Background of the Invention

Field of the Invention

The present invention relates to inductors, and more particularly to inductors for use in high frequency, high voltage circuits, such as, for example, the output stage of the power supply circuit for an electrostatic precipitator.

Description of the Related Art

Electrostatic precipitators have taken on considerably greater importance in recent years, particularly in view of the increased emphasis upon maintaining a clean environment. That increased importance includes the need for more effective air pollution control by maintaining clean exhausts from industrial processes that involve either the combustion of fuels or the reaction or transformation of materials in

chemical processing operations that result in the generation of particulate matter as a consequence of carrying out the process. The techniques and structural elements incorporated in modern electrostatic precipitators, particularly the electrical control apparatus for controlling the power provided for imparting a charge to the particulate matter to be collected, as well as the power provided to the collection surfaces, have been continually refined to more completely remove undesirable particulate materials from stack gases and also to provide longer useful operating life for the precipitator

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components. The stack gases in connection with which electrostatic precipitators are often necessary to meet environmental regulations include chemical process exhaust gases, fossil fuel electric generating plant exhaust gases, and exhaust gases from steam generation boilers, such as those commonly associated with paper mills for processes such as paper web drying, where scrap "black liquor" from wood processing operations and other fossil fuels are often the fuel sources.

The theory behind the operation of an electrostatic precipitator involves the generation of a strong electrical field through which stack gases pass, so that the particulates carried by the stack gases can be electrically charged. By charging the particles electrically they can be separated from the gas stream and collected, and thereby not enter and pollute the atmosphere. The generation of such electrical fields requires electrical power supplies that can provide a high DC voltage to charge the particulate matter and thereby permit its collection. The existing systems are most often based upon AC corona theory, using a single phase transformer-rectifier set to

rectify AC power to DC power and provide a high DC potential between a charging

electrode and a collection surface, usually a plate, to charge the particles by subjecting the stack gases to the maximum possible current without complete breakdown. That approach is believed to produce the maximum ionization of the particles, and thereby provides the maximum efficiency in effecting removal of such particles. The emphasis in particulate removal is generally placed on increasing the current flow between a grid and a plate defining the electrostatic precipitator collection surfaces, to a current level that produces a maximum of sparking between

the grid and the plate. In fact, some precipitators incorporate a grid structure that contains barbed wire or special pointed rods, specifically to enhance such sparking. The sparking inside a precipitator is believed to be necessary as an indicator that the maximum possible current is being drawn, and therefore that the maximum possible ionization of the gases and particles is taking place. In fact, the practice of encouraging sparking is emphasized, even though it is known that sparking produces stresses upon the electrical components of the system, it causes increased precipitator maintenance because of the production of agglomerated particles, sometimes called, "ash balls" or "klinkers," and it also causes difficulty in insuring that the "rappers," which are devices that vibrate the precipitator plates to remove collected particles, are, in fact, operative and are removing collected particulate material.

A problem that results from operating an electrostatic precipitator at levels at which sparking occurs is the prevention of damaging arcing. An automatic controller for the input power to the transformer-rectifier set must sense incipient arcing and

immediately reduce the voltage on the precipitator collector plate, because any spark can quickly create an arc between the plate and the electrode, with a resultant high current flow. The high current flow can cause severe damage to the precipitator grid or plate. Additionally, arcing can cause the transformer-rectifier set to fail, it can cause the controller to fail, or it can open the overcurrent protectors that are provided in the incoming power line. Any of those incidents will cause a section of the

precipitator to be temporarily off-line, with the resultant undesirable passing of greater amounts of particulates into the atmosphere until the damage to the precipitator has

been repaired. Repair can be a matter of minutes, or it can be weeks if the transformer-rectifier set or controller has to be replaced.

Heretofore, the prevention of arcing has been attempted by providing complicated sensing and control circuits that add expense to the cost for an electrostatic precipitator. Examples of such circuits are shown in U.S. Patent Nos. 4,290,003, which issued on September 15, 1981, to Philip M. Lanese; 4,390,831, which issued on June 28, 1983, to William Byrd et al.; 4,587,475, which issued on May 6, 1986, to James A. Finney, Jr., et al.; and 5,255,178, which issued on October 19, 1993, to Guglielmo Liberati. However, the presently available circuits, although effective to some degree, still permit sparking and arcing to occur, thereby requiring more frequent periodic maintenance of the precipitator to repair the damage that is caused by such sparking and arcing. Maintenance involves down time for the precipitator.and usually for the system in which the precipitator is installed, thereby increasing the cost for producing the product of the system in which the precipitator is employed. In many electrostatic precipitators sulphur trioxide or ammonia, or both, must be injected into the gas stream in order to keep the opacity of the stack gases as low as possible. However, the use of such gases is undesirable because of their caustic nature, that over time causes damage to the precipitator and to the stack, again necessitating repair and consequent down time of the process or equipment with which the precipitator is employed.

It is an object of the present invention to provide a more uniform electrostatic precipitator output voltage, having reduced voltage ripple and high frequency energy

to reduce the occurrence of sparks and arcs, and thereby reduce the frequency of

precipitator maintenance and downtime.

It is a further object of the present invention to provide apparatus that can be readily incorporated into existing electrostatic precipitators to improve their efficiency of operation by reducing the occurrence of sparks and arcs.

It is another object of the present invention to provide apparatus that helps to more efficiently and more effectively reduce the opacity of stack emissions from coal- fired, and other fossil fuel boilers, by reducing the amount of caustic gases that might be required to meet air quality limits and to enable such devices to less expensively meet opacity level maximums specified by regulatory agencies.

Summary of the Invention

Briefly stated, in accordance with one aspect of the present invention, an improved inductor is provided for incorporation into the output power circuit of a standard transformer-rectifier set as employed in an electrostatic precipitator. The inductor can be positioned between the rectifier output and the precipitator electrodes, either with or without one or more additional filter elements.

The improved inductor device in accordance with the present invention is a two-stage device that includes a body member having an input terminal and an output terminal that is spaced from the input terminal. A first inductor element for blocking

current ripples in a first, low-to-moderate frequency range is carried by the body member, the first inductor member having a first terminal and a second terminal, and

wherein the first terminal of the first inductor element is electrically connected with the body member input terminal.

A second inductor member is also carried by the body member. The second inductor is positioned between the first inductor member and the output terminal of the improved inductor device and downstream of the first inductor member for blocking voltage and current changes in a second, moderate-to-high frequency range.

The second frequency range is important because it operates to reduce the source of the energy that is necessary for sparks to form, and for sparks to reach the magnitude

of an arc. The second inductor member also has a first terminal and a second terminal.

T e first terminal of the second inductor member is electrically connected with the second terminal of the first inductor member, and the second terminal of the second inductor member is electrically connected with the body member output terminal. In accordance with another aspect of the present invention, the two-stage inductor device is incorporated in the high voltage output side of a precipitator power supply between the transformer/rectifier set and the precipitator electrodes, and is coupled with a choke input filter, which can be a T-type filter including choke coils

and a capacitor.

Brief Description of the Drawings Figure 1 is side elevational view of an inductor in accordance with the present

invention.

Figure 2 is a transverse cross-sectional view of the inductor shown in Figure 1, taken along the line 2-2 thereof.

Figure 3 is a longitudinal cross-sectional view of the inductor of Figure 1, taken along the line 3-3 thereof. Figure 4 is a circuit diagram showing the output portion of a power supply circuit for an electrostatic precipitator from a transformer-rectifier set to a precipitator electrode, in which an inductor in accordance with the present invention is installed before the precipitator electrodes along with a T-type filter.

Description of the Preferred Embodiments Referring now to the drawings, and particularly to Figures 1, 2 and 3 thereof, there is shown an improved high voltage inductor assembly 10 in accordance with the present invention. Inductor 10 is a two-stage inductor and includes a hollow elongated body member 12 that carries on its exterior a first inductor member 14, and that carries within its interior a second inductor member 16 (see Figure 3). Second inductor member 16 is spaced longitudinally along body member 12 from first inductor

member 14. A lead wire 18 extends between output terminal 20 of first inductor member 14 and an end of a conductor 22 that extends interiorly within body member

12 to support second inductor member 16 to provide an electrical connection between first and second inductor members 14 and 16 to connect them in series. Body member 12 is an elongated carrier that is shown in the form of a hollow tubular member, such as a hollow cylinder. Although shown as of cylindrical cross

section in Figure 2, body member 12 can be of any convenient cross-sectional shape, so long as it is capable of firmly carrying and supporting both first inductor member 14 and second inductor member 16. Preferably, body member 12 is made from an electrically non-conductive material that is capable of withstanding high ambient temperatures of the order of at least about 260°C without deformation or collapse. In that connection, it has been found that epoxy-impregnated fiberglass tubing provides the necessary physical attributes for satisfactory operation in connection with the present invention. Other materials are also suitable, as will be appreciated by those skilled in the art. A pair of electrically conductive terminal connectors are securely carried at

each end of body member 12, to permit connection of the two-stage inductor with the associated precipitator power supply circuit components, as will be hereinafter described. Typically, an input terminal connector 24 and an output terminal connector 26 are securely carried at the respective ends of body member 12. Terminal connectors 24 and 26 can be made from brass, or from other electrically conductive materials, usually metallic materials, and can provided in the form of unions of

appropriate configuration, such as standard brass pipe unions, to firmly physically and electrically engage with the circuit elements with which the inductor member is connected. When provided in the form of brass unions, terminal connectors 24 and 26 can be soldered or brazed to copper conductors (not shown). The precise form of terminal connector will be dependent upon the nature of the cable or other components to which the inductor of the present invention is to be connected. The

terminal connectors provide a convenient method for connecting the inductor with the transformer/rectifier precipitator bus, which typically includes V* inch unions carried by the bus and the inductor so that a simple mechanical connection can be made the also effects the electrical connection. First inductor member 14 is a high-voltage, air core inductor that is of toroidal overall form and is defined by a multiplicity of turns of a continuous length of wire. The wire winding defining inductor member 14 is wound circularly about the central longitudinal axis passing through the central opening of the resulting toroid, in overlapping relationship, and it is securely carried on the outermost surface of body member 12, adjacent to input terminal connector 24 for convenient electrical connection thereto. Input terminal 28 permits electrical connection of inductor member 14 with input terminal connector 24 by a lead wire 30, or the like.

From the standpoint of inductance, first inductor member 14 can have an inductance that varies from 0.5 henry to over 25 henries, depending upon the

particular power level for the circuit in which the inductor assembly is connected. In terms of size, first inductor member 14 generally ranges from three to five inches in width, and up to ten inches in outer diameter when formed from insulated copper wire that is wound to a large number of turns to provide the desired inductance to prevent the formation of spark leaders, sparks, and arcs in the particular precipitator circuit. Additionally, the multiplicity of turns of wire should be capable of carrying the required current without overheating because of I R loss, and they are preferably

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vacuum-impregnated with a high temperature epoxy material of a type well known to those skilled in the art.

Second inductor member 16 is carried within the interior of body member 12, as more clearly seen in Figures 2 and 3. As shown, second inductor member 16 is defined by a plurality of end-to-end ferrite beads 32 having a generally tubular configuration. Each of ferrite beads 32 is carried on copper conductor 22 that is

substantially coaxially supported within the interior of body member 12 by means of a forward support member 34 of generally disk-like form that can be retained within tubular body member 12 by means of a press fit, for example, and output terminal connector 26. Conductor 22, which can also be provided in the form of a brass rod, is substantially aligned with the axis of tubular body member 12 between output terminal connector 26 and support member 34. Between support member 34 and first inductor member 14 conductor 22 passes transversely through the wall of body member 12 to be electrically connected with an end of lead wire 18 so that first and second inductors 14 and 16 are electrically connected in series and with each of input terminal connector 24 and output terminal connector 26.

Each of the ferrite beads 32 includes a generally centrally-provided axial passageway to permit the beads to be carried on the outer surface of conductor 22, which passes centrally through each of the beads to support the beads in axially aligned relationship. The number of ferrite beads 32 can be varied to obtain the total amount of ferrite to produce the inductance needed to provide the circuit impedance

to offset the ripple current that is present in the output of the power supply for a particular electrostatic precipitator.

In one embodiment of the present invention, the beads defining second inductor member 16 have an inner diameter of V* inch, an outer diameter of ³ /β inch, and a length of lVβ inches. As shown in Figure 3, eleven such ferrite beads are positioned in axially aligned, end-to-end, contacting relationship on conductor 22. Preferably, conductor 22 is a solid copper conductor having an outer diameter of slightly less than about V* inch, for slidably receiving each of ferrite beads 32 thereon, and for supporting the several ferrite beads within body member 12. In that regard, as much ferrite as necessary to provide the correct inductance should be carried in the second inductor member without reaching the saturation limit for the direct currents that pass through the device, and for those alternating currents of different frequencies that also pass through the device. Preferably, the ferrite beads have the structure and composition as disclosed in U.S. Patent No. 2,594,890, which issued on April 29, 1952,

to W.B. Ellwood, the disclosure of which is incorporated herein by reference to the same ^" extent as if fully rewritten. The Ellwood patent provides an expression for calculating the inductance in henries for a conductor that carries a ferrite bead, wherein the inductance is based upon the size of the cylindrical ferrite bead and the

permeability of the particular ferrite material. As shown in Figure 3, the end of conductor 22 that is closest to input terminal connector 24, and that extends axially of body member 12 for a small distance beyond forward support member 34, passes transversely through an aperture 35 provided in

the wall of body member 12. Aperture 35 is at an axial position, along the axis of body member 12, that lies between first inductor member 14 and second inductor member 16, and permits electrical connection of conductor 22 with output terminal 20 of first inductor member 14 by means of lead wire 18. In that regard, lead wire 18 can be a high-voltage silicon lead wire, which is a preferred form of lead wire for use as part of the inductor in accordance with the present invention when it is installed in an electrostatic precipitator, to withstand the environmental operating conditions of such an inductor member when so employed. If desired, and instead of passing conductor 22 through aperture 35, lead wire 18 can have a sufficient length to permit it to pass through aperture 35 for direct connection with forward support member 34, to provide the electrical connection between first inductor member 14 and conductor 22.

As seen in both Figures 2 and 3, the outermost cylindrical surfaces 36 of ferrite beads 32 are preferably spaced from the innermost cylindrical surface 13 of body member 12 to provide an air gap therebetween, and to permit dissipation of any condensation that might occur within second inductor 16. Additionally, the ferrite beads defining second inductor member 16 are preferably carried within body member 12, which because it is non-electrically conductive, avoids a possible shorting path that could result between ferrite beads 32 and adjacent structural elements. In that connection, because the ferrite beads are either reflecting or absorbing high frequency

voltages, a voltage drop will develop across each bead and therefore the interposition of a non-electrically conductive material is desirable, because if a short circuit were

to exist between the ferrite beads and adjacent structure, the effect of the ferrite would be eliminated.

Referring once again to Figure 3, the end of conductor 22 that is most distant from first inductor member 14, and that is closest to connector 26, is electrically connected with output terminal connector 26. From a structural standpoint, connector 26 can include a transversely extending wall member 38 to which conductor 22 is connected in order to maintain conductor 22 in spaced relationship relative to inner surface 13 of body member 12. As shown, wall member 38 is a part of connector 26, although it can be a separate and independent supporting arrangement, if desired, so long as there is electrical connection between conductor 22 and connector 26.

When inductor assembly 10 is installed as an element in a power-carrying conduit, first inductor member 14 provides an impedance sufficient to block or reduce low to moderate frequency voltage fluctuations (from about 1 Hz. to about 10 KHz.). Second inductor member 16 provides an impedance sufficient to block moderate and high frequency voltage fluctuations (from about 10 KHz. to about 10 MHz.). Such voltage fluctuations can be found in the power output provided by a transformer/rectifier set of an electrostatic precipitator power supply.

When installed in the output side of the power supply circuit for an

electrostatic precipitator, inductor assembly 10 operates to block (by reflection or

absorption) or to reduce voltage ripples that would otherwise flow from the output of

the transformer/rectifier set to an electrostatic precipitator electrode, and that could lead to sparking, and possibly damaging arcing. Typically, ripple voltages in such

devices are about 50% of the nominal voltage (or 25 kv. of voltage ripple for a nominal 50 kv. transformer/rectifier set), and the present inductor invention operates to reduce that ripple voltage by up to about 50%, to provide a significant reduction in precipitator operating cost and maintenance, as well as a significant reduction in stack gas opacity. When installed in the power supply circuit for an electrostatic precipitator, it is preferred from an efficiency of operation standpoint that the two- stage inductor in accordance with the present invention be placed as close to the precipitator electrode as possible.

One type of circuit in which the two-stage inductor assembly 10 in accordance with the present invention can be advantageously utilized is shown schematically in Figure 4. As shown, a transformer/rectifier set 40 includes a high-voltage transformer 42 that has its output connected with a high-voltage rectifier bridge 44. One output terminal 46 of bridge 44 is grounded and the other output terminal 48 is electrically connected with a high voltage electrostatic precipitator electrode 50, to provide with

grounded electrode 52 a field through which combustion or other process gases are passed and from which particulates are desired to be separated.

Immediately before precipitator electrode 50 in the power supply circuit is a

two-stage inductor assembly 10 in accordance with the present invention. Optionally,

the connection between inductor assembly 10 can be effected through a T-type filter 54, which provides further reduction of the ripple component of the output voltage

from transformer/rectifier set 40. Filter 54 can include a pair of series-connected inductors 56, 58 with a capacitor 60 extending from a point between the two inductors

to ground. Inductors 56 and 58 can have an inductance of from about 1 to about 25 henries, and capacitor 60 can have a capacitance of from about 0.1 to about 25 μfd. It will be recognized by those skilled in the art that the schematic diagram shown in Figure 4 is a vastly simplified circuit diagram. Examples of the circuit elements sometimes provided between a transformer/rectifier set and electrostatic precipitator electrodes are shown in U.S. Patent No. 4,996,471, which issued on February 26, 1991, to Frank Gallo. However, inductive elements l^ and L ₄ as shown in the Gallo '471 patent are commonly provided by the manufacturer of the

transformer/rectifier set, and they provide no filtering but serve only to limit output current peaks to a value deemed "safe" based upon peak current surge rating of the rectifiers CR2, CR4, CR6, and CR8.

The incorporation of a two-stage inductor assembly in accordance with the present invention in the output portion of the power circuit of a transformer/rectifier set in an electrostatic precipitator has been found to greatly reduce particulate

emissions resulting from fossil fuel combustion in boilers, and the like. For example, in actual testing in an electrical generating station, the incorporation into the power

output portion of the circuit of a two-stage inductor assembly having the construction shown in Figures 1 through 3 was found in actual operation to provide a reduction in

the stack gas opacity of from about 20% to about 13%. At the same time, the power required to operate the precipitator was reduced by about 40%. Moreover, because the inductor assembly in accordance with the present invention provides substantial smoothing of the output voltage from the transformer/rectifier set, arcing was

significantly reduced, resulting in an increase in effective collection time for the precipitator. Additionally, downtime for the generating plant caused by arcing was also reduced, thereby improving both the operating efficiency of the generating plant and also the particulate separation efficiency of the electrostatic precipitator. Furthermore, in actual commercial practice many electrostatic precipitators must have sulfur trioxide and ammonia injected into the combustion product stream in order to assist in maintaining the stack gas opacity as low as possible. The present invention can reduce the need for the injection of such caustic gases, thereby reducing the cost for plant operation, and reducing the otherwise necessary maintenance resulting from the use of caustic gases.

In addition to the beneficial results flowing from the incorporation into an electrostatic precipitator of the two-stage inductor assembly in accordance with the present invention, it has been found that the addition to the power output circuit of a T-type filter as shown in Figure 4 results in an additional reduction in the AC ripple current and voltage, thereby further improving the collection efficiency and power consumption of the precipitator, while simultaneously reducing the tendency toward

sparking and arcing. In that regard, filter 54, in addition to the T-type filter arrangement shown in Figure 4, can also be a filter of the Pi, series, parallel, or tank

circuit type. With a properly chosen filter, ripple voltage can be reduced by two to five times or more as compared with the reduction obtained by installing the two-stage inductor assembly alone. Typical ripple voltage with inductor assembly 10 and filter

54 installed is reduced from about 25,000 volts peak to peak to less than about 10,000 volts peak to peak in a 50 kv. transformer/rectifier set.

It will be apparent to those skilled in the art that various changes and modifications can be made without departing from the spirit of the present invention. Accordingly, it is intended to encompass within the appended claims all such changes and modifications that fall within the scope of the present invention.