Inductor devices – Combined – With connector
Reexamination Certificate
2003-06-09
2004-06-22
Nguyen, Chau N. (Department: 2831)
Inductor devices
Combined
With connector
C336S173000, C174S142000
Reexamination Certificate
active
06753750
ABSTRACT:
RELATED U.S. APPLICATIONS
Not applicable
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not applicable
REFERENCE TO MICROFICHE APPENDIX
Not applicable
FIELD OF THE INVENTION
This invention relates to a porcelain bushing for transformers and, more particularly, relates to means for improving the ability of such a bushing to withstand lightning impulse voltages rated at 45 kV and 1.2/50 &mgr;s form without external flashover or puncture.
BACKGROUND OF THE INVENTION
A bushing is a device that enables one or several conductors to pass through a partition such as a wall or a transformer tank, and insulates the conductor from it. Bushings commonly comprise an insulating bracket, a conductor, and provision for mounting the bracket to the transformer to insulate the conductor from the transformer surface. The bushings are used to hold the electrical conductors and transfer current from outside of the transformer to coils on the inside of the transformer without shorting out on the transformer surface.
Bushings are rated on how well they insulate the transformer wall from current flowing through the conductor. One factor that must be considered in the rating of bushing terminals is the presence of contaminants the bushing. Contaminants on the bushing provide a conductive path, which can cause electricity to leak out onto the transformer wall and cause electrical arcing or striking. Arcing occurs when the difference in potential between the transformer wall and the conductor becomes sufficiently large. The air ionizes between the transformer wall and the conductor, and this creates a path of relatively low resistance through which current can flow. The resulting blast of electricity can cause a short circuit to occur and severely damage the transformer.
Another factor used to rate bushings is electrical “creepage” across the insulating bracket. Creepage is the electrical leakage on a solid dielectric surface. Creepage distance is the shortest distance on a dielectric surface between two conductive elements. The current will essentially track or crawl across the insulating bracket onto the transformer wall. The onset of creepage can produce similar effects to contamination in that a short circuit to the transformer wall can occur and cause damage to the transformer itself.
One solution utilized to minimize these problems is the use of a material with a high insulating property such as porcelain to create the bushing. However, porcelain is not durable and is easily broken. A broken or damaged bushing requires costly replacement and down time of the transformer. Utilization of Ex-Mount bushing a t a higher voltage will result in higher incidents of arcing, striking, or creepage across the insulating bracket. The following are patents related to transformer bushings.
Shirakawa, et al. in the U.S. Pat. No. 6,466,425 provided an arrester excellent in a limit voltage characteristics which can withstand a contamination phenomenon of a porcelain bushing surface, and a surge absorption. Metal plates each having a thickness greater than a zinc oxide element portion does using high withstand voltage zinc oxide elements are arranged to suppress a rise in temperature of the zinc oxide elements. By employing the above means, it is possible to realize an arrester capable of suppressing a rise in temperature of the zinc oxide elements and which is low in a limit voltage and has higher performance.
Smith, et al. in the U.S. Pat. No. 6,140,573 disclosed an improved hollow core composite bushing for use in, for example, a dead tank circuit breaker, is smaller and lighter and offers other advantages over previous designs. The hollow core bushing includes therein a fiber reinforced plastic (FRP) elongated tube that operates to increase the dielectric constant between a conductor and a transition shield. By placing the transition shield external to the hollow core bushing and providing the FRP tube between the conductor and transition shield, the dielectric constant between the shield and conductor is increased thus allowing the inner diameter of the shield and outer diameter of the bushing to be decreased. In addition, the transition shield may be an integral part of the flange that is mounted to the bushing current transformer support column or interrupter enclosure of the dead tank circuit breaker thus further reducing the diameter and overall size of the bushing. Further, the transition shield maybe an integral part of the FRP tube.
Whited, et al. in the U.S. Pat. No. 5,153,383 disclosed a flexible sheet of material for mounting about a vertical or horizontal type porcelain insulator of electrical equipment to physically protect the insulator from falling objects during maintenance when the electrical equipment is de-energized. The sheet is formed of flexible PVC material and is foldable into a conical configuration and secured about the insulator by VELCRO strips attached to the sheet. Additional rectangularly-shaped sheets of similar material also are provided with VELCRO fasteners are formed into a cylindrical configuration and placed around the insulator and secured to the conically wrapped sheet to provide physical protector for taller porcelain insulators.
Eggleston in the U.S. Pat. No. 4,985,599 disclosed a bushing cap provided for mounting to a transformer bushing or to a lightening arrestor. The bushing cap when mounted to the bushing acts as an umbrella for the bushing to protect the same from airborne contaminates such as salt spray which can form a conductive surface on the bushing, while providing adequate bottom and side clearance from the bushing to ensure that a conductive surface between the cap and the bushing is avoided.
Thiel, et al, in the U.S. Pat. No. 4,760,216, disclosed a method for making an electrical bushing having an enlarged intermediate portion and reduced portions at opposite ends. The method is characterized by the steps of mounting an annular flange around one of the reduced portions to form a peripheral gutter adjacent to the one portion, mounting a cap on the other of the reduced portions to form a peripheral gutter adjacent to the other portion, placing a solid solder ring in each gutter, and with the open sides of the gutters upright, heating the solder rings to a molten temperature and cooling the molten solder to below its melting temperature in order to provide a joint between the flange and the one portion and the cap and the other portion.
Wood, et al. in the U.S. Pat. No. 4,670,625 disclosed a bushing arrangement for conveying an electrical conductor through the casing of high voltage electrical apparatus exposed to the weather comprises a rigid member of synthetic resin cast around, so as to be bonded in a gas tight manner to, the conductor, and incorporating an exterior portion protruding outside the casing, the member being enclosed, over at least the major part of its exterior portion, by a plurality of insulting axially-overlapping weather-resistant collars and a stress cone each having a creepage flange or shed, the member being secured to the casing by a flanged collar. The insulating member may be made of a relatively cheap non-weather-resistant epoxy resin and the collars of EPDM rubber.
Gamble in the U.S. Pat. No. 4,016,359 disclosed an insulating bushing assembly for use with electrical apparatus and adapted to be installed through an aperture in a wall thereof, characterized by an electrically insulating bushing extending through an aperture in the wall and sealed in place by an elastic gasket between the bushing and the wall on one side thereof, the bushing having an annular groove on the other side of the wall and a garter spring disposed in the groove and having a smaller portion thereof extending beyond the groove and in contact with the other side of the wall, whereby the elastic gasket is retained in fluid-tight compression between the bushing and the wall.
West, et al. in the U.S. Pat. No. 5,281,767 disclosed an improved, failure-resistant, low mechanical stress electrical connector bushing is provided which includes an elong
Harrison & Egbert
Nguyen Chau N.
Prolec, S.A. de C.V.
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