Metal working – Method of mechanical manufacture – Electrical device making
Reexamination Certificate
1999-12-13
2001-10-30
Young, Lee (Department: 3729)
Metal working
Method of mechanical manufacture
Electrical device making
C029S606000, C029S607000, C336S206000, C336S206000, C336S206000, C336S205000
Reexamination Certificate
active
06308401
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Scope of Invention
This invention relates generally to large electrical power transformers, not to smaller distribution transformers, and more particularly to shell-form or pancake conductor coils having improved lateral and longitudinal turn strength.
2. Prior Art
The shell-form style of transformer construction is essential to current modes of electrical power generation and transmission. The inherent mechanical strength of the design to resist forces generated during external short circuits and other electrical surges is excellent. These transformers are called upon to either step up or step down voltage on a highly efficient level.
Such step up or step down transformers incorporate a number of individually wound shell-form or pancake coils which are connected “start-to-start” and “finish-to-finish” to form a “group” of high or low voltages. These groups of coils are placed in certain sequences with appropriate insulation barriers when designed as either single phase or three phase transformers. Because equal electrical current flows through each group of coils in the same direction, magnetic fields which are developed are canceled between each of the coils within the group, thus creating mechanical forces of attraction which drastically increase when large short-circuit current flows during electrical faults occurring external to the transformer.
As a result of these increased attractive forces, some of which are non-axial (radial) because of varying coil inside and outside dimensions to the electrical core steel, the edge margins of the coil can laterally or longitudinally deform, either momentarily or permanently, to the extent that a short may occur between spaced adjacent coils or between turns in the same coil.
Additionally, during coil manufacture, a single length of a conductor groups forming an electrical turn is spiral wound on a horizontal surface around a central mandrel having a shape of a core of the transformer into which the flat coil will be installed as part of a group. Multiple turns of the conductor groups are required in the spiral winding to complete the coil. The conductor groups of each turn are pre-wrapped with an insulating paper material which may include an epoxy thermo set adhesive that, when cured, bonds all conductor groups to add strength to the otherwise somewhat fragile coil as it is lifted and transferred into its assembled position after the coil forming process is complete.
In U.S. Pat. No. 4,489,298, Hall teaches the utilization of a dielectric cover for enclosing the end strands whereby the end strands of adjacent layers are sufficiently insulated to prevent electrical breakdown therebetween. However, this device is not subjected to high mechanical forces, nor does it increase the resistance to deformation during fault current situations.
Trunzo, in U.S. Pat. No. 3,934,332 teaches a method of making electrical coils having improved strength and oil permeability. It is here that an aspect of the teaching of the addition of an adhesive (epoxy) resin to the insulation wrapped around the elongated conductor to be spiral wound into a coil is taught. Another form of heat cureable adhesive addition for strength and insulation is taught by Palmer in U.S. Pat. No. 3,170,134. However, the epoxy resin is used for coil turn stability in these distribution transformers as a manufacturing aid rather than to control mechanical forces.
The following additional U.S. patents are shown to represent the current state of the art in distribution transformer coil construction, method of manufacture and/or utilization:
U.S. Pat. No. 3,662,461 to Lake et al.
U.S. Pat. No. 4,239,077 to Dixon et al.
U.S. Pat. No. 4,492,944 to Toba
U.S. Pat. No. 4,173,747 to Grimes et al.
U.S. Pat. No. 5,621,372 to Purohit
U.S. Pat. No. 2,817,065 to Horelick
U.S. Pat. No. 3,634,164 to Van Hirtum
U.S. Pat. No. 3,555,670 to Zickar et al.
U.S. Pat. No. 4,859,978 to Feather et al.
U.S. Pat. No. 5,220,304 to Ho
U.S. Pat. No. 4,460,885 to Hansen et al.
The present invention provides an improved structure and method of manufacture of shell-form or pancake coils utilized in forming coil groups in large scale power transformers for electrical power transmission. The improved coil adds superior strength to the inner and outermost coil or conductor turns where mechanical forces can be at their maximum by providing spread wrapping of two or more of groupings of coil turns. The embodiment of the tape utilized is that of a polyester heat-shrinkable structural tape which, when properly installed and heat shrunk, adds significantly of the resistance of lateral and longitudinal deformation of the inner and outermost coil turns during external transformer faults. Similarly structured coils for other uses are also envisioned to be within the scope of this invention.
BRIEF SUMMARY OF THE INVENTION
This invention is directed to an improved large scale power transformer coil of the type characterized as a shell-form or pancake coil. The improvement adds superior strength to provide coil geometry stability as a result of handling the coil during transformer phase assembly and as a result of fault current surges in operation. The improvement is directed to tightly spread taping the inner and outermost coil turn groups together with a special grade of heat shrinkable structural tape. This spread tapeing is periodic or spaced along the outer periphery of the coil and may be staggered in terms of the number of coil turn groups wrapped together. The innermost conductor turns are also spread taped in this same manner.
It is therefore an object of this invention to provide a transformer coil of the shell form or pancake type which has inherently greater strength to provide stable coil geometry and resist lateral and longitudinal deformation of the inner and outermost turns of each coil during both transfer at manufacture and during periods of high fault current within the transformer.
It is another object of this invention to provide an improved transformer coil and method of manufacture utilizing spread tape techniques with a high tensile strength heat shrinkable polyester structural tape.
In accordance with these and other objects which will become apparent hereinafter, the instant invention will now be described with reference to the accompanying drawings.
REFERENCES:
patent: 2817065 (1957-12-01), Horelick
patent: 3170134 (1965-02-01), Palmer
patent: 3555670 (1971-01-01), Zickar et al.
patent: 3634164 (1972-01-01), Johannes et al.
patent: 3662461 (1972-05-01), Lake et al.
patent: 3839653 (1974-10-01), Beddows
patent: 3934332 (1976-01-01), Trunzo
patent: 3980906 (1976-09-01), Kuris
patent: 4173747 (1979-11-01), Grimes et al.
patent: 4239077 (1980-12-01), Dixon et al.
patent: 4319155 (1982-03-01), Nakai
patent: 4329077 (1982-05-01), Bouplon
patent: 4347454 (1982-08-01), Gellert
patent: 4376255 (1983-03-01), Kleinschmidt
patent: 4420876 (1983-12-01), McDermott
patent: 4460885 (1984-07-01), Hansen et al.
patent: 4489298 (1984-12-01), Hall
patent: 4492944 (1985-01-01), Toba
patent: 4598223 (1986-07-01), Glennon
patent: 4641053 (1987-02-01), Takeda
patent: 4736130 (1988-04-01), Puskas
patent: 4859978 (1989-08-01), Feather et al.
patent: 4864547 (1989-09-01), Krsna
patent: 5220304 (1993-06-01), Ho
patent: 5621372 (1997-04-01), Purohit
patent: 6124660 (2000-09-01), Umeda
Kim Paul D
Ohio Transformer
Prescott Charles J.
Young Lee
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