Electrostatic precipitator voltage controller having improved electrical characteristics

A control system for controlling high power from an AC source for electrostatic precipitators. The AC power is gated both on and off during the same half-cycle of the AC sources. The gating off of the AC power occurs at a time substantially different from the time of the zero crossings of the AC source. The AC source may be gated on and off respectively before and after each peak to provide high voltage to the precipator electrodes while the period of such pulsing is kept short enough to prevent arcing. Additionally, the source may be gated on after one peak and gated off before the next peak, thereby providing high voltage to the electrodes without applying the peak voltage of the AC. Further in accordance with the invention, such gating may be performed using gate turn-off thyristors. The pulses may be symmetric about the peaks or about the zero-crossings of the source. The source may also be gated on and off a plurality of times during each half cycle.

BACKGROUND OF THE INVENTION 
A. Field of Invention 
The present invention relates to controlling a high power alternating 
source for an electrostatic precipitator. 
B. Background Art 
It is known in the art to control an AC energy source for electrostatic 
precipitators using silicon control rectifiers (SCR). See for example 
Laugesen U.S. Pat. Nos. 4,326,860 and 4,390,830. 
In this prior art a turn-on signal was applied to the gate of the SCR to 
turn the SCR on, usually after the peak of a half-cycle, and energy was 
applied to the electrodes of the electrostatic precipitator by way of the 
SCR. See, for example, the prior art waveform shown in FIG. 2E or SCR 
Manual, Fifth Edition, General Electric Company, Chapter 9 (AC Phase 
Control). 
Since the current through an SCR must be decreased to substantially zero to 
turn the SCR off, after the SCR was turned on energy was supplied to the 
electrodes for the remainder of the half-cycle during which the SCR was 
turned on. Thus the SCR could control the energy from one end of the AC 
half-cycle only in the direction indicated by the arrows of FIG. 2E. 
The SCR was usually turned on after the peak of a half-cycle because arcing 
of the electrodes is most likely at the peak of the AC signal. This delay 
in turning the SCR on avoided applying energy to the electrodes during the 
portion of the half-cycle most likely to cause arcing. 
However, this also resulted in poor utilization of the waveform, since the 
portion of the half-cycle between a zero-crossing and a peak could not be 
applied to the electrodes. This was so because the turn-off time of the 
SCR was too long to turn an SCR on in the portion of the half-cycle before 
the peak and reliably turn it off before the peak to prevent arcing. See 
for example SCR Manual, Fifth Edition, General Electric Company, page 123 
for a list of parameters which affect the turn off time of SCR's. Forced 
commutation circuits to accomplish this type of turn off were very complex 
and extremely expensive. 
Additionally, the harmonic content and the DC ripple of the pulses produced 
in these SCR power supplies for electrostatic precipitators were 
objectionable when this arrangement was used because of the way that the 
DC waveform was chopped, especially with high current loads. 
Furthermore, because it was difficult to turn off the SCR, it was difficult 
to terminate the supply of energy to the electrodes quickly under arcing 
or other emergency conditions. A further problem associated with shutdown 
upon arcing or other emergency shutdown was that this type of sudden 
shut-down a large amount of energy to be dumped into the precipitator, 
stressing precipitator components. 
In addition to these difficulties, since the voltage rose during the early 
portions of the half-cycle before the SCR was turned on to supply current 
to the load, the voltage and current were out of phase resulting in a poor 
power factor. 
It has also been known in the prior art to use gate turn-off thyristors 
(GTO) to operate from a DC voltage rail to obtain a variable frequency AC 
output. See for example, "Gate Turn-Off Thyristors: Their Properties and 
Applications", W. Bosterling, H. Ludwig, R. Schimmer, M. Tscharn; 
AEG-Telefunken, Primary Technical Information, October, 1983. However, 
this method was not useful for ESP technology because it would have to be 
applied to the energy supply after step-up and rectification where the 
voltage level is in the range of one hundred to two hundred kilovolts. 
SUMMARY OF THE INVENTION 
A control system for controlling high power from an AC source for 
electrostatic precipitators. The AC power is gated both on and off during 
the same half-cycle of the AC source. The gating off of the AC power 
occurs at a time substantially different from the time of the zero 
crossings of the source. The AC source may be gated on and off 
respectively before and after each peak to provide high voltage to the 
precipitator electrodes while the period of such pulsing is kept short 
enough to prevent arcing. Additionally, the AC source may be gated on 
after one peak and gated off before the next peak, thereby providing high 
voltage to the electrodes without applying the peak voltage of the AC. 
Further in accordance with the invention, such gating may be performed 
using gate turn-off thyristors. In another embodiment gate turn-off 
thyristors are used to shape AC waveforms.

DETAILED DESCRIPTION OF THE INVENTION 
Referring now to FIG. 1 there is shown electrostatic precipitator (ESP) 
system 10. System 10 includes a high power alternating source 12 which 
applies AC signal 16 to switch 18 by way of lines 14. The power provided 
by high power source 12 may be in the range of five kilowatts to two 
hundred fifty kilowatts. Switch 18 receives AC signal 16 and shapes AC 
signal 16 into pulses such as pulses 22 (mode A), pulses 23 (mode C), or 
pulses 24 (mode B). Pulses 22,23,24 are applied to transformer 25 by way 
of lines 20 and thereby to full wave rectifier 26. Rectified voltage is 
then applied to electrodes 28. 
Referring now to FIG. 3, there is shown a more detailed representation of 
switch 18 including gate turn-off thyristors (GTO) 88 which are connected 
in antiparallel. GTO's 88 operate during opposite polarities of signal 16 
and have the ability to block reverse voltage as described in 
International Rectifier Aplication Notes AN-315 "Applying International 
Rectifiers 160 PFT Type Gate Turn-Off Thyristors". Each GTO 88 is 
controlled at its respective gate control terminals 96 by a respective 
firing circuit 84 and current trip 86 which cause GTO's 88 to be turned on 
and off as required to produce pulses 22, 23, 24. Details of voltage 
controller 31, which produces timed signals as required for pulses 22, 23, 
24, are set forth below. 
Because GTO's 88 may be turned off quickly they may be used to chop up 
signal 16 and produce the high voltage waveforms applied to electrodes 28 
by way of lines 20 as compared with conventional control using silicon 
control rectifiers which could only be reliably turned off by a reversal 
of supply current, usually at a zero-crossing, or forced to turn off by 
commutating circuits. 
Damping resistors 92, charging diodes 90 and capacitors 94 provide 
conventional directionally controlled snubber circuits for GTO's 88. 
During turn off of a GTO 88 when a negative voltage is provided from the 
gate to the cathode of a GTO 88, current to the load is diverted to the 
snubber circuit. During these conditions it is desired to charge 
capacitors 94 as quickly as possible to more quickly stop current to the 
load. Thus forward biased diodes 90 are provided in order to by-pass 
resistors 92. When the GTO 88 is turned back on, diodes 90 are back biased 
and current from capacitors 94 pass through resistors 92. 
Conventional power supplies 82 provide power for firing circuits 84 as well 
as current trip circuits 86 which permit GTO's 88 to be turned off very 
quickly during the shaping of pulses 22, 23, 24 as well as during arcing 
of electrodes 28. Supplies 82 may provide 0, +5, and +15 volts. Current 
trips 86 are also conventional. 
Switch 18 may gate signal 16 through to lines 20 to produce pulses 22 by 
turning on shortly before the peaks of signal 16 and turning off shortly 
after the peaks of signal 16. Pulses 22 are preferably symmetric about the 
peaks of signal 16. Because the likelihood of arcing at electrodes 28 is 
highest at the peaks of signal 16, the duration of pulses 22 is kept 
shorter than the amount of time required for electrodes 28 to arc. This 
permits high peak voltages to be applied to electrodes 28 while preventing 
electrodes 28 from arcing. This is useful in systems in which high voltage 
at electrodes 28 is required because of the resistivity of the particles 
being precipitated. pitated. 
Switch 18 provides pulses 24 by turning on a predetermined period of time 
after each peak of signal 16 and turning off a predetermined period of 
time before the next peak of signal 16. This predetermined period of time 
may be lengthened, causing the turn-on and turn-off times to move 
outwardly from the zero crossings, in order to apply a desired average DC 
to electrodes 28 without causing the increased risk of arcing associated 
with applying the peak voltages of signal 16 to electrodes 28. Higher 
average DC results in increased precipitator efficiency. 
Furthermore, switch 18 may provide pulses 23 by combining pulses such as 
pulses 22, 24. Thus, pulses 23 may contain portions in which energy is 
gated on around each peak of signal 16 as previously described for pulses 
22 as well as portions in which energy is gated on after one peak and 
gated off before the next peak as previously described for pulses 24. 
Additionally, to provide higher average DC, further energy may be provided 
by pulses 23 by further gating of switch 18 between the pulses described 
for pulses 22, 23 as will be described in detail below. 
Referring now to FIGS. 2A-D, there is shown in more detail signal 16 as 
well as pulses 22, 23 and 24. Signal 16, provided by supply 12, typically 
is in the range of 440 to 575 volts AC and has peaks 30, 32 and 
zero-crossings 31a,b,c. Pulses 22, 23, 24, after the output of switch 18 
has been applied to transformer 25, and may be in the range of five 
kilowatts to over two hundred and fifty kilowatts. Pulses 22, 23, 24 may 
have a peak DC voltage in the range of ninety to one-hundred fifty 
kilovolts while the RMS voltage on the primary side of transformer 25 may 
be in the range of four hundred forty to six hundred volts. Thus first the 
switching is performed on the alternating source voltage and the voltage 
is then stepped up and rectified. 
Pulses 22 (mode A) are provided by causing switch 18 to turn on at time 34 
and to turn off at time 36 preferably by means of GTO 88. The time 
difference between turn-on time 34 and the time of peak 30 may be selected 
to be equal to the time difference between the time of peak 30 and 
turn-off time 36. Thus, the pulse produced when switch 18 is in mode A, 
which turns on at time 34 and off at time 36, may be symmetrical about the 
positive-going peak 30 of signal 16. 
Likewise, switch 18 turns on at time 38 and turns off at time 40 in which 
times 38, 40 may be selected to cause a pulse 22 which is symmetrical 
about the time of negative-going peak 32 of signal 16. 
The total time difference between turn-on time 34 and turn-off time 36 in 
mode A, as well as the total time difference between turn-on time 38 and 
turn-off time 40, may be as short as permitted by circuit parameters 
(typically fifty to seventy-five microseconds) or as wide as the entire 
half cycle of signal 16. In general, these durations are selected to be 
short enough to prevent electrodes 28 from arcing. In high resistivity 
particle environments, it is often desired that a high DC value be 
provided to electrodes 28 while still preventing electrodes 28 from 
arcing. An example of such a high resistivity environment is precipitation 
of some types of coal dust. 
Times 34, 36, as well as times 38, 40, may be adjusted outwardly from the 
times of peaks 30, 32, as shown by the directions of the arrows of FIG. 
2B, to provide greater average DC to electrodes 28 while stopping short of 
a pulse width which would cause electrodes 28 to arc. 
Referring now to FIG. 2C, there is shown in more detail pulses 24 (mode B). 
To provide pulses 24, switch 18 is turned on at time 44 and turned off at 
time 48. During the time between times 44, 48, signal 16 passes through 
zero-crossing 31b. Because switch 18 is designed to include GTO's 88 
rather than silicon controlled rectifiers, shutoff of power to electrodes 
28 at a the zero-crossing is prevented. An example of such a shutoff 
during the zero-crossing, which is avoided in the present invention, is 
shown in the prior art waveform of FIG. 2E. (For simplicity, the waveform 
of FIG. 2E is shown as if the load supplied with energy is purely 
resistive). Pulses 24 may continue after the zero-crossing by firing the 
GTO 88 of the opposite polarity because the portion of pulse 24 thus 
produced may then be terminated before the next peak of signal 16 by GTO 
88 control circuits 84, 86. Thus control of the turn-off point of 
individual GTO's 88 permits complete control of termination of pulses 24 
to maintain equal volt-seconds for each segment of each pulse of pulses 24 
as well as equal volt-seconds for each pulse of pulses 24. 
Switch 18 thus causes signal 16 to be gated off from time 48 until time 50. 
At time 50, switch 18 gates signal 16 on again as previously described for 
time 44. The pulse produced when switch 18 turns on at time 50 continues 
past zero-crossing 31c into the next half-cycle (not shown) of signal 16 
until switch 18 is again turned off. Similarly, in a half-cycle (not 
shown) prior to zero-crossing 31a, switch 18 is turned on. Switch 18 is 
then turned off at time 42 in the manner previously described for time 48. 
The average DC of pulses 24 when switch 18 is operating in mode B may be 
increased by adjusting times 42, 44, 48, 50 in the direction indicated by 
the arrows of FIG. 2C. For example, a GTO 88 may be turned on before time 
44 and turned off after time 48. Thus, the utilization of signal 16 may be 
increased without applying energy to electrodes 28 at peaks 30, 32 of 
signal 16. Times 42, 44 may be symmetric about the time of peak 30 and 
times 48, 50 may be symmetric around the time of peak 32. 
Referring now to FIG. 2D, pulses 23 (mode C) are produced by applying the 
techniques used to produce pulses 22, 24. For example, by turning switch 
18 on at time 64 and off at time 66, a pulse similar to pulses 24 is 
produced in which switch 18 turned on at time 44 and off at time 48 as 
previously described. Likewise, turning switch 18 off at time 54 ends a 
pulse similar to pulses 24 in a manner similar to that described for time 
42 of FIG. 2C, and turning switch 18 on at time 78 begins a pulse in a 
manner similar to that described for turning switch 18 on at time 50. 
Symmetric to positive-going peak 30, switch 18 may be turned on at time 34 
and off at time 36 within pulses 23 in a manner similar to that previously 
described for pulses 22. Likewise, during the negative half-cycle of 
signal 16, switch 18 may turn on at time 38 and off at time 40 when 
operating in mode C to produce a portion of pulse 23 in a manner similar 
to that described for pulses 22. 
Thus, pulses 22, 24 may be combined by having switch 18 gate signal 16 on 
and off a plurality of times during each half cycle. Additionally, switch 
18 may be turned on at time 56 and off at time 58 in the same manner as 
previously described for times 34, 36. Likewise, switch 18 may be turned 
on at time 60 and off at time 62, on at time 70 and off at time 72, and on 
at time 74 and off at time 76 to provide additional portions of pulses 23. 
A plurality of such pulses may be provided between pulses 22, 24 when 
combining pulses 22, 24 as required for the optimum operation of system 
10. Thus each GTO 88 may be fired several times within the half-cycle that 
it is forward biased. This is useful when impedance matching system 10. 
The turn-off current of switch 18 when providing pulses 22, 23, 24 may be 
aproximately 600 amps. 
Thus it will be understood by those skilled in the art that pulses 22,24 
may be combined to form pulses such as pulses 23. Pulse 23 are a direct 
combination of pulses 22,24. 
Referring now to FIG. 4 there is shown a flow chart for selecting one of a 
plurality of programs for providing pulses 22 (mode A), pulses 24 (mode 
B), and pulses 23 (mode C). Each of the programs is set forth in a table 
below in a structured format understandable to those skilled in the art. 
Each mode, A, B, C, or D may be manually input as shown in block 112. If 
mode A is manually selected, as determined at decision 116, execution 
proceeds through the program of Table 2 as shown in block 114. If mode B 
is manually selected, as determined at decision 118, execution proceeds to 
the program of Table 1 as shown in block 120. If mode C is manually 
selected, as determined at decision 124, execution proceeds to the program 
of Table 3 as shown in block 122. If mode D is selected, as determined in 
decision 126, execution proceeds to the program of Table 4 as shown in 
block 128. Mode D is a mixed mode which permits variable selection of one 
of the preceding modes A, B, C by the main program from cycle to cycle. It 
will be understood by those skilled in the art that the waveforms formed 
by the programs of Tables 1,2,3 may be described in either an inverted 
form or a non-inverted form. For example the program of Table 3 provides a 
waveform which is the inverse of that shown as pulses 23. 
Table I 
05 FOR N=0 TO 1 
10 ON PULSE (EN) FOR X DEGREES 
20 AT X DEGREES, OFF PULSE (EN) FOR (180-2X) DEGREES 
30 ON PULSE (EN) FOR BALANCE OF HALF CYCLE 
40 NEXT N 
50 READ NEW X FROM MAIN PROGRAM 
55 IF X=0, RETURN TO MAIN PROGRAM 
60 GOTO 05 
Table 2 
95 FORN=0 TO 1 
100 OFF PULSE (EN) FOR X DEGREES 
200 AT X DEGRESS ON PULSE (EN) FOR (180-2X) DEGREES 
300 OFF PULSE (EN) FOR BALANCE OF HALF CYCLE 
400 NEXT N 
500 READ NEW X FROM MAIN PROGRAM 
505 IF X=0, RETURN TO MAIN PROGRAM 
600 GOTO 95 
Table 3 
1145 FOR N=0 TO 1 
1150 A=90/(Y+Z): I=INT(A) 
1160 FOR N1=0 TO (I-1) 
1165 OFF PULSE (EN) FOR Z DEGRESS 
1170 ON PULSE (EN) FOR Y DEGREES 
1175 NEXT N1 
1180 OFF PULSE (EN) UNTIL 90+(A-I) DEGREES 
1200 FOR N2=0 TO (I-1) 
1205 ON PULSE (EN) FOR Y DEGRESS 
1210 OFF PULSE (EN) FOR Z DEGREES 
1215 NEXT N2 
1300 NEXT N 
1400 READ NEW Y, NEW Z FROM MAIN PROGRAM 
1450 IF Y=0 AND Z=0, RETURN TO MAIN PROGRAM 
1460 GOTO 1145 
Table 4 
2000 READ MODE$ FROM MAIN PROGRAM 
2005 IF MODE$=A, GO TO 05 
2010 IF MODE$=B, GO TO 95 
2015 IF MODE$=C, GO TO 1145 
2020 IF MODE$=0, RETURN TO MAIN PROGRAM 
2025 GOTO 2000 
Referring now to FIG. 5, routine 150 for synchronizing pulses 22, 23, 24 
with the zero crossings of signal 16 is shown. A conventional zero 
crossing detector (not shown) is used in system 10 for detecting the zero 
crossings of signal 16, such as 31a, 31b, 31c. This conventional zero 
crossing detector outputs a pulse (not shown) at each zero crossing of 
signal 16. 
The zero crossing pulses are received in input block 152 and clock timing 
computations are performed in block 154. These timing computations may 
include for example a computation of the time between time 34 and time 36 
or between time 38 and time 40 when system 10 is in mode A. 
The computations which are used in the program of Table 2 determine the 
value of X which represents the period of time between zero crossing 31a 
and time 34. In Table 1, X represents the period of time between 
zero-crossing 31a and time 42. In Table 3, Z represents the period of time 
between zero-crossing 31a and time 54 while Y represents the period of 
time between time 54 and time 56. 
Thus, in blocks 154,156 turn-off time 42 and turn-on time 44 are determined 
when system 10 is in mode B and these times are used in the program of 
Table 2. Thus, the time periods required for producing pulses 22, 23, 24 
are produced and synchronized with signal 16 in blocks 154,156. The main 
program of b1ock 156 ana1yzes feedback variables from the 
transformer/rectifier set and ESP electrodes 28 in determining optimum 
values of X, Y, and Z. In an alternate embodiment of system 10, timing 
computation 154 may be performed by hardware (not shown). 
Voltage controller 31, which executes the main program receives feedback by 
way of current feedback line 27 and voltage feedback line 29. By sensing 
the voltage across resistor 27a, current feedback line 27 provides a 
signal representative of the current through electrodes 28. By sensing the 
voltage across electrodes 28, divided down by voltage divider 29a, voltage 
feedback line 29 provides a signal representative of the voltage across 
electrodes 28. 
The determinations made in accordance with the feedback signals of lines 
27, 29 may be determinations such as those set forth in U.S. Pat. Nos. 
4,326,860 and 4,390,830 which are herein incorporated by reference. These 
determinations, in addition to being used to shape pulses 22, 23, 24, may 
be used by voltage controller 31 to provide emergency shutdown of energy 
to electrodes 28, for example during arcing. Furthermore, voltage 
controller 31 may adjust timing periods, such as periods X, Y, and Z, to 
tailor and fine tune pulses 22, 23, 24 to the specific parameters of a 
particular electrostatic precipitator and the materials being 
precipitated. 
Execution then proceeds from flowchart 150 by the way of off-page connector 
158 to the on-page connector 111 of FIG. 4 to the program to select mode 
A, B, C, D as previously described. 
Referring now to FIG. 6, enable routine 160 is shown. As previously 
described, the programs of Tables 1-3 enable the firing of GTO's 88 to 
shape pulses 22, 23, 24. GTO's 88 therefore must be enabled when, for 
example, instructions 10, 20 of Table 1 are executed or instructions 100, 
200, 300 of Table 2 are executed. When any of these instructions is 
executed, or any of the instructions of Table 3 whcch turn GTO's 88 on or 
off are executed, enable routine 160 is executed. 
It will be understood by those skilled in the art that the pulses produced 
by the PULSE (EN) instructions of Tables 1-3 to cause firing of a GTO 88 
may be logically inverted. It will be further understood that the 
processor of voltage controller 31 (not shown) executing the programs may 
produce these pulses. Furthermore, the operations shown in Tables 1-3 and 
in FIGS. 4-6 in software form may be implemented using hardware such as 
conventional logic circuits (not shown). 
Execution of enable routine 160 begins when a PULSE (EN) instruction is 
executed by way of on-page connector 162 and in decision 164 determination 
is made whether signal 16 is in the on period of the first GTO. Each GTO 
88 of switch 18 has an on period during one of the half cycles of signal 
16. 
If a determination is made that signal 16 is in the on period of the first 
GTO 88, execution proceeds to output block 166 in which an output is 
transmitted to the first firing module by way of control bus 21, for 
example a firing module 84 as shown in switch 18. If signal 16 is not in 
the on period of the first GTO 88, signal 16 must be in the on period of 
the second GTO 88 as determined at decision 168. When signal 16 is in the 
on period of the second GTO 88 execution proceeds to block 170 in which an 
output to the second firing module 84 is provided by way of control bus 
21. Modules 86, which sense current through GTO's 88 by way of current 
sensing elements 95, may also cause firing circuits 84 to turn off GTO's 
88 independently of controller 31. Current sensing elements 95 may 
comprise resistors, current transformers (not shown) and Hall effect 
devices (not shown). 
Referring now to FIG. 7, an alternate embodiment 18a of switch 18 is shown. 
Switch 18a is used for GTO's 88 which cannot block reverse voltage. In 
switch 18a, GTO's 88 are connected cathode to anode. A conventional 
snubber circuit, including diode 90, resistor 92 and capacitor 94 is 
provided across each GTO 88 as previously described. Each GTO 88 is also 
provided with an anti-parallel diode 102 connected across it to prevent 
build-up of reverse voltage. Furthermore, each GTO 88 is provided with an 
additional series diode 104 to provide the reverse voltage blocking 
capability lacking within GTO 88. Power supplies 82, firing circuits 84 
and current trips 86 may be similar to those described for switch 18. 
Referring now to FIG. 8 there is shown, switch 18b which is an additional 
alternate embodiment of switch 18. Switch 18b may be used when GTO's 88 
lack reverse voltage blocking capability. In switch 18b, GTO's 88 are 
connected cathode to cathode and the series diode of switch 18b may then 
be omitted. Anti-parallel diodes 102 are provided as in switch 18a to 
prevent build-up of reverse voltage. Snubber circuits, power supplies 82, 
firing circuits 84, and current trips 86 are provided as previously 
described. 
In system 10 the following components have been used for the operation and 
function as described and shown. 
______________________________________ 
Reference Numeral 
Type 
______________________________________ 
84 International Rectifier GK2B 
86 Megatran Electronic Power 
G74024 
88 International Rectifier 
160 PFT 140 
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