Note: Descriptions are shown in the official language in which they were submitted.
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AN ELECTRICALLY VARIABLE CURRENT LIMITING
REACTOR FOR ~K~;cL~l~lATORS
DESCRIPTION
BACKGROUND OF THE INVENTION
5 1. Field Of The Invention
Continuing ~ ~-c; q on environmental quality and
recent new ,' ~cic on air quality in particular have
resulted in increasingly stringent regulatory control of
industrial .omi Ccif)nc~ One technique which has proven
10 highly effective in controlling air pollution is the
removal of undesired particulate matter from a gas stream
by electrostatic precipitation.
An electrostatic precipitator is an air
pollution control device designated to electrically charge
15 and collect particulates generated from industrial
processes such as those occurring in cement plants, pulp
and paper mills and utilities. Particulate-laden gas
flows through the precipitator where the particles acquire
a charge. These charged particles are attracted to, and
20 collected by, oppositely-charged metal plates. The
cleaned process gas may then i~e further processed or
safely discharged to the atmosphere.
The electrostatic precipitation process involves
several complicated, interrelated physical - -h~n;rmc:
25 The creation of a nonuniform electric field and ionic
current in a corona discharge; the ionic and el~ctronic
*
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charging of particles moving in ~: ' in~d electro- and
hydrodynamic fields; and the t~lrblllf~nt transport of
charged particles to a collect~Dn surf ace . Because of J
this, many practical considerations can act to reduce
5collection ef f iciency .
To maximize the particulate collection, a
precipitator should operate at the highest practical
usahle energy level, increasing both the particle charge
and collection ~ ~r~hi 1 i tles of the system. At the same
10time, there is an energy level above which arcing or
"sp~rkin~", a temporary short which creates a conductive
gas path, occurs in the system. M~cimi7.in~ the efficiency
of an electrostatic precipitator re auires operating the
system at the highest poseihle usahle energy level.
Ideally, the electrostatic precipitator should
operate constantly at its point of greatest f~f f ~ c~-~r~ry.
Unfortunately, conditions unaer which an electrostatic
precipitator operates, such as temperature, comhustion
rate, and the rhf~mJc~l composition of the particles heing
20collected, change constantly. This ~ tes
calculating parameters critical to a precipitator ' s
operation .
2. Description Of The ~rior Art
This invention relates to electrostatic
25precipitators in general and cp~cifi~lly to precipitator
power supplies. Prior art precipitator power supplies
have used either saturahle core reactors or silicon-
controlled rectif iers ( SCRs ) paired with a f ixed-value
current-limiting reactor (CLR). This invention relates to
30 an illl~L~ V~..~llt of the CLR.
Prior art ~:LRs have an inductance of f ixed value
with several taps for selecting other values. The number
of taps available is limited, typically to three.
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Adjusting the inductance of the CLR requires that the
precipitator f ield section be powered down and taps
manually changed.
A CLR of the correct value contributes to
5 protecting the precipitator power supply f rom the
destructive ef f ects of arcing or spark currents and
ensures greater electrical and particulate collection
Pf f i ~-iPn~iPc .
Prior art devices useful for voltage and current
10 control of power supplies have been disclosed in various
patents i n~ 1in g U.5. Patent No. 1,372,653 issued March
22, 1921 to F. Dessauer on an Electrical Transformer
System; U.S. Patent 1,702,771 issued Feb. 19, 1929 to Y.
Groeneveld on an Amplifying Transformer; U.S. Patent No.
1,73Z,715 issued Oct. 22, 1929 to F. ~PcsAllpr et al on an
Electromagnetic Induction ApRaratus; U. S . Patent No.
1,896,480 issued Fel:. 7, 1933 to A. Christopher on a
BAlAnrPd Inductance Device; U.S. Patent No. 2,878,455
issued March 17, 1959 to C. Lamberton et al on a Three
20 Winding Transformer; U.S. Patent No. 3,483,499 issued Dec.
9, 1969 to L. Lugten on an Inductive Device; U.S. Patent
No. 4,020,438 issued April 26, 1977 to A. MAnir^-lPthU on
an Autotransf ormer With Series And Tertiary Wlndings
Havlng Same Polarity T _~'An-~e; U.S. Patent No. 4,513,274
issued April 23, 1985 to M. Halder on a Current
Transformer For Measuring In:,LL, -ts; U.S. Patent No.
4,590,453 issued May 20, 1986 to A. Weissman on an
Autotransformer With Common Winding Having Oppositely
Wound Sections; U.S. Patent No. 4,916,425 issued April 10,
30 1990 to N. Zabar on an Ele~ n~otic Device and U.S.
Patent No. 4,973,930 issued November 27, 1990 to U. Mai et
al on a Twin Coil.
An alternative to the silicon-controlled
rectif iers paired with a f ixed-value current limiting
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reactor is a saturable core reactor. The saturable core
reactor ( or saturable reactor ) was originally developed in
Germany and was used in the United States extensively from
1945 foreward. The principal application has been to
5 control the power applied to heating elements. Saturable
reactors are electrically and ~hiln;c~lly rugged. In
recent years, their ~unctions have been largely taken over
by silicon-controlled rectifiers; as a consequence, the
saturable reactor has been relegated to obscurity.
SUM~ARY OF T~IE INVENTION
The present invention generally provides a
current limiting reactor f or use within a power supply
system for a electrostatic precipitator wherein the
15 inductance of the current limiting reactor can be
electrically, automatically and cont~n~lollcly modified
responsive to system conditions. By continuous monitoring
of the correct system conditions the variation in the
inductance of the current limiting reactor can increase
20 the average voltage and current within the precipitator
f ield . The ultimate result of this more caref ul and
accurate control is that the destructive effects of spark
currents on equipment are minimi7ed and the electrical and
particle collection eff;r!i~nc~c are ~nh~n~-r~d.
2 5 Furthermore the overall average voltage and current in the
precipitator fields can be increased before spark over
actually occurs such as to permit a higher overall power
level bef ore spark over . Fur~hf~ _ e it is particularly
important that the variable currer~t limiting reactor of
3 0 t~e present invention be constructed such as to
automatically attain its maximum inductance value if an
open circuit condition occurs in the control circuit or
control wlnding. In this manner the automatic protection
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of equipment will be achieved if excitation of the control
winding is lost.
The basic precipitator power supply i n~ c a
silicon controlled rectifier stack which preferably
- 5 includes two individual silicon control rectif iers
connected in an inverse parallel conf iguration in series
between a line voltage power source and the current
limiting reactor. An automatic control can be operative
to modify the output of the silicon control rectifier
stack to modify the power output of the silicon controlled
rectif ier stack . When operated at maximum power the
silicon controlled rectifier stack output ;n~]ll~ c a
sin--cr);~lAl AC current waveform. However when operated
below the rating thereof there is a naturally occurring
deterioration of the wavef orm in addition to the power
output .
The current limiting reactor is positioned in
series with respect to the silicon controlled rectif ier
stack . In prior art conf igurations this current limiting
reactor was of a fixed inductance value or had various
taps to allow some element of modif ication of the
inductance thereof between fixed values. Changing of the
inductance value normally required powering down the
system in order to make the change in the current limiting
reactor. With the present invention this current limiting
reactor is dynamic and continuously responsive to system
parameters in order to vary the inductance thereof.
The operative current limiting reactor is
connected to a transf ormer rectif ier set . Initially the
primary of the transformer receives the low voltage and
high current signal an~ transforms this to a high voltage
and low current signal in the secnnA~ry of the
transformer. The output of the step-up trans~ormer
s~ n~l~ry is provided to a rectifier which provides a high
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voltage DC signal to the precipitator to f acilitate
collection of particulate matter.
In one conf iguration of the dynamic current
limiting reactor of the present invention the control
winding is cQnnected to a variable DC power source. This
control windlng is adapted to vary the inductance of the
current limiting reactor responsive to variations in the
DC power source . With this conf iguration electrical
coupling between the control winding and the inductor
winding or windings of the current limiting reactor is
achieved through a magnetic core. In the preferred
physic21 configuration two identical inductor windings are
wound about a magnetic core. ~he core extending through
each inductor winding extends throus~h the control winding
in opposite directions to yield a resultant instantaneous
f lux through the control winding of zero . As such with
this conf iguration the inductance of the CLR control
device is a function of the magnitude of the DC current
passing through the control winding.
Operation of the control winding can be
automatic responsive to sensed system conditions such as
the dynamic variables wlthin the precipitator f ield .
These dynamic variables can depend upon the type of
material being precipitated, the temperature or ~les:-uL~
conditions or other various dynamic conditions. Variation
in the DC power source can be achieved manually by an
operator responsive to visual reading of the parameters or
can be automatically variable.
Pref erably variation in DC power supply to the
control winding is responsive to the shape of the AC
waveform at the input of the primary of the transformer
rectif ier set or is responsive to the shape of the
rectif ied AC wave at the output of the transf ormer
rectifier set. Both the maintenance of a low form factor
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and the maintenance of a high secondary f ractional
cor~luct i nn have been shown to be excellent parameters f or
maintaining accurate control o_ variations in the
inductance of the current limiting reactor as will be
5 shown in more detail below.
As an alternative conf iguration the present
inventiorl can include a somewhat 'i f; Pfl automatic system
for controlling the inductance of the current limiting
reaetor wherein a eurrent transformer utilizes the primary
10 current passing in series from the silicon controlled
rectif ier to the transf ormer rectif ier set as the primary
with a transformer secondary winding extending thereabout.
The output signal of the current transformer sec--n~lAry
winding is rectified by a conventional full wave bridge
15 rectif ier and is provided to the control winding of the
current limiting reactor control winding. The DC current
through this control winding will then modify the
inductance of the inductor winding which is in series
between the silicon controlled rectif ier stack and the
20 current transformer primary. In this manner the
inductance value of the inductor winding of the current
limiting reactor will be proportionally responsive to the
current at the primary of the transf ormer rectif ier set .
It is an object of the present invention to
25 provide an electrically variable current limiting reactor
wherein utilization with an electrostatic precipitator is
greatly PnhAn-'Pd.
It is an object of the present invention to
provide an electrically variable current limiting reactor
30 wherein variation in the inductance therein is made
possible responsive to system parameters.
It is an object of the present invention to
provide an electrically varlable current limiting reactor
particularly usable with a power supply for an
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electrostatic precipitator wherein a low f orm f actor of
the input current signal at the primary of the transformer
rectifier set is maintained. ,
It is an object of the present invention to
5 provide an electrically variable current limiting reactor
particularly usable with a power supply f or an
electrostatic precipitator wherein a high q~f-nn~lAry
fractional conduct~on at any power level is achieved at
the output of the full wave rectifier o~f the transformer
lO rectifier set.
It is an object of the present invention to
provide an electrically variable current limiting reactor
particularly usable with a power supply for an
electrostatic precipitator wherein the destructive effects
15 of arcing or spark currents are minimized.
It is an object of the present invention to
provide an electrically variable current limiting reactor
particularly usable with a power supply for an
electrostatic precipitator wherein greater electrical and
20 par~;c1~lAte collection eff;c~i~nr;~ are achieved.
It is an object of the present invention to
provide an electrically variable current limiting reactor
particularly usable with a power supply f or an
electrostatic precipitator wherein '; f i ~Ations of the
25 inductance of the current limiting reactor can be achieved
without having the precipitator field powered down.
It is an object of the present invention to
provide an el~c~rin~lly variable current limiting reactor
particularly usable with a power supply for an
3 0 electrostatic precipitator wherein the overall average
voltage and current in the precipitator f ield is increased
before spark over occurs thereby permitting a higher
overal~power level before spark over.
It is an object of the present invention to
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provide an electrically variable current limiting reactor
particularly usable with a power supply for an
electrostatic precipitator wherein the current limiting
reactor automatically goes to maximum inductance value
responsive to an open circuit occurring within the control
circuit or the control winding.
It is an object of the present invention to
provide an electrically variable current limiting reactor
particularly usable with a power supply f or an
electrostatic precipitator wherein automatic protection of
all e~auipment is provided if the control winding
excitation is lost.
BRIEF DESCRIPTION OF T~IE DRAWINGS
While the invention is particularly pointed out
and distinctly claimed in the conrll~Ai n~ portions herein,
a preferred ~ i t is set forth in the following
detailed description which may be best understood when
read in connection with the ~ ying drawings, in
which:
Figure 1 is a schematic illustration of a
typical precipitator power system;
Figure 2 is a graph of a conventional ~:~n~lcoiiii-
wavef orm;
Figure 3 is a vector diagram for ~et~rm;n;n~ the
;mp.or-7i~nf~ of the current limiting reactor;
Figure 4 is a graph of kilovolts vs. m; l l; i 5
showing the advantages of the variable current limiting
reactor over the prior art f ixed current limiting reactor;
- Figure 5 iE a schematic of an: ' -';- t of an
automatic electrically variable current limiting reactor;
- Figure 6 is a schematic illustration of an
embodiment of the general coil and core conf iguration f or
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an electrically variable current limiting reactor;
Figure 7 is a perspective illustration of an
; `,oAir^nt of the electrlcally variable current limiting
reactor shown in Figure 6;
Figure 8 is a perspective illustration of an
~ho~l; r t of the general eoil and core eonf iguration f or
an electrically variable current limiting reactor; and
Figure 9 is a graph of a transfer function of an
electrically variable current limiting reactor.
DETAILED DESCRIPTION OF T~E ~ ;KK~ ~M72f~DIM~T
The present invention is designed to provide a
precipitator f ield 10 where particulate matter is actually
collected. It is made up of collecting plates connected
to one side of the precipitator power supply. The other
15 side of the supply is conneeted to discharge electrodes 58
which are l~n; fnrmly spaeed from the colleetion plates.
The field, in effeet, forms a capacitor, two conduetors
separated by an insulating material. The precipitator
power supply is operated at a very high direet-current
20 voltage which charges particulates e~tering the field as
well as causing them to be attracted to the collecting
plates. As the voltage of the preeipitator power supply
is increased, particulate collection increases. The
voltage cannot be increased infinitely, however; the
25 practical high-voltage ceiling is limited by the
electrical ratings of the e~uipment and by the occurrence
of sparking in the f ield.
Sparking in the f ield occurs when the voltage is
high enough to ionize the gas between a discharge
30 electrode and a collectlng plate. Ionized gas is a
conductor, so the result is a localized eleetrical
breakdown of the gas causing energy stored in the
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capacitive f ield to be discharged through the breakdown,
somewhat like lightning. This event defines the maximum
energy level that can be sustained in the precipitator
f ield at the time it happens . When a spark occurs, it is
5 effectively a short across the se~nfl~ry of the
transformer-rectifier (TR) set 22. If the precipitator
power supply is not interrupted when a spark occurs, the
spark may be maintained, caus ing current f low in the
precipitator to become very high as energy is gained rom
10 the power supply. Spark currents are wasted energy; they
do not contribute to the collection of particulates.
Uncontrolled, they damage precipitator system c, ~e~ts,
both mechanical and elcctrical, and greatly reduce
collection ef f iciency .
To determine the size of the precipitator field,
many factors must be considered: The type of material
being collected, the size and resistivity of the
particles, and the operating temperature are principal
among them. In most industrial pr~cipitators, more than
20 one ield is used. A typical application will ind
precipitator fields arranged one behind another as inlet
ield, second field, third field, outlet field, etc.
A transformer rectifier (TR) set 22 is a
combination step-up transformer and full-wave rectifier.
25 The transformer transforms the primary voltage to a very
high secondary voltage and transf orms the primary current
to a low se~ontl~ry current. The rectifier converts the
alternating current (AC) output from the ~er~nfl~ry of the
transformer to full-wave rectified DC. A typical TR set
30 used in a precipitator application is filled with oil or
cooling and insulation. T~pical ratings might be:
RMS Primary voltage: 400 VAC
RMS Primary current: 240 Amps (A)
Average secondary voltage: 45,000 VDC
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Average secnnAAry current: 1500 m~ l l i Ar-,rq
(mA)
Transformer turns ratio: 1:135
This example of a typical TR set will be used in much of
S this document. ~
The same factors cnnqi ~ red in sizing the
precipitator field affects the selection of a TR set,
along with the size of the field itself. In most
industrial precipitators, one TR set is connected to one
10 or two precipitator f}~l~ sections.
Eower control for a precipitator is accomplished
by silicon-controlled rertifiPrs 16 and 18 (SCRs). An SCR
is a solid-state device that acts like a switch because it
has a "gate" that allows it to be turned on electrically.
15 A f irst silicon-controlled rectif ier 16 and a second
sill~on co~LLolled rectifier 18 are connected in an
inverse-parallel conf iguration in series between the line
voltagQ power source 14 and ahead of the current-limiting
reactor 24 and the precipitator high voltage transformer.
20 Each SCR conducts alternately, one on the positive half-
cycle, the other on the negative half-cycle. some form of
automatic SCR voltage control 20 ( typically
miuLu~ ucessor-based) de~-~rmi n~q which SCR is switched on
and at what point in the half -cycle of the wavef orm . An
25 SCR which is switched on remains on until the current
f lowing through it decays below what is called the
~holding current", usually at or near the end of the half-
cycle; it cannot be switched of f in any other manner .
A complete sine ~sinusoidal) wave cycle 28, one
30 positive followed by one negative half-cycle, is measured
in its progress by degrees from zero to 360 (Figure Z~. A
half-cycle is measured in its progress ~rom zero to 180
degrees. The point at which an SCR is turned on, or
"fired", is measured in degrees from the b~inn~ng of the
.
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half -cycle and hence is called the f iring angle . The part
of the half-cycle during which the SCR conducts is also
measured in degrees from the firing point until confll~ctit n
ceases and is called the conduction angle. Power control
- 5 is achieved with SCRs by varying the point in the half-
cycle at which each SCR is switched on. The nature of the
SCR device is such that the output f rom the stack is no
longer a sine wave 28 because each half-cycle is "chopped"
at the point in that cycle where an SCR is "fired" or
switched into a conductive state.
Det~rminin3 the SCR stack rating also involves
several considerations. The SCRs 16 and 18 must each have
a current rating that exceeds that of the TR set 22 with
which they will be used. ~he blocking voltage of each SCR
must be approximately three times the line voltage to
prevent inadvertent conduction of the SCR resulting f rom
voltage breakdown. The rate of change of voltage with
respect to time ( expressed as dv/dt ) must also be
sufficient to prevent inadvertent conduction. "Snubber"
circuits are normally used on the SCR stack in
precipitator applications to reduce or "snub" the dv/dt to
a level d~ u~l iate to commercially-available SCRs .
The SCR automatic voltage control ~ AVC ) measures
the primary and seconflAry voltages and currents ( some also
monitor form factor and se~r)nflAry fractional conduction),
and is connected to the SCR stack 12. The AVC provides
the triggering pulses which f ire the SCRs, putting them
into a state of conduction. It detf~rmi n~.C where in the
electrical half-cycle to fire a particular SCR, thereby
achieving power control. For example, if the AVC fired
each SCR 16 or 18 at 90 degrees into the electrical half-
cycle, the firing angle would be 90 degrees, the
conduction angle would be 90 degrees, and exactly half of =
the AC power would be applied to the TR set 22. It is in
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this manner that the AVC 20 provides power control to
ensure operation within the electrical limits of the
equipment . Further, if the AVC does not f ire an SCR f or a
half-cycle, then the output of the precipitator power
5 supply is interrupted for that half-cycle. This permits
interrupting or "quenching" sparks when the AVC detects
them .
The current-limiting reactor 24 ( CLR) of prior
art is an inductor of f ixed value . ~any CLRs used in
lO precipitator `A~Flic~t;r~nc have taps which can be changed
manually to provide a limited selection of inductance
values .
The CLR 24 limits the current flow during
sparking. If a spark occurs while an SCR is conducting,
15 the spark continues until the SCR stops conducting near
the end o~ the half-cycle. During this time, the TR set
22 effectively has a short on its secr~n~Ary due to the
spark and this is ref lected into the primary. A properly
t1e.ci qn~ TR set 22 has some circuit i ~ nre, even with a
20 spark, but it is not enough to significantl~ limit the
current. Since the SCR 12 is fully turned on and the TR
set 22 presents a low 1~re~lAnre due to the spark, the only
circuit element r i n 1 nq to control current f low is the
CLR. Because of this, it is important that the CLR 24
25 have the right inductance value to control spark currents.
Another function of the CLR 24 is to shape the
voltage and current waveforms. For optimum electrical and
collecting ef f iciencies, the wave shape of the voltage and
current presented at the primary of the TR set 22 must be
30 a sine wave 28. Because the SCRs 16 and 18 chop and
thereby distort the current waveform, the CLR 24 is needed
to ~ilter and restore the waveform to some approximation
of the sine wave. Selecting the proper inductance value
Qf the CLR 24 is important ~or this function as well.
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Flistorically, the inductance value of the CLR 24
has been detsrm;n~sd by using a figure of 50 percent of the
-~9nre of the TR set 22. Vector analysis of the
voltages in the primary circuit of the TR set 22
illustrates this in Figure 3.
The voltage on the primary of the TR set 22 is
assumed to be at a zero-degree phase angle such that TR
set 22 is purely resistive. The voltage is set at its
maximum value, which is the primary voltage rating of the
example TR set 22, or 400 VAC. The voltage across the CLR
24 is assumed to be at a 90-degree phase angle such that
the CLR 24 is purely inductive. The voltage is to be
det~rm; ne~.
Since the CLR 22 and TR set voltages are in a
90-degree phase angle relationship to one another, the
problem presents itself as a right triangle. The voltage
output from the SCR stack 12 forms the hypotenuse of the
triangle. If the SCRs 16 and 18 are assumed to be at or
near full conduction, i.e., a zero-degree firing angle and
a 180-degree conduction angle, the magnitude of the
hypotenuse will be approximately equal to the line
voltage. For a 460 to 480 VAC line, 450 VAC can be
assumed .
The Pythagorean theorem is used to f ind the
unknown side of a right triangle with the formula c2-
a2=b2. In this instance, substitution provides 4502_
4002=CLR voltage2, and the CLR voltage is found to be the
square root of 42,500, or 206 volts, approxlmately half
the voltage on the TR set primary.
Next, it is nsc~o~sAry to determine the
inductance of the CLR 24 that will yield the calculated
voltage. Since the voltage across the CLR 24 is half that
across the primary of the TR set 22, the i _-1Anre of the
CLR 24 is approximately half that of the TR set 22.
-
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The i -~Ar~p of the TR set 22 is de~ermi nP~ by
dividing the primary voltage rating by the primary current
rating. In this example, that is 400V/240A=1.67 ohms.
Half of this figure, about 0.84 ohms, is the desired
5 ; -~Ant~P . The needed irductance is detPrm; nPtl by
calculating the inductive reactance using the f ormula
L=XI~/ ( 27rf ) . By substitution, this becomes
L=0 . 8 4 / ( 2x3 . 1 4 16x6 0 ), giving an inductance I L ) of 2 . 2
mi llihenr ie s ( mH ) .
If the ef_ect of a CLR 24 with a value of 50
percent of the i - 'Ance of the TR set 22 at spark-level
currents is PY~mi~d, it is found that, at the rated TR
set current limlt, the ;~re~1An~ e of the TR set 22 is x and
the i -'An~e of the CLR 24 is 0.5x. These ;rpP~An~eS are
not in time phase an~ cannot be added arithmetically, so
the total circuit ;~re~Ant~e in the primary is l.llx. When
a spark occurs, the TR set; ~ 'An-'e iS assumed to drop to
zero f or all practical purposes, and the resulting circuit
nre is now 0.5x. Since the im7P~Anre in the primary
dropped by a factor 1.11/0.5, or 2.22, the primary current
would increase by a factor of 2.22. In fact, since the TR
set 22 still has some i - 'An~P, the current does not
actually increase that much, but a significant increase
does occur.
The CLR value has been selected f or operation at
the current limit rating of the example TR set. For
operation at a lower current, a correspon~i; nqly larger
inductance value could be used. This would have the
practical e_fect o~ reducing spark currents, si~n;fi~Antly
lengthening the lif~s of e~auipment. However, this would
also limit the amount of current that could be applied to
the TR set 22 and therefore restrict its output to a lower
current. ~qany TR sets 22 are operated below their rated
limited .
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Measuring Resultant Precipitator Wave Shapes -
Once the values and ratings of the c, - Ls of the
precipitator power supply are det~rm; n~d, the
characteristics of the primary and secondary voltages and
5 currents can be measured to determLne if those values and
ratings are correct. Recall that the CLR inductance value
was calculated to provide nearly full conduction of the
SCR stack output when the TR set is operating at its
maximum ratings. This will provide a primary current wave
10 shape that will be very nearly a sine wave 28. The
secontlAry current wave shape will be very nearly a full-
wave rectif ied sine wave . Two electrical measurements can
be made to determine how closely the wave shapes
correspond to the desired sine waveform.
One measure of how closely the primary current
waveform approximates a sine wave 28 is the primary
current form factor. The form factor is de~erm;n~d by
measuring both the root-mean-square (RMS) and average
primary current and then dividing the RMS value by the
average. Expressed as an equation, this means ~orm
factor=RMS/Average. For an ideal sine wave 28, these are
the relat;onch;p-c between R~S and average values and form
f actor:
RMS value: 0.707 of peak value
Average value: 0.636 of peak value
Form factor: 0.707/0.636=1.11
Precipitator power supplies operating at maximum ratings
are normally designed to operate at a form factor of 1.2.
How closely the s~on~l~ry current waveform
approaches a rectified sine wave is the secondary current
fractional conduction. This is det~r~in~d by measuring
the duration of the 5~ nn~1~ry current waveform and
dividing it by the maximum possible duration. For a line
frequency of 60 Hertz (Hz), the maximum pocc;hl~ duration
WO 92/16302 PCr~US9l/01745
208205~ 18
is 8.33 m;lli~ nnfl~ (ms), the period of a single half-
cycle . Hence, secondary f ractional conduction=t/T, where
t is the duration of the secondary current waveform and T
is the maximum pQ':~; hl ~ duration . Precipitator power
5 supplies operating at maximum ratings are normally
designed to operate with a ~ n~Ary fractional conduction
of 0.86. Secondary fractional conduction relates to form
factor as secondary fractional conduction=(1.11/Form
factor ) 2 .
Importance Of Precipitator Wave Shapes - To
illustrate the importance of precipitator wave shapes, the
, r L values and ratings for a precipitator electrical
system, and particularly the CLR 24, were selected for
operation at the maximum ratings of the equipment. The
15 table presents actual, measured values for a precipitator
power supply, t n~ nq form factor and ~ nn~Ary
fractional conduction data. These indicate how closely
the waveforms approximated a sine wave 28 at the primary
of the TR set 22 and a full-wave rectified sine wave on
20 the s~c~n~Ary. The T~ set 22 has the ratings presented on
page 13, and a turns ratio of 1:135.
RMS Primary Amps 40 80 120 160 200 220
RMS Primary Volts 152 203 243 282 312 327
Avg .cF~c~nt~Ary Mi 11; i _ ~ 158 369 609 873 1155 1307
25Avg 5~Qn~lAry Kilovolt 25 27 29 30 32 33
Form Factor 1.79 1.56 1.44 1.35 1.29 1.26
Fractional Conduction 0 . 33 0 . 45 0 . 54 0 . 63 0 . 76 0 . 81
SCR Firing Angle 130 115 103 92 82 77
( in degrees )
30SCR cr~n~lc~ n Angl 50 65 77 88 98 103
( in degrees )
For each point, multiplying the average
WO 92/1
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19
secondary current by the turns ratio of the TR set 22 and
the form factor will equal the RMS primary current. As an
eSIuation, this is represented as Average secondary current
x Turns ratio x Form factor = RMS Primary current. This
demonstrates clearly that the secondary current output
varies directly with the form factor. Maximum electrical
ef f iciency occurs when there is maximum output f rom
minimum input. As the table shows, maximum electrical
efficiency occurs when the form factor is lowest, at 1.2.
As the f orm f actor increases, the output decreases with
respect to its input.
Because of this, it is a primary objective of
this invention to maximize electrical Pffiri~nry by
devising a variable CLR and CLR control 26 for the purpose
of maintaining a low forrn factor and a high ce~n~lAry
f ractional conduction at any given power level, thereby
increasing the average voltage and current in the
precipitator field for a given input.
The 5P~nr~rlAry voltage is not subject to
coL~ ding analysis because of the capacitive nature of
the precipitator f ield . ~owever, the voltage-current ( VI )
graph ( Figure 4 ) illustrates that the se~ n~l~ry voltage
also increases as the form factor decreases. The graph on
Figure 4 is f or a precipitator power supply used in a
refuse bur~ing application. Its ratings are:
R~S Primary Voltage: 460VAC
RMS Primary Current: 6 l A
Average Se~ n~lAry Voltage: 50 ,000 VDC
Average Seron~Ary Current: 400 mA
There are two plots on the graph. The first
shows the voltages and currents in the precipitator f ield
with the f ixed-value CLR supplied by the manuf acturer . At
the primary current limit of 61A, the se~ n~lAry current
limit of 400 mA could not be attained. The maximum
WO 92/1
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~ ~8~0~6 20
c~cnn~Ary current possible was 332 mA.
The second plot shows the voltages and currents
in the precipitator field with a prototype variable CLR.
An increase of both secondary current and voltage across
5 the operatlng range is clearly indicated, as well as the
fact that the secnr~Ary current limit could be achieved.
It is therefore a primary objective of this
invention to maximize particulate collection efficiency by
devising a variable CLR f or the purpose of malntaining a
lO low form factor and a high secondary fractional conductlon
at any given power level, thereby increasing the average
voltage and current in the precipitator f ield. This in
turn will cause more particulate collection to occur
because the particle charge is increased, as is the
15 attraction to the plates.
The practical limit to which the high voltage
can be raised is governed by the electrical ratings of the
eçluipment or by sparking in the precipitator f ield.
Sparking will occur when the spark-over voltage is
20 reached. This voltage is determined by several actors,
; nn] ~ ng gas chemistry. When this voltage level is
reached, voltage cannot be raised beyond Lt. An ideal
precipitator power supply will apply power in such a
manner that the peak value of the s~cnn~iAry voltage and
25 current are near the average value. This will produce the
maximum average secondary voltage and current before
spark-over occurs.
If the precipitator wavef orms have very high
peaks and very low averages, measuring the precipitator
30 wave shapes will show a high form factor and a low
cenOn~lAry fractional conductlon. ~rArkin~ will occur on
the peaks and the field will have little average s~cnn~9Ary
voltage and current needed for particulate collection.
Therefore, this invention is designed to
WO 92/16302
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.
21 ' 2082
maximize particulate collection efficiency by devising a
variable CLR 24 for the purpose of maintaining a low form
factor and a high ser~n~lAry fractional conduction at any
given power level, thereby increasing the average voltage
- 5 and current in the precipitator field before spark-over
occurs .
As it has been demonstrated, sparking in the
precipitator field, energy management, or any condition
that causes operation of the TR set below its rated limits
will cause an increased form factor and a decreased
secondary fractional conduction, resulting in operating
ineffiri~nri~s~ The voltage level at which a spark occurs
changes constantly because of dynamics of the gas
chemistry, temperature, and other related precipitator
parameters. To maintain the desired electrical and
particulate collection ~ffiriF~nr;~q, the imn~ nre of the
CLR 24 must be dyn rAlly adjustable.
It is therefore a primary objective of this
invention to maximize electrical and particulate
collection effir; ~nrj~s by devising a variable CLR 24 that
can be dynA-n; rA l l y adjusted by being varied electrically
and automatically f or the purpose of maintaining a low
form factor and a high secon~l~ry fractional conduction at
any power level.
This precipitator power supply is designed to
have a full-wave rectified sine wave output from the TR
set 22. This will contribute to the electrical and
particulate collection effir;~nr;es. SCRs 16 ana 18
paired with a f ixed-value current limiting reactor 24 have
been shown to be superior to saturable core reactor
systems . However, even SCR-CLR systems become inef f icient
when operated at any power level other than the limits f or
which the - -~nts were rated. This is because at any
lower power level the SCRs have a reduced conduction angle
-
WO92/163 2
O PCr~US91/01745
2~8'~
22
resulting in a high f orm f actor and a low secondary
fractional conduction. It is therefore the objective of
this invention to create current limiting reactor 24 that
can be varied electrically and/or automatically for the
5 purpose of overcoming these inef f iciencies .
The electrically variable current-limiting
reactor (EVCLR) is an improvement over the prior art
f ixed-value CLRs and saturable reactor systems . The EVCLR
is much like a ~t~1r~hl e reactor. Both devices have a
lO control winding 32 which is connected to a source of DC
energy. Both devices are h~;c~lly inductors, the
i~re-l~n- e of which can be varied electriaally. The speed
at ~ which a change applied to the control winding appears
as a change in the i -~n~-e of the device is slow in both
15 devices. The range of variability of the inductance of
the EVCLR is not as great as that of the saturable
reactor .
The principal advantage of the EVCLR over the
saturable reactor is that the EVCLR causes virtually no
20 distortion to the primary current waveform, while the
saturable reactor causes much distortion. The distortion
caused by the EVCLR can be held to low values, on the
close order of less than one percent.
Since the EVCLR is slow like the saturable
25 reactor and has a limited range of inductance adjustment,
it is not suitable as a control element if used by itself.
However, in precipitator systems that use SCRs paired with
a fixed-value CLR, the EVCLR can replace the fixed-value
CLR and yield c~n~ r~hl ~ advantage. In this
30 application, adjustment of the CLR can now be accomplished
electrically and automatically. This accomplishes all of
the objectives of the invention.
The concept of EVCLR operation that is
contemplated is that the ;~r~lAn~-e of the EVCLR would be
WO 92/
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208205~
23
ad~usted to its minimum i n~ tAn~-e value when the TR set
22 is operating at its rated limit. This would be
approximately 50 percent of the T~ set i _-~An~'e, and
would provide the optimum form factor of l. 2 and s~ Ary
- 5 fractional conduction of 0. 86 . When the TR set 22 is
operated below its rated limit, the EVCLR can be ad~usted
electrically to increase its inductance, thereby
maintaining a low form factor and a high ~ ror~lAry
fractional conduction. This configuration will have the
following advantages:
l ) It will increase averag~ voltage and current
in the precipitator field, thereby increasing parti ~1l1 Ate
collection;
2) It will minimi2e the destructive effect of
spark currents on eS~uipment;
3) It will increase electrical efficiency by
delivering maximum electrical output for minimum input;
and
4 ) It will increase the average voltage and
current in the precipitator field before spark-over
occurs .
The basic configuration of the EVCLR is as shown
in Figures 6 and 7. In the schematic shown in Figure 6
the control winding 3 2 is operatively connected with
respect to a variable DC power source 42. The control
winding is coupled with respect to the inductor winding
means 30 which preferably takes the form of a first
inductor winding means 34 and a second inductor winding
means 3 6 which are basically identical with respect to one
another. The first inductor winding means 34 as shown
best in Figure 7 is wound about a f irst core 3 8 . In a
similar manner the second inductor winding means 36 is
wound about a second core 40. Preferably both the first
core 38 and the second core 40 extend through the control
WO 92/16302
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p~,820~6
24
winding 3 2 in opposite directions to eancel the
instantaneous 1ux therein. This is shown further below.
This conf iguration results in the inductanee of the EVCLR
device being a f unction of the magnitude o the DC current
5 passing through the control winding which itself is
variable responsive to different types of controls.
various controls for modifying the DC current
through the control winding 32 can include a manual
adjustment which is based upon manual reading of form
10 factor and/or se~nn~i~ry fractional conduction readings.
This manual adjustment furthermore eould be based upon any
applieable physical signal or combination of physical
signals sueh as boiler load, eoal type or temperature,
etc. Furthermore the adjustment of the DC power source 42
15 and thus the eontrol o the amount of DC eurrent passing
through eontrol winding 32 ean be varied by an automatic
adjustment responsive to the same above-identified
parameters . In another possible ronf i~ration as shown in
Figure 6 an automatie electrieally variable eurrent
20 limiting reactor can be designed Uti 1 i 7:1 n~ the eurrent at
the primary of the transformer reetifier set 22 as the
power souree.
In the EVCLR as shown in Figures 6 and 7 as the
DC power souree 42 eonneeted to the eontrol wlndlng 32 is
25 reduce~, the inductance increases. I a fault condition
occurs which causes a loss of control winding excitation,
the inductor 3 0 def aults to its maximum inductance value .
This limits the primary current f low to its lowest and
safest value. Therefore, it is a primary ob~ective of
30 this invention to devise a variable current limiting
reactor which will automatically attain its maximum value
o ; n~ tAnre to provide automatic protection of equipment
if a fault occurs which causes a loss of control winding
excitation .
WO 92/16302 PCr/US9l/01745
25 ` ~` `2`~82~
The automatic elect}ically variable current-
limiting reactcr 44 (AEVCLR) can be constructed according
to schematic illustrated in Figure 5. The primary windin~
48 of a current transformer 46 is placed in series with
the AEVCLR. The sPco~lAry winding 50 of the current
transformer is connected to a full-wave bridge rectifier
52. The DC cutput of the full-wave bridge rectifier is
connected to the control winding 56 of the AEVCLR.
This conf iguration provides f or automatic
adjustment of the current-limiting reactor 24. The
inductance will be inversely proportional to the primary
current. As the primary current increases, the DC signal
to the control winding 56 increases. This causes a
proportional decrease in the inductance of the CLR
inductcr winding means 54 of the current limiting reactor.
Conversely, as the primary current decreases, the DC
signal to the control winding 56 decreases. This causes a
proportional increase in the inductance of the CLR
inductor winding 54 of the current-limiting reactcr.
This configuration will automatically adjust the
inductance of the AEVCLR 44 by responding to changes in
operating conditions of the TR set 22, thereby maintaining
a low form factcr and a high secondary fractional
conduction at any given power level and thus achieving all
of the stated objectives of this invention.
Design And Construction Of The EVCLR - The
design considerations for an electrically-varlable current
limiting reactor (EVCLR) are:
Nominal system voltage;
3 0 Rated current;
Inductance required at rated operating current;
Inductance re~[uired at cne-half of rated
operating current;
Maximum temperature rise of the EVCLR;
WO 92/16
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20820~6 ~
26
Non-saturation of the inductor when the full
primary voltage is impressed across it;
The inductance over the general operating range
(from one-half to full operating current)
shall be Lnversely proportional to the
operating current, ensuring that the
inductance is nearl~ optimal; and
Distortion should be kept at a minimal level
over the entire operating range.
The design procedure for a representative EVCLR
is shown in Figures 6, 7 and 8. Figures 6 and 7 present
the general coll and core configuration of the device.
Two identical inductor windings 34 and 36 are mounted on
two cores 38 and 40 and connected in parallel as shown.
Alternating currents in the lnductor windlngs 30 result in
an alternating f lux in each core . The windings are
connected so that the instantaneous f lux coupled to the
control winding, which is common to both cores, is always
zero. Hence, if everything is bAlAr~ d, there is no
induced voltage in the control winding. In actual
practice, the center leg of the core can be magnetically
coupled. ~wo separate core structures are not reguired.
A magnetomotive force caused by DC current in
the control windlng 3 2 does, however, cause e~ual magnetic
drops in both cores 38 and 40. These drops cause changes
in reluctance of the magnetic paths and hence changes in
inductance. As such, the inductance value of the device
is a function of the magnitude of the direct current in
the control winding 32.
It should be noted that the E~CLR as illustrated
is two inductors in parallel, each of which conducts half
of the load current . Each individual inductor, theref ore,
must be designed f or twice the reguired inductance and
half of the rated current.
WO 92/16302
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27 2~2056
The EVCLR must be ~tF~c; qn~d not to saturate when
the full primary voltage Ls impressed across it. During
sparking, the full primary voltage appears across the
EVCLR. In this example, the maximum AC flux density will
5 therefore be limited to 16 kilogauss (one kilogauss equals
1000 lines of flux per square centimeter) at full primary
voltage for M-6 29-gauge electrical steel. This density
( B ) can be calculated as f ollows:
B=3875Ep/NAf
10 where Ep is the system primary voltage, N is the number of
turns, A is the inductor core area in square inches, and f
is the line frequency in Hertz ~cycle per second).
The individual inductors must be ~t~ nf~d f or
half the maximum continuous current expected.
Generally, a 110-degree Celsius (C) temperature
rise is acceptable for this type of device. For a 110-
degree rise, it is important to use a 180-degree
insulation system. This allows for a rise of llO-degree
rise above a 40-degree ambient temperature as well as a
20 30-degree "hot spot". For higher ambient temperatures,
adj ustments must be made ln the design .
The choice of ~1 ; or copper f or windings is
entirely discretionary. If ~1 t is used, a current
density of approximately 1000 amps (A) per square inch is
25 a good starting point. For copper, the figure should be
1450A/in2. Coil watt-densities for either conductor
should be approximately 0 . 4 watts per square inch at 20
degrees Celsius . It should be noted that signif icant
losses will occur in the windings owing to fringing around
3 0 the gaps under the inductor windings .
The general requirement for inductance for the
example EVCLR will be 1. 5 x mH at rated current and 3 . 0 x
mH at one-half of rated current, providing a desirable and
usable control range.
WO 92/16302
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2 0 ~ ~ 2 8
To accomplish "automatic control", the AC line
current in the lines is transf ormed to a suitable level,
then rectified. This DC signal is supplied to the control
winding of the EVCLR. The DC signal has little ripple
because of the high inductance inherent in the control
winding. The control current is therefore proportional to
the average of the primary load current. However, it
should be noted that the control current is proportional
to the R~S of the load current only if the form factor
remains constant. To operate effectively, the EVCLR must
also be operated in the more linear portion of its range
as shown in the graph in Figure 8. As illustrated, the
design range for the example inductor must be
approximately 4 to l . The inductance will, theref ore, be
four times as high with no control current ( 0 amps ) as it
is when the device is fully saturated.
To meet the above requirements and still ensure
low harmonic distortion, the inductor is constructed with
two dif f erent air gaps . Figure 8 shows the general
construction used. Each of the pair of inductors has two
large air gaps and two small ones.
The general design criteria are:
AX/AU--2 . 4
lc/lgx~60
lc/lgu~500
where lc is the mean length of the magnetic path ( steel),
lgx and lgu are the lengths of the air gaps in the X and U
portions of the core, respectively, and Ax and Au both are
the area of steel in the x and u portions, respectively.
The inductance range can then be calculated to
be from sectiD~s u and X both being completely unsaturated
(high relative p~rlT~'~Ahi 1~ ty) to section U being completely
saturated. In this condition, it is as if ~ section u does
not exist.
WO 92/163
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29
Derivation of the design equations proceeds in
this manner:
L = N~/I
~ = NI/R
where N is turns, ~b is total f lux lines, I is current and
R is reluctance
( magnetic resistance )
by substitution:
L = N2/R
for an air gap, iron-core circuit;
R = ( lC/~Uo~rAc ) + ( lg/iUoAC )
where lc is the core mean length, lg is the air gap
length, JUO is the p~ -h;l;ty free space (3.19 x 10-8
H/IN" ), iur is the relative permeability of
steel, and Ac is the core area
Thus, the general inductance equation:
L=(3.19xlO=8)N2Ac/[lc/~ur)+lg]
For the purposes here, ,ur will be con~ red either very
high ( inf inite ) or very low ( zero ) .
The inductance equation can then be simplified
to
L=( 3 . l9xlO=8 )N2/lg/AC
This equation will be used to calculate the two
extreme conditions of inductance: Section U completely
saturated, and sectlon u not saturated.
Let Ru=lgu/Au
and Rx=lgx/Ax
Since reluctance in magnetic circuits is
analogous to resistance in electrical circuits,
3 o RT=RuRx/ ( Ru+Rx )
The high and low inductance limits are now
calculated using the following equations in conjunction
with previously-cited general criteria:
~in=(3-19X10 8)N /Rx
WO 92/1
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,
2082aS~ 30
where
Rx=lgx/Ax
and
LmaX=( 3 . l9x10-8 )N2~RT
where
RT=RURX/ ( RU+Rx )
= ( lgulgx/Au~x ) / [ lgU/AU ) + ~ lgX/AX ) ]
It is important to recall that the goal of this
sequence is to achieve a relationship f or the example
10 inductor wherein Lmax is 4 x Lmin~
The control winding must be ~ nP~l and matched
to the primary load current with several factors,borne in
mind:
Temperature rise of control wlnding;
Correct ampere-turns for proper full-current
inductance; and
AvaLlable current transformer.
Design assumptions:
Load current f orm f actor of 1. 2 .
For 100 degrees (C) temperature rise, 0.55 watts
per square inch at 20 degrees should be used on the
control winding.
The DC current should be calculated by using:
B = ( 0 .155NIDC) / ~ 313lgu)
Use B - 20 kilogauss.
While particular ~mhor~; ts of this invention
have been shown in the drawings and described above, it
will be apparent, that many changes may be made in the
form, arrangement and positioning of the various elements
30 of the combination. In con~ ration thereo~ it should be
understood t~at preferred ~mhQ~ ts of this invention
disclosed herein are intended to be 1~ l ustrative only and
not intended to Iim~t the scope of the invention.
.
