or reducing to a safe operating
level, power delivery to the SCR gate 36.
Spark rate control logic 68 is provided to detect the occurrence of
the spike type pulses 48 which occur at point 44 (FIG. 1) when
sparking occurs at the precipitator plates. The input to the logic
68 is along lead 67, the opposite end of this lead being connected
to point 44 (FIG. 1).
Referring to FIG. 5, there are illustrated a series of spark pulses
48, which provide an input along lead 67 to the logic 68. The
output of the logic is a pulse train 72, also shown in FIG. 4, each
pulse having constant width and amplitude. However, the repetition
of the pulses in the square wave train depend upon the repetition
rate of the spark pulses. An operational amplifier 74 has a first
input connected to the output of logic 68 so that the operational
amplifier receives the pulse train. A second input to the
operational amplifier is connected to a reference or spark rate
adjustment. The purpose of the operational amplifier is to provide
an integration or averaging of the pulse train pulses. This average
is a DC level 70 indicated in FIG. 5. Thus, the average voltage 70
is the signal that is present at the output 76 of the operational
amplifier 74.
FIG. 6 illustrates in schematic form, the soft start circuit
generally indicated by reference numeral 58 in FIG. 4. Input leads
78 are connected to the power supply 56 as shown in FIG. 4. A
transformer 80 transforms the input voltage to the secondary 82 of
the transformer. The resulting voltage is rectified by diode 84 and
the signal is filtered by the parallel RC combination 86, 88. The
capacitor 88 charges almost immediately after power is applied to
the soft start circuit. The resistor 90 and shunt connected
capacitor 92 provide a time constant to slowly charge the capacitor
92. This charging process occurs each time there is an interruption
of power from the power supply 56 and thus the power lines 18' and
16' in FIG. 4. Such interruptions occur when the system is first
turned on, or in the case of emergency disruptions. The base of
buffer transistor 96 is indicated by reference numeral 100 and is
connected to the junction between resistor 90 and capacitor 92.
Point 102 on the opposite end of capacitor 92 as well as point 100
are connected to the logic circuit 68, as will be explained
hereinafter. However, point 102 is connected as a first input to
the gate drive 60, while the emitter of buffer transistor 96
furnishes a second input to the gate drive 60. The collector of
transistor 96 receives the DC level output from the amplifier 74.
Typical output signals from the gate drive are shown at output
leads 138 and 140 which are connected to the SCR gate 36 (FIG. 4).
The signals on the leads 138 and 140 are trigger signals for the
SCR gate 36 and their phase relation to the AC line signal
determines the power output from the SCR gate 36. The soft start
circuit shown in FIG. 6 becomes reset when the capacitor 92
discharges rapidly through the shunt connected diode 94.
FIG. 7 illustrates the circuitry involved in the logic circuit 68.
Upon the occurrence of the spark pulse 48 at input lead 67, a
connected one shot 106 generates a squarewave pulse 112, which, by
way of example, may be one-tenth second in duration. The output
lead 69 from one shot 106 drives the base of the transistor 110.
The collector of the transistor 110 is indicated by reference
numeral 100. The emitter of the transistor is indicated by 102.
Points 100 and 102 are connected to similarly indicated points in
FIG. 6. Thus, the logic 68 is connected with the soft start circuit
to achieve a soft start power buildup in the event spark pulses
occur, indicating sparking at the precipitator plates. The one shot
106 serves to detect quench time or off time of the individual SCRs
142 and 144 in the SCR gate generally indicated by 36 in FIG. 4.
The one shot 108 has its input connected with the input of one shot
106. The one shot 108 generates a pulse train 72 for the purposes
of spark rate detection and control. The pulse train is fed to the
operational amplifier 74 which has already been explained as
achieving an integrating or averaging of the pulse train. Referring
back to transistor 110, it should be noted that signals from the
output leads 100 and 102 that are connected to similarly indicated
points in FIG. 6, turn off transistor 96 and keep the transistor
turned off for the duration of pulse 112.
FIG. 8 illustrates the circuitry utilized in the current limiter 62
(FIG. 4). As previously mentioned, a conventional current
transformer 64 is slipped over a line section 63 of the transformer
primary 22 (FIG. 1). The leads from this current transformer 64 are
connected to the primary 116 of a high step-up transformer 114. The
secondary of the transformer 118 and a parallel connected resistor
119 are connected to a diode rectifier 120 for performing full wave
rectification of the stepped-up voltage signal across the secondary
118. The rectifier 120 has its output points connected across a
current limit adjust potentiometer 122. The output from the
potentiometer 122 is connected to a resistor 134 that has a shunt
capacitor 124 connected thereto to form an RC combination for
filtering the signal derived from the full wave rectifier. A Zener
diode 126 is connected between the node of the RC combination and
the base of the transistor 128. The Zener establishes a threshhold
reference. When the signal presented to the Zener exceed the
threshhold, transistor 128 is caused to conduct at collector lead
130 and emitter lead 132. By adjusting the potentiometer 122, the
threshhold value at which the transistor 128 will conduct can be
adjusted.
In operation, referring to FIG. 4, the current limiter 62 has its
first output lead 130 connected to the lead 136 while the other
output lead 132 is connected to a common.
In operation of the current limiter 62, when the current
transformer input from 64 indicates a high current flow in the
primary 22, the output from th soft start circuit 58 is bled by the
current limiter so that the gate drive 60 does not trigger the SCR
gate 36, or the current limiter 62 retards the conduction angle and
reduces the power output. The current limiter referred to in this
invention works with proportional action, and does not necessarily
shut off the SCRs 142 and 144, all the way.
Referring to the gate drive 60, the particular components which
comprise the drive will not be discussed herein. Rather, reference
will be made to a patent which discloses a similar type of gate
drive. Reference is made to my previous U.S. Pat. No. 3,304,438
which is entitled "Phase Shift Gate Drive Circuit". Referring to
FIG. 4 of the present invention, the input power lead 147 is
similarly connected to points 10, 53, and 54 in FIG. 8 of my
previous patent. The input 136 of gate drive 60 is connected in a
manner shown by reference numeral 20 in FIG. 7 of my previous
patent. The output leads 138 and 140 from the gate drive 60 are
connected in a manner similar to 15 and 15' in FIG. 7 of my
previous patent.
FIG. 9 illustrates the output signal 146 from the gate drive 60,
relative to the AC line signal 150. The leading edge of the gate
drive output signal is indicated by reference numeral 148.
Depending upon the phase of the leading edge 148 more or less of
the AC line signal 150 will be incorporated under a common area as
shown shaded in FIG. 9. Thus, the gate drive 60 controls the power
to the SCR gate by virtue of varying the phase relation. FIG. 10
illustrates an alternate method of detecting a spark. The discussed
circuitry detected a spark by employing a resistor 38 (FIG. 1)
which developed a signal that was transmitted to the logic 46.
Although this approach operates satisfactorily, it would be more
desirable to detect spark pulse voltage signals, such as 78, in the
primary circuit of FIG. 1 rather than the high voltage secondary
circuit discussed. Accordingly, the circuitry of FIG. 10 offers an
extra advantage of isolation between the high voltage secondary
circuitry and the logic circuitry. When using this alternate
approach, the resistor 38 of FIG. 1 is removed so that a short
circuit exists between point 44 and ground (42). The remainder of
the circuitry discussed is identical.
However, in order to detect a spark, a second current transformer
152 is connected via leads 154 to the primary 156 of a step-up
transformer. The secondary 158 imposes a voltage across the
parallel connected resistor 160. A diode bridge 162 rectifies the
signal appearing across resistor 160. A rectified signal is tapped
off at bridge points 166 and 168. These points are connected across
the potentiometer 164. The lower terminal of the resistor is
connected to ground as indicated at 170. The tap-off wiper 167 of
the potentiometer 164 is connected to the logic 46 along with a
lead from point 170. In effect, these output leads present a spark
signal 78 in the same way that comparable leads supplied such a
signal from across resistor 38 in the previous embodiment. In FIG.
10, the spark pluse voltage signal 78 occurs due to the transformer
coupling of the secondary-primary windings.
It should be understood that the invention is not limited to the
exact details of construction shown and described herein for
obvious modifications will occur to persons skilled in the art.
* * * * *