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 resin cast dry transformer coil and a method of manufacturing the same
INVENTORS
Baburao Keshawatkar, Wagle Anant Madhukar and Venkatasami Athikkan, all of Crompton Greaves Limited, CG Global R and D Centre, Kanjur Marg (E), Mumbai -400042, Maharashtra, India, all Indian nationals
APPLICANTS
Crompton Greaves Limited, CG House, Dr Annie Besant Road, Prabhadevi,
Mumbai 400030, Maharashtra, India, an Indian Company
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 a resin cast dry transformer coil and a method of manufacturing the same.
PRIOR ART DESCRIPTION
A dry transformer comprises a resin cast primary coil and a resin cast secondary coil. In order to facilitate air cooling of the transformer, cooling air ducts are provided in the transformer coils at predetermined locations. The air ducts are generally formed by locating insulating material tubes in the coils at predetermined locations during winding of the coils on formers. The wound coils alongwith the insulating material tubes are placed in vacuum leak tight moulds and impregnated or encapsulated with a resin in a casting chamber under vacuum of the order of 4 mm/Hg to 40 mm/Hg and at a temperature of the order of 65 to 75 °C. On completion of encapsulation or impregnation, the coils are removed from the casting chamber and gelled at about 80 to 90°C followed by curing at 120 to 150°C. The coils are then allowed to cool down to ambient temperature. In such a dry transformer, the insulating material tubes form a part of the coils. As a result, the weight and cost of the coils increase. In case of any mismatch between the thermal conductivity of the insulating material tubes and the casting resin, a temperature gradient will be developed in the coil. This may create cracks and air gaps between the insulating material tubes and the casting resin and lead to ingress of moisture into the transformer and partial discharge and failure of the transformer thereby reducing the reliability and life of the transformer. The insulating material tubes generally protrude outside the resin casting. If there is a mismatch of the colour of the insulating material tubes and the casting resin, the aesthetic look of the transformer will be adversely affected. Also in order to prevent

the casting resin entering the insulating material tubes during casting, the open ends of the tubes are to be plugged. The plugs are removed after the casting is over. This is inconvenient and cumbersome and requires additional labour.
OBJECTS OF THE INVENTION
An object of the invention is to provide a resin cast dry transformer coil which is light weight and cost effective.
Another object of the invention is to provide a resin cast dry transformer coil which is reliable and has increased life.
Another object of the invention is to provide a resin cast dry transformer coil which has improved aesthetic look.
Another object of the invention is to provide a resin cast dry transformer coil which is simple in construction.
Another object of the invention is to provide a method of manufacturing the above resin cast dry transformer coil.
DETAILED DESCRIPTION OF THE INVENTION
According to the invention there is provided a resin cast dry transformer coil comprising cooling air ducts integrally formed or cast in the coil at predetermined locations.

According to the invention there is also provided a method of manufacturing a resin cast dry transformer coil, wherein the coil is wound on a former with cooling air duct forming rods corresponding to the size of the cooling air ducts to be formed in the coil located at predetermined locations in the coil and the rods are removed from the coil during gelling of the coil so as to integrally form the cooling air ducts in the coil, the cooling air duct forming rods being made of a material which is inert to the casting resin and which does not deform at the gelling temperature.
The following is a detailed description of the invention with reference to the accompanying drawings, in which the sole Fig 1 is a cut section isometric view of a resin cast dry transformer coil according to an embodiment of the invention. The transformer coil 1 as illustrated in the accompanying drawings, which may be primary coil or secondary coil of the transformer, comprises a plurality of cooling air ducts 2 integrally formed in the resin mass 3. 4 and 5 are the conductor turns and interlayer dielectric insulation of the coil. 6 is the former on which the coil is wound. The cooling air ducts are formed in the coil by placing cooling air duct forming rods (not shown) corresponding to the size of the air cooling ducts at determined locations in the coil during winding of the coil on the former. The cooling air duct forming rods are pulled out from the coil during gelling thereof. The rods are made of materials which are inert to the casting resin and which do not deform at the gelling temperature. Therefore, they do not react with the resin and can be easily pulled out from the coil during gelling and the air cooling ducts formed will retain their shape. The air ducts forming rods are made of materials such as ultra high molecular weight high density polyethylene. The number of cooling air ducts and the geometry and configuration of the cooling air ducts can vary depending upon the design

requirements of the transformer. The cooling air duct forming rods can be made of any other material. Such variations of the invention are obvious to those skilled in the art and are to be construed and understood to be within the scope of the invention.
According to the invention the insulating material tubes have been eliminated. Instead the cooling air ducts are integrally formed in the casting resin mass itself. Therefore, cost and weight of the coil are reduced. Further no temperature gradient is developed across the resin crosssection during operation of the transformer. Therefore possibility of crack or gap formation within the resin mass is eliminated. Ingress of moisture into the resin mass is eliminated and reliability and life of the transformer are improved. The aesthetic look of the transformer is also improved. Also labour involved in the plugging and unplugging of the insulating material tubes is also eliminated. The invention is thus advantageous in several respects.

We claim :
1) A resin cast dry transformer coil comprising cooling air ducts integrally formed or cast in the coil at predetermined locations.
2) A method of manufacturing a resin cast dry transformer coil, wherein the coil is wound on a former with cooling air duct forming rods corresponding to the size of the cooling air ducts to be formed in the coil, located at predetermined locations in the coil and the rods are removed from the coil during gelling of the coil so as to integrally form the cooling air ducts in the coil, the cooling air duct forming rods being made of a material which is inert to the casting resin and which does not deform at the gelling temperature.
Dated this 13th day of March 2008.

of Khaitan &Co
Agent for the Applicants

Abstract
A resin cast dry transformer coil comprising cooling air ducts (2) integrally formed or cast in the coil at predetermined locations. Also a method of manufacturing the resin cast dry transformer coil. The coil is wound on a former with cooling air duct forming rods corresponding to the size of the cooling air ducts to be formed in the coil, located at predetermined locations in the coil and the rods are removed from the coil during gelling of the coil so as to integrally form the cooling air ducts in the coil. The cooling air duct forming rods are made of a material which is inert to the casting resin and which does not deform at the gelling temperature (Fig 1).