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
PROVISIONAL SPECIFICATION
(See section 10 and rule 13)
TITLE OF THE INVENTION
Cooling ducts for dry type transformers
APPLICANTS
Crompton Greaves Limited, CG House, Dr Annie Besant Road, Worli,
Mumbai 400 030, Maharashtra, India, an Indian Company
Inventors
Anthony Marcel Lobo & Athikkan Venkatasami, both of Condition Monitoring &
Diagnostic Centre, Global R&D, Crompton Greaves Ltd, Kanjur Marg, Mumbai, Maharashtra, India, both are Indian Nationals

PREAMBLE TO THE DESCRIPTION
The following specification describes the invention


The present invention relates to a process of formation of cooling ducts for dry type cast resin Transformers.
A typical transformer comprises a laminated, ferromagnetic core, a high voltage winding and a low voltage winding. Traditionally two major techniques are used to construct windings of dry type transformer with primary voltages over 600 volts i.e. conventional dry, resin encapsulated and solid cast. In the conventional dry method, some form of vacuum impregnation is done using a solventless type varnish on a completed assembly consisting of the core and the coils or individual primary and secondary coil. In case of resin-encapsulated method, encapsulation of a winding is done with resin impregnation/resin encapsulation under vacuum. The windings are placed in the mold and impregnated and/or encapsulated with a resin under a vacuum, which is then allowed to cure before the mold is removed. Since the resin or other process material is retained during the curing process under vacuum, winding will be free of voids. Cooling channels can be formed as part of the encapsulated/molded windings/coils.
Several methods are available for the formation of cooling channels. In the traditional method of formation for cooling channels a few channel forming molds are inserted during winding of the coil at predetermined locations. They are extracted by force after the resin has partially cured. This method of duct formation includes a few major drawbacks. The major drawback involves the limitation in the number of ducts formed. The limitation in the number of ducts can be because of several other factors such as, the
2

insertion of molds at predetermined location, maintenance of uniform spacing between two consecutive ducts, and many more.
Due to this, the number of cooling channel molds are significantly reduced, which results in drop in air pressure, reduction in the amount of heat dissipation, etc.
To overcome all the drawbacks as mentioned above a novel method of duct formation is disclosed in accordance with the invention.
An object of the invention is to provide a process of cooling channel formation to increase the number of cooling channels in the transformer winding.
Another object of the invention is to provide a process of cooling channel formation, which can increase the amount of heat dissipation to a significant level.
Yet another object of the invention is to provide a process of cooling channel formation, which can increase the flow of air pressure in considerable manner.
Another object of the invention is to provide a process of cooling channel formation, which will improve the mechanical strength of the moulded/casted transformer windings.
3

Another object of the invention is to use available material, which can significantly reduce the inventory.
Yet another object of the invention is to use cheaper material, which is a cost effective solution.
In accordance with the invention as disclosed herein the coil can be wound on winding mandrel on a coil winding machine. An absorbent reinforcing material such as dry fiber glass, a chopped stand mat, a woven fiberglass mat or any other family member of the same becomes the first layer. Insulated conductors either copper or aluminum can be wound on the absorbent reinforcing materia/. The insulated conductors will continue to be wound over the previous layer and the process can be repeated until a predetermined number of conductor turns and layers are reached. The terminals of the insulated conductors are brought out to provide connections.
In the present invention the second layer of absorbent material such as fiber glass, chopped stand mat or any other family member of the same absorbent reinforcing material can be wrapped over the conductor winding to full linear length covering the entire coil diameter. A cooling channel mold can be placed on another layer of absorbent material such as fiber glass, chopped stand mat or any other family member of the same in a zig-zag fashion bypassing the absorbent material in and out of alternate cooling channel molds to for a U-shape of the absorbent material around the diameter of the coil. Due to the zig-zag wrapping a certain number of duct molds get covered with the
4

absorbent material on upper side of the cooling channel mold and certain number of duct molds alternately get covered with the absorbent material on the lower side of the duct mold. Fibre glass tape is tied along the diameter at both ends of the coil so that the cooling channel molds are held in place. A last layer of absorbent material such as fiber glass, chopped stand mat or any other family member of the same absorbent reinforcing material is wrapped over the conductor winding to full linear length covering the entire coil diameter which will cover the exposed cooling channel molds along the linear length not being covered by the zig-zaz winding of the absorbent material. The cooling channel molds after this stage are fully covered to the linear length with the absorbent material. This step increases the number of cooling channels as they are formed close to each other along the diameter with a layer of absorbent material between the duct mold and this formation will increase the heat dissipation to a considerable amount.
The remaining layers are wound in a similar manner until the coil is complete, with absorbent mat forming the final layer. If additional layers are required, it can be done using the same method. Once the process is done, the coil can be sent for resin impregnation/ casting.
After the process of impregnation/casting, the molds are withdrawn and this results into a continuous cooling channel. These cooling channels are in immediate proximity to the conductor winding and can therefore provide effective convection cooling.
The preferred thermoplastic design of the cooling channel mold is a solid piece of
5

material having a uniform cross section, either machined, thermoformed or extruded. The thermoplastic material used should not react with nor bond to the coil bonding resin. It should have a heat distortion temperature high enough to resist heat associated with the resin curing process.