FORM 2
THE PATENTS ACT, 1970
(39 of 1970)
As amended by the Patents (Amendment) Act, 2005
&
The Patents Rules, 2003
As amended by the Patents (Amendment) Rules, 2006
COMPLETE SPECIFICATION (See section 10 and rule 13)
TITLE OF THE INVENTION
A solid cast resin coil for transformers with an improved insulating layout to facilitate uniform resin flow during the impregnation process
APPLICANTS
Crompton Greaves Limited, CG House, Dr Annie Besant Road, Worli, Mumbai 400 030, Maharashtra, India, an Indian Company
INVENTORS
Lobo Anthony and Venkatasami Athikkan of Crompton Greaves Ltd, CMDRC, CG Global R&D Centre, Kanjur (E), Mumbai 400042, Maharashtra, India, both are Indian Nationals
PREAMBLE TO THE DESCRIPTION
The following specification particularly describes the nature of this invention and the manner in which it is to be performed:

FIELD OF THE INVENTION
This invention relates to an improved insulating layout to facilitate uniform resin flow
and thereby reducing air gaps in a solid cast resin coil for transformers.
BACKGROUND OF THE INVENTION
The present invention relates to the field of dry type electrical transformers, and more particularly to a resin-encapsulated transformer coil manufactured using solid cast method. Manufacturing of dry type transformer coil by solid cast method initially involves forming of a coil within a mould of metallic sheets. A metal sheet is wound around a former following which multiple layers of insulating material and a first layer of conducting coil is wound to form a low voltage coil or 'primary coil'. The primary coil is further wound with multiple layers of insulating layers and a second conducting coil to form a high voltage coil or 'secondary coil'. The secondary winding is further wound with multiple layers of insulating material and enclosed in a metal sheet mould. The coil as formed between the metal sheets is impregnated with a electrical grade resin under vacuum. The resin permeates through the insulating and conducting layers and binds all the layers of conducting and insulating material. The solid cast method utilizes a mold (metal sheets) around the coil, which makes this method significantly different from the other methods of manufacturing transformer coils. Once the coil gets solidified the metal sheets are removed to obtain a hardened coil. Since all of the resin or other process material is retained during the curing process, there is a greater likelihood that the windings will be free of gaps or voids. It is desired to improve the mechanical strength of the coil formed by a solid cast method without compromising on the electrical properties of the transformer coil.

OBJECTS OF THE INVENTION
An object of the invention is to provide a solid cast transformer coil with increased
mechanical strength,
Another object of the invention is to provide a solid cast transformer coil which offers a uniform resin flow to during the impregnation process thereby reducing the occurrence of gaps or voids in the cast.
DETAILED DESCRIPTION OF THE INVENTION
According to the present invention the mechanical strength of the coil is improved by providing additional layers of insulator windings for both the primary and secondary coils. The insulating material as proposed by the invention for providing additional strength is a fiber glass mat which is made up of horizontal and vertical strips of fiber glass. Additional layers of fiber glass mat is to improve the mechanical strength of the coil .The gauge of the vertical strips of the fiber glass mat is more than that of the horizontal strips and the layer formed of it creates a porous cross-section through which the resin can flow during the impregnation process. Thus in addition to provide better mechanical strength, the fiber glass mat also facilitates for better encapsulation by resin during the impregnation process. Such a fiber glass mat further allows for a uniform flow of the resin during impregnation along the cross section of the fiber glass mat. These and other aspects, features and advantages of the invention will be better understood with reference to the following detailed description, accompanying drawings and appended claims, in which,

Figure 1 is a representation of a cross section view of the solid cast transformer coil comprising of a low voltage coil and a high voltage coil according to the present invention.
Figure 2 represents the top view of the solid cast resin coil highlighting the fiber glass mat layers orientation as used in solid cast transformer coil according to the present invention.
Figure 3 represents the orientation of fiber glass mat as used in solid cast transformer coil according to the present invention.
Figure 1 represents the cross section view of the solid cast transformer coil. High voltage coil 1 and low voltage coil 2 together comprise the solid cast transformer coil. Both the coils are manufactured similarly and placed concentric to each other separated by a radial gap. The winding of the low voltage coil is described further. The insulating layers as wound include a fiber glass mat 3, fiber glass chopper strand mat 4, and a fiber glass mesh 5. An offset edge strip 6 is wound over the insulating layers above which conductor plate 7 is wound. Conductor 7 is again wrapped around with a layer of fiber glass mesh 5, fiber glass chopped stranded mat 4 and a fiber glass mat 3. The winding structure as formed is impregnated with resin under gravity. Resin permeates through all the windings and binds the layers together to give the windings solid structure. Fiber glass mat 3 gives the transformer coil additional mechanical strength and is formed of horizontal and vertical strips of fiber glass.

Figure 2 illustrates the top view of the transformer coil (high voltage coil 1 and low voltage coil 2) when wound with fiber glass mat according to the invention. In viewing figure 2 in conjunction with figure 1 one can appreciate that there are pores in the fiber glass mat layer 3 for each of the coils.
Figure 3 illustrates a planar view of fiber glass mat 3 and its orientation as used in the transformer coils of Figure 1. Viewing Figure 3 in conjunction with Figure 2 one can appreciate that horizontal stripes 10 and vertical stripes 11 of fiber glass mat 3 create pores which allow for uniform movement of the resin. Such a uniform movement of resin avoids formation of air gap in the hardened coils.
Although the invention has been described with reference to a specific embodiment, this description is not meant to be construed in a limiting sense. Various modifications of the disclosed embodiment, as well as alternate embodiments of the invention, will become apparent to persons skilled in the art upon reference to the description of the invention. It is therefore contemplated that such modifications can be made without departing from the scope of the invention as defined in the appended claims.

We claim
1. A solid cast resin coil for a transformer comprising a primary coil and a secondary
coil wound concentrically with each of the primary and secondary coils comprising
of:
a winding structure formed with a metal layer between a plurality of insulating layers arranged concentrically in radially spaced apart relationship and is impregnated with a resin in vacuum; and
wherein the innermost and outermost insulating layers of the plurality of insulating layers is a fiber glass mat giving mechanical strength to the winding structure, the fiber glass mat made of interleaved horizontal and vertical fiber glass strips, the vertical strips being aligned to the vertical central axis of the solid cast resin coil to facilitate uniform resin flow during impregnation.
2. The solid cast resin coil as claimed in claim 1, wherein the plurality of insulating
layers comprises of:
a fiber glass mat; and
a fiber glass chopped strand mat; and
a fiber glass mesh; and

3. The solid cast resin coil as claimed in claim 1, wherein the metal layer is made up of aluminum or copper windings.