DETAILED DESCRIPTION OF THE INVENTION

[0031] The present invention relates to removing the electric field from the internal volume of high voltage interconnects, such as bushings, connectors, and cables. The space of interest, for example, is that between the center conductor and the solid dielectric insulator that normally provides the primary electrical insulation from the outer conductor. This invention provides a conductive surface on the inside surface of the principal solid dielectric insulator surrounding the center conductor and connects the center conductor to this conductive surface. This simple approach removes the electric field across an inherently weak region and places it in the primary insulation. The advantage of removing the electric fields from the weaker dielectric region to a stronger area improves reliability, increases component life and operating levels, reduces noise and losses, and allows for a smaller compact design. While various techniques can be utilized to metalize desired regions, such depend, for example, on material, size, thickness, and surface accessibility, and the simplest method uses a standard electroless plating technique. Other known metalization techniques include, but are not limited to, vapor and energetic deposition, plasma spraying, conductive painting, and other controlled coating methods. The metalization approach in accordance with the present invention can be easily applied to existing high-voltage interconnect designs, and such receive an instant reduction of corona activity and related effects.

[0032] FIGS. 1A and 1B illustrate cross-sectional views of a generic high-voltage bushing, 100, having a cylindrically symmetric design and generally used to insulate electrical conductors passing through metal walls of protective enclosures. A generic embodiment is used to show common features in bushings and to illustrate the essence of this invention as it applies to all highly electrical-stressed structures. The bushing insulator is typically constructed of a single continuous solid dielectric insulator material, 102, having an external (upper) section and an internal (lower) section. An external connection point, 101, is the high-voltage input to or output from an arbitrary high-voltage device located inside an enclosure, 112. The external solid dielectric insulator, 102, provides the primary electrical insulation between the internal bushing conductor, 103, and the enclosure, 112. The external solid dielectric insulator, 102, is typically shaped to prohibit surface flashover and provide structural support of the internal bushing conductor, 103, of the bushing, 100. Internal bushing conductor, 103, provides the electrical connection from external connection point, 101, to the internal connection point, 106. Internal bushing conductor, 103, can be constructed of rigid metal or flexible cable. The enclosure, 112, is typically at ground potential. In the case of oil encapsulated systems, such as transformers, a metalized coating, 104, on the outer surface of the lower or internal solid dielectric insulator, 102, will extend the ground potential well inside the enclosure, 112, and below the oil fill line, 113. This metalized coating, 104, eliminates the triple-junction that would otherwise be created by the internal or lower section of the solid dielectric insulator, 102, as it penetrates the enclosure, 112, through the enclosure flange, 110, and relocates the triple point region, 114, to a higher dielectric strength environment of oil. The upper end flange, 107, and the lower end flange, 111, support the internal bushing conductor, 103, and provide a means to attach input and output connections. The void region, 108, extends between the solid dielectric insulator, 102, and the internal bushing conductor, 103, constituting a region of weaker dielectric material. In current art, this weaker dielectric region becomes a troublesome source of electrical corona discharge activity that reduces the reliability and operating level of the high-voltage bushing.

[0033] In the present invention, a metalized surface, 109, is applied to the inner surface of the cylindrically shaped solid dielectric insulator, 102. This metalized surface, 109, includes end sections which provide an electrical connection between the upper end flange, 107, the lower end flange, 111, the internal bushing conductor, 103, the external connection point, 101, and the internal connection point, 106, assuring that they are all at the same electrical potential. In this fashion, the void region, 108, is completely surrounded by metallic surfaces at the same potential, therefore no electric field can exist within the void region, 108, and thus there is no initiating source of electrical corona activity in this region. FIG. 1B shows symmetry between the external connection point, 101, of bushing conductor 103; the void region, 108; the metalized surface, 109; and the solid dielectric insulator, 102.

[0034] FIGS. 2A and 2B illustrate an expanded view of a bushing similar to that of FIGS. 1A-1B, showing the internal section of a high-voltage bushing, 200, below the oil-fill line, 113, without the improvement called out in this invention. This is representative of current art and depicts equipotential lines developed in the void region, 204. The internal bushing conductor, 201, and the lower end flange, 202, are electrically connected and provide for one of the two electrodes defining the equipotential lines developed in the void region, 204. The electrically opposing electrode is extended below the oil line, 113, by a metalized outer surface, 205. Fields are distributed across the void region, 204, and the solid dielectric insulator, 207, as a relationship of their discrete capacitance. The resulting equipotential lines developed in the void region, 204, reach the intrinsic breakdown level of the ambient air or oil before the intrinsic level of the solid dielectric insulator, 207, is reached. As a result, the internal section of the high-voltage bushing, 200, goes into electrical corona activity on the inside of the bushing opposite the triple point region, 206, or as it penetrates the enclosure, 112, opposite the enclosure flange, 110 (see FIG. 1A). When the exposed insulator surface, 208, is of sufficient length to inhibit the surface breakdown; electrical corona activity on the inner surface of the high-voltage bushing, 203, erodes the solid dielectric insulator, 207, until the unit catastrophically fails.

[0035] FIGS. 3A and 3B illustrate the internal section of and improved high-voltage bushing, 300, as an expanded view of FIG. 1A showing the bottom section of a typical high-voltage bushing, 100, below the oil fill line, 113 (see FIG. 1A). The internal bushing conductor, 301, lower end flange, 302, and the metalized inner surface of the high-voltage bushing, 303, are electrically connected by being in direct contact and are therefore at the same electrical potential as stated in the above description. The metalized outer surface, 305, of the solid dielectric insulator, 307, defines the lower section of the high-voltage bushing, 100, of FIG. 1A. The dashed lines in FIG. 3B indicate equal-potential gradient lines and are shown to be in the stronger dielectric material of the bushing solid dielectric insulator, 307, and not in the weaker dielectric material filling the void region, 304. The solid dielectric insulator, 307, is considered to be made of homogenous material without significant flaws or imperfections. The metalized inner and outer surface, 303 and 305, respectively, on portions of the cylindrical high-voltage bushing, 303, provide controls on the shape and density of the electrostatic field lines within the solid dielectric insulator, 307, between the two surfaces. When the metalized outer surface 305, ends at the triple region, 306, the electric fields within the solid dielectric insulator, 307, extend out into the surrounding oil environment, as governed by the geometric shape of the enclosure and nearby objects, as well as the relative dielectric constants. The triple point region, 306, will have the highest field-enhancement and is the likely point from which electrical failure will occur. The reason electrical breakdown begins from this location relates directly to the rapid change in the equipotential lines at the sharp edge of the metalized outer surface, 305. When the electric field stress at the triple point region, 306, exceeds the strength of the oil environment, electrical will occur in proximity to the insulator and an arc will develop across the exposed insulator surface, 308, and make attachment to the lower end flange, 302, resulting in an electrical failure that will likely cause permanent damage to the solid dielectric insulator, 307.

[0036] FIGS. 4A and 4B show the internal section of an improved high-voltage bushing, 400, previously shown in FIG. 3A. The improvement is realized by contouring the solid dielectric insulator, 407, in the vicinity of the triple point region, 306, as shown in FIG. 3A. The metalized region of the inner surface, 403, and metalized outer surface, 405, confine the radial electric field inside the solid dielectric insulator. A gradual increase in the electric field inside the body of the insulator, 407, is caused as the embedded electrode, 409, reshapes the internal fields, however, the electric field stress need not exceed the electrical strength of the insulator material, 407. The embedded electrode, 409, does serve to reduce the electric field enhancement at the triple point region, 406. As previously stated, the initiating mechanism for breakdown depends on the field-stress at this triple-junction and the reduced electric field provided at this point by the contoured embedded electrode, 409, improves reliability and operating levels. The contoured embedded electrode, 409, is designed to have a slope of about 135 degrees with relief to the metalized outer surface, 405, to allow air bubbles that might otherwise be trapped in the recessed portion of the embedded electrode, 409, to float up and away from the triple point region, 406.

[0037] Several methods of metalizing dielectric surfaces are possible. The preferred method of metalizing the surfaces 109, 303, 403, and 409 of the bushing insulator 102, 307, and 407 employs a strictly chemical process for plating copper and copper alloys on surfaces exposed in a chemical bath, also known as electroless chemical plating. Other methods for metalizing surfaces of ceramic dielectric materials can be employed, such as electroplating, depositing and baking metal-based inks, physical vapor deposition, chemical vapor deposition, plasma-assisted vapor deposition and plasma spraying of metal powders in a vacuum chamber, conductive painting, and other controlled coating techniques. Other material for metallic coatings can be employed, such as aluminum, nickel, silver, platinum, palladium, and gold, as well as alloys thereof. The metalized surface may have a thickness in the range of atomic level of the material to a millimeter. The dielectric material may be composed of ceramics, plastics, glass, fiberglass, mica, and rubber. Also, the conductive material may be of semiconductor materials, such as cuprous oxide, germanium, gallium arsenide, gallium phosphide, indium arsenide, lead sulfide, selenium, silicon, and silicon carbide.

[0038] Another method for passing electrical current through conductive walls of enclosures is by connectors. FIG. 5 shows a representative coaxial connector, 500, of an improved design in a typical configuration, such as a vacuum feedthrough. The outer conductive shell, 501, of the connector attaches to the enclosure wall, which is typically grounded. The primary insulator, 502, is typically constructed of a solid dielectric material, such as plastic or ceramic. The void region, 503, between the insulator, 502, and the center conductor, 504, is typically filled with gas or liquid from the ambient environment, which is most often a weaker electrically insulating material than the stronger solid dielectric insulator, 502. A vacuum seal, 505, attaches the center conductor, 504, to the insulator, 502. The inner surface of the insulator, 502, is metalized from the vacuum seal, 505, to the region of 507. The result is that the center conductor, 504, and inner surface of adjoining dielectric, 506, are electrically connected, thereby ensuring that no electric field exists in the void region, 503, of weaker dielectric material.

[0039] FIG. 6 illustrates an expanded or enlarged view of the region 507 of FIG. 5 and of controlling stress in coaxial type connectors, 600. The outer conductive shell, 601, provides the termination of the connector to the enclosure wall, as mentioned above. In addition, a vacuum seal is required between the outer conductive shell, 601, and the insulator, 602. The increased thickness of the insulator, 602, adjacent to the vacuum region, 605, provides added electrical strength to this section of the insulator. The vacuum triple-point region, 606, defines the highest region of stress in this vicinity. The center conductor, 603, and the inner surface region of metalization, 604, as mentioned above, are electrically connected, and, as such, remove the electrical fields in the weaker ambient region, 609. Metalizing the inner surface, 604, of insulator, 602, as shown, also removes the electrical fields from the dielectric support 608. Properly ending the metalization 604, defines the ambient atmosphere triple-point region 607. The atmosphere triple-point region 607, represents the significant electrical field enhancement in this connector.

[0040] Typical connectors use plastics as the principal dielectric material. Some vacuum feedthrough connectors will use ceramics as an insulator to be more compatible with the vacuum environment. Connectors using ceramics can easily use the same metalization process as described for high-voltage bushings. Plastics can be more challenging in quality plating. Platable acrylonitrile-butadiene-styrene (ABS) plastic has been shown to be a very useful dielectric. Another widely used plastic for connectors is Teflon (polytetrafluorethylene), presently requiring energetic deposition techniques. One possible lay down process for conductive surfaces on Teflon is by controlled explosive copper wire techniques.

[0041] Another area for application of the present invention for improving performance of highly stressed electrical insulation structures includes improvement to metalized film and metalized film/foil capacitors. Multilayer film capacitors have several different construction techniques. Common to all current art capacitor structures involves alternating layers of conductive and nonconductive dielectric material. FIGS. 7A, 7B, and 7C illustrate cut-away views of a metalized film capacitor using the disclosed improved manufacturing technique. FIG. 7A and 7B show a cross-section of the first and second metalized dielectric film, 700 and 706. The dielectric films 701 and 707 are continuous high-quality film material of homogenous material. The metalization process involves a quality conductive surface applied to both sides of the dielectric film with a masked region on opposite and opposing sides to provide nonconductive regions 710. The top layer of the first dielectric film 701 extends to a summing electrode for the positive electrode 703. In contrast, the bottom metalized layer 709 of the second dielectric film 707 extends to the same summing electrode for the positive electrode 703. The opposing electrodes, defined as the lower metalization 704 of the first metalized dielectric film 701 and the upper metalization 708 of the second dielectric 707, both extend to the summing electrode for the negative electrode 705. A detail to the manufacturing process is that the tape defining each of these metalized film/foils is the same, and in the roll-up process one of the tapes is turned over so that the metalized surfaces of the two adjacent metalized surfaces are in the same direction. The effect of this is illustrated in FIG. 7C by placing the first, 701, and second, 707, dielectric films in close proximity to each other. The void region, 711, typical to the above-mentioned regions, represents the weak area of dielectric material. During the roll-up assembly process the adjacent metalized surfaces make electrical contact by physical pressure and remove the electrical field across this weak void region 711. The current art for this type of capacitor is to have metal on one side of the dielectric with alternating extension of the electrode to define the capacitor electrodes. In contrast to this invention, the effect of the current art assembly process traps weaker dielectric material without a removal mechanism, as shown in FIG. 7C. As part of this invention, the process of eliminating weak electrical regions of stress is obtained by having metalized surfaces surrounding these regions of concern.

[0042] FIG. 8 illustrates an example of a rolled up construction of an axial lead metalized film capacitor. Metalized surface, 801, on dielectric film, 804, and metalized surface, 802, on dielectric film, 803, are rolled adjacent to each other providing one of the capacitor electrodes. This process has removed electric fields that would normally be in this region and increases the reliability and operating levels. Similarly, metalized surface, 807, on dielectric film, 804, and metalized surface 808, on dielectric film 803, provide the other capacitor electrode. Similar connection to each of the extended electrodes is made with a conductive disk, 809. Surface areas, 805 and 806, provide insulation between the two electrodes defining the capacitor.

[0043] FIG. 9 shows the inclusion of a metal foil, 901 and 902, rolled between the two otherwise adjacent metalized surfaces. The purpose of the foil provides a lower internal resistance, allows ease of electrode extension over the edge of the dielectric film, and allows high-current operations.

[0044] The optimum metalization technique for the FIGS. 8 and 9 application is vapor deposition with controlled masking in desired areas. The process can be done on both sides during a single pass or on a single side in a double pass.

[0045] It has thus been shown that the present invention enables the removal of electric field in weak areas of high-voltage interconnects, such as bushing, connectors, cables, capacitors, etc. In contrast to prior attempts to remove electric fields in weak regions, the invention provides a conductive surface on the inside surface of the principal solid dielectric insulator surrounding the center conductor and connects the center conductor to this conductive surface.

[0046] The use of this invention in manufacturing and field management applies to all high-voltage interconnects. In addition to improved life of existing high-voltage interconnects, a reduction of corona induced noise is anticipated. Some of the immediate applications include: accelerators, high-voltage bushings in power supplies, high-voltage connectors, compact high-voltage sources, and highly-reliable high-voltage structures. The use of this invention will improve equipment life expectancy, provide added compactness for the same operating levels, lighten the environmental impact from alternative field management controls, and improve the electromagnetic capability of equipment. Uses include power grid components, such as transformers, feed-through bushings and switch gear, high-voltage power supplies, modulators, high-voltage capacitors and tubes, and space-based communication systems.

[0047] While particular embodiments, materials, parameters, etc., have been described and/or illustrated to exemplify and teach the principles of the invention, such are not intended to be limiting. Modifications and changes may become apparent to those skilled in the art, and it is intended that the invention be limited only by the scope of the appended claims.