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
Method and heater for uniformly curing a resin impregnated electrical bushing
APPLICANTS
Crompton Greaves Limited, CG House, Dr Annie Besant Road, Worli, Mumbai - 400 030,
Maharashtra, India, an Indian Company
INVENTORS
Mandlik Manoj Kisan, Crompton Greaves Limited, Global R&D Centre, Kanjur (East), Mumbai 400042, Maharashtra, India and Thekkeveettil Sethumadhavan, Crompton Greaves Limited, Vacuum Interrupters and Instrument Transformers Division, Power Systems, Aurangabad, Maharashtra, India, both Indian nationals
PREAMBLE TO THE DESCRIPTION
The following specification particularly describes the invention and the manner in which it is to
be performed:

FIELD OF THE INVENTION
This invention relates to a method and heater for uniformly curing a resin impregnated electrical
bushing
BACKGROUND OF THE INVENTION
Electrical bushings find their applications in electrical devices or apparatuses like transformers, circuit breakers or gas insulated switchgears. Electrical bushings are very essential and critical components to facilitate or establish electrical connectivity between outside supply and internal parts of electrical apparatuses or devices and to avoid electrical break down with the enclosures of the apparatuses or devices. An electrical bushing comprises a wound structure formed by winding electrical grade paper like crepe paper or craft paper on a central conductor with conductor foils inserted at regular intervals radially between the layers of the paper insulation The wound structure is then impregnated with an electric grade oil or with an electric grade epoxy resin. The resin impregnated wound structure is cured by heating. The former is known as oil impregnated or OIP bushing and the latter is known as resin impregnated paper or RIP bushing. The central conductor is hollow in the case of the RIP bushing.
RIP bushings are dry and explosion proof and have high reliability besides being pollution-free. RIP bushings are especially suitable for outdoor applications and in areas prone to fire. The resin impregnation and curing of an RIP bushing is generally carried out in an apparatus comprising a casting chamber provided with a heater element on the outer surface thereof. The wound

structure to be impregnated and cured is located in a mould and the mould with the wound structure is located in the casting chamber at the base thereof. Degassed resin composition at about 60°C is fed into the mould from the bottom thereof and the wound structure is allowed to be impregnated with the resin composition at about 60°C by heating the casting chamber from outside at 60°C. The resin composition rises in the mould by capillary action and impregnates the wound structure. Following impregnation, the resin is allowed to gel at 90°C and cure at 120°C by raising the temperature of heating of the casting chamber.
During gelling and curing by heating the casting chamber externally, heat energy travels from outside to the inside of the mould and the wound structure and curing by solidification of the resin takes place from outside towards the inside of the wound structure. As a result, shrinkage of the resin takes place from outside towards inside of the wound structure and invariably causes separation or crossectional delamination between the paper layers and conductor foils and between the central conductor and resin impregnated paper body and formation of microcavities within the wound structure and also radial stretching of the wound structure. Further the central conductor will pick up the heat travelling from outside the casting chamber inwardly towards the mould and get heated up. As a result, gelling and curing of the resin may take place in both the radial directions of the wound structure namely from outside radially inwardly and from inside radially outwardly. This will cause shrinkage of the resin to take place in both the directions namely from outside radially inwardly and from inside radially outwardly and separation of the conductor foils and paper insulation and central conductor and resin impregnated paper body within the wound structure. Because of all this, proper adhesion and bonding between the conductor foils and the paper insulation and between the resin impregnated paper body and

central conductor within the wound structure will not be maintained, especially during working of the bushings as an effect of temperature cycles and different expansion and contraction rates of the current carrying central conductor and resin impregnated wound paper solid body. As a result, the life and reliability of the bushing are reduced.
In our patent application No 2529/MUM/2008 filed on 3 December 2008, we have described a method and apparatus for resin impregnation and curing of electrical bushing by heating the wound structure externally and internally at 60°C during the resin impregnation followed by gelling and curing the resin impregnated wound structure by heating the resin impregnated wound structure internally and externally at 90. to 80°C and at 120 and 115°C respectively simultaneously so as to allow shrinkage of the resin to take place from inside the resin impregnated wound structure uniformly radially outwardly. The apparatus comprises a casting chamber having an openable top portion and atleast one looking window in the top portion. The casting chamber further has a resin inlet at the bottom thereof extending outwardly from the bottom of the casting chamber. An external heater comprising a heater element is provided around the casting chamber. A first control panel is connected to the heater element and to a first temperature sensor located within the casting chamber. An internal heater is located within the hollow central conductor of the wound structure and is extending along the length thereof. A second control panel is connected to the internal heater and to a second temperature sensor located within the central conductor in the proximity of the internal heater. Shrinkage of the resin takes place from inside radially outwardly so as to ensure proper and uniform adhesion between the central conductor and resin impregnated paper body and conductor foils and the paper insulation so as to improve the dielectric strength and electrical properties and performance

efficiency of the electrical bushing. As the curing progresses within the entire wound structure radially at the same curing temperature from top to bottom of the wound structure, there is possibility for separation or crossectional delamination between the conductor foils and paper insulation and between the resin impregnated paper insulation and central conductor and formation of micro cavities within the wound structure and radial stretching of the wound structure. There is thus need for improvements in the curing of resin impregnated paper electrical bushing.
OBJECTS OF THE INVENTION
An object of the invention is to provide a method for uniformly curing a resin impregnated electrical bushing, which method ensures proper and uniform adhesion between the central conductor and paper insulation and between the conductor foils and paper insulation so as to improve the electrical strength and electrical properties and performance efficiency of the electrical bushing and the life and reliability of the bushing.
Another object of the invention is to provide a method for uniformly curing a resin impregnated electrical bushing, which method is simple and easy to carry out,
Another object of the invention is to provide a heater for uniformly curing a resin impregnated electrical bushing, which heater ensures proper and uniform adhesion between the central conductor and paper insulation and between the conductor foils and paper insulation so as to

improve the dielectric strength and electrical properties and performance efficiency of tht electrical bushing and the life and reliability of the bushing.
Another object of the invention is to provide a device for uniformly curing a resin impregnated electrical bushing, which heater is simple in construction and easy and convenient to operate.
DETAILED DESCRIPTION OF THE INVENTION
According to the invention there is provided a method for uniformly curing a resin impregnated electrical bushing; the resin impregnated wound structure comprising a winding of electrical grade paper wound on a hollow central conductor with conductor foils inserted at regulat intervals radially within the paper insulation and impregnated with an electrical grade resin, the method comprising precuring the resin impregnated wound structure by heating simultaneousl> equal sections of the wound structure from bottom to top of the wound structure internally through the hollow conductor at progressively reducing temperatures from bottom to top sections of the wound structure starting with a temperature of 90 - 110° C for the bottom section and progressively reducing the temperature of precuring for the intermediate sections up to the top section by a temperature gradient of 4-6°C and curing the wound structure by heating the equal sections of the wound structure internally through the hollow conductor at progressively reducing temperatures from bottom to top sections of the wound structure starting with a temperature of 110-130°C for the bottom section and progressively reducing the temperature of curing for the intermediate sections upto the top section by a temperature gradient of 4-6°C.

According to the invention there is also provided a heater for uniformly curing a resin impregnated electrical bushing, the heater comprising a vertically disposed thermal conductor material hollow cylindrical body having a plurality of heater elements of different wattages wound over the cylindrical body one above another corresponding to equal sections of the wound structure from bottom to top of the wound structure, the wattages of the heater elements reducing progressively from bottom heater element to the top heater element to generate temperatures ranging from higher temperature to lower temperatures from the bottom heater element to the top heater element through the intermediate heater elements upto the top heater element with a temperature gradient, each of the heater elements being associated with a temperature sensor located on the cylindrical body and control panel connected to the heater elements and temperature sensors.
The following is a detailed description of the invention with reference to the accompanying drawings, in which:
Fig 1 is a crossectional view of the heater for uniformly curing a resin impregnated electrical bushing according to an embodiment of the invention; and
Fig 2 is a crossectional view of a resin impregnated wound structure of an electrical bushing with heater of Fig 1 located within the central conductor of the wound structure.
The heater 1 as illustrated in Figs 1 and 2 of the accompanying drawings comprises a vertically disposed thermal conductor material hollow cylindrical body 2 having a plurality of heater

elements 3 of different wattages wound over the cylindrical body one above another. The wattages of the heater elements reduces progressively from the bottom heater element to the top heater element such that the bottom heater element has the highest wattage and the top heater element has the lowest wattage and the intermediate heater elements from bottom to top have progressively reducing wattages between the highest and lowest wattages. Each of the heater element is associated with a temperature sensor 4 located on the cylindrical body. 5 is a control panel connected to the heater elements and temperature sensors by lead wires 6a, 6b and 7a, 7b, respectively. 8 is a resin impregnated wound structure comprising a winding of electric grade paper like crepe or craft paper 9 wound on a central hollow conductor 10 with conductor foilsl 1 inserted at regular intervals radially within the paper layers. The wound structure is impregnated with an electrical grade epoxy resin (not marked) and the heater of Fig 1 is inserted in the hollow central conductor. The heater elements from bottom to top of the heater correspond to sections or segments of the wound structure from bottom to top of the wound structure. On passing a current through all the heater elements simultaneously, the bottom heater element generates highest temperature and the temperatures generated by the intermediate heater elements upto the top heater element reduces progressively by a constant temperature gradient. The resin impregnated wound structure is precured at the desired temperature range and temperature gradient by heating the resin impregnated wound structure internally through the hollow conductor by passing current through the heater elements. The heat generated by the various heater elements is sensed by the control panel through the respective temperature sensors and is controlled at the required temperatures by regulating the currents in the heater elements by the control panel. On completion of procuring, the resin impregnated wound structure is cured at the desired temperature range and temperature gradient by heating the wound structure internally through

the central conductor by operating the heater. The curing temperature range used for curing the precured wound structure is slightly higher than the precuring temperature range. Curing temperature at the bottom heater element is highest and progressively reduces from bottom heater element to top heater element by a constant temperature gradient. Preferably, the precuring temperature for the bottom heater element and bottom section of the wound structure is 90 to 110°C and progressively reduces from bottom to top heater elements and sections by a temperature gradient of 4 to 6°C. Preferably the curing temperature for the bottom heater element and bottom section of the wound structure is 110-130°C and progressively reduces from bottom to top heater elements and sections by a temperature gradient of 4 to 6°C. Preferably the centra! conductor and conductor foils are made of aluminium. The cylindrical body is made of any thermal conductor material and is preferably made of aluminium. The heater elements are preferably heater coils, preferably made of aluminium or copper.
According to the invention the precuring and curing of the resin impregnated wound structure is carried out from bottom to top of the wound structure at a higher to lower temperatures with a constant temperature gradient. Therefore, solidification of the resin takes places both during precuring and curing gradually from bottom to top. Most of the solidification of the resin wound structure takes place during the precuring stage itself. Curing at a higher temperature ensures that the solidification is complete and the entire solvents and moisture in the wound structure have completely evaporated out. As the solidification gradually proceeds from bottom to top, the resin at the top settles down and fills the micro cavities being formed at the bottom sections of the resin impregnated wound structure. As a result, chances for delamination and formation of micro cavities within the wound structure and radial stretching of the wound structure are

completely avoided and the wound structure is uniformly cured. The electrical strength and electrical properties and performance efficiency of the electrical bushing are improved. The reliability and life of the bushing are increased.
It is understood that the number of heater elements of the heater can vary based on the height and number of sections or segments from bottom to top of the wound structure. Such variation in the configuration of the heater is to be construed and understood to be within the scope of the invention. The invention is in the precuring and curing of the resin impregnated wound structure by heating it simultaneously from bottom to top with a constant temperature gradient along various sections of the wound structure from bottom to top. The temperature range and gradient will depend upon the resin used. Variations in the temperature ranges and temperature gradient are also to be construed and understood to be within the scope of the invention.

We claim:
1. A method for uniformly curing a resin impregnated electrical bushing, the resin impregnated wound structure comprising a winding of electrical grade paper wound on a hollow central conductor with conductor foils inserted at regular intervals radially within the paper insulation and impregnated with an electrical grade resin, the method comprising precuring the resin impregnated wound structure by heating simultaneously equal sections of the wound structure from bottom to top of the wound structure internally through the hollow conductor at progressively reducing temperatures from bottom to top sections of the wound structure starting with a temperature of 90 - 110° C for the bottom section and progressively reducing the temperature of precuring for the intermediate sections up to the top section by a temperature gradient of 4-6°C and curing the wound structure by heating the equal sections of the wound structure internally through the hollow conductor at progressively reducing temperatures from bottom to top sections of the wound structure starting with a temperature of 110-130°C for the bottom section and progressively reducing the temperature of curing for the intermediate sections upto the top section by a temperature gradient of 4-6°C.
2. A heater for uniformly curing a resin impregnated electrical bushing, the heater comprising a vertically disposed thermal conductor material hollow cylindrical body having a plurality of heater elements of different wattages wound over the cylindrical body one above another corresponding to equal sections of the wound structure from bottom to top of the wound structure, the wattages of the heater elements reducing progressively from bottom heater element to the top heater element to generate temperatures ranging from higher temperature to lower temperatures from the bottom heater element to the top heater element through the intermediate

heater elements upto the top heater element with a temperature gradient, each of the heater elements being associated with a temperature sensor located on the cylindrical body and control panel connected to the heater elements and temperature sensors.
3. The heater as claimed in claim 1, wherein the wattage of the bottom heater element generates a temperature of 90 - 130°C and the wattages of the intermediate heater elements upto the top heater element generate progressively reducing temperatures with a temperature gradient of 4-6°C.
4. The heater as claimed in claim 2 or 3, wherein the cylindrical body is made of aluminium.
5. The heater as claimed in anyone of claims 2 to 4, wherein the heater elements are heater coils.
6. The heater as claimed in claim 5, wherein the heater coils are aluminium or copper coils.