FORM 2
THE PATENTS ACT, 1970
(39 of 1970)
As amended by the Patents (Amendment) Act, 2005
AND
The Patents Rules, 2003
As amended by the Patents (Amendment) Rules, 2005
COMPLETE SPECIFICATION
(See section 10 and rule 13)
TITLE OF THE INVENTION
Ultra High Voltage Corona Shield
APPLICANTS
Crompton Greaves Limited, CG House, Dr Annie Besant Road, Worli, Mumbai 400 030, Maharashtra, India, an Indian Company
INVENTOR
Selvaraj Saravanan of Crompton Greaves Ltd, Analytics Centre, Global R&D Centre, Kanjur (E), Mumbai 400042, Maharashtra, India; Indian National
PREAMBLE TO THE DESCRIPTION
The following specification particularly describes the nature of this invention and the manner in which it is to be performed:

TECHNICAL FIELD OF THE INVENTION :
The present invention generally relates to a ultra high voltage corona shield that protects the bushing terminal of an ultra high voltage apparatus. More particularly, the present invention relates to an ultra high voltage corona shield for reducing electric stress during the testing of ultra high voltage apparatus.
BACKGROUND OF THE INVENTION :
Ultra high voltage apparatus, such as transformers, are usually equipped with bushings that carry electric current to high potential. When the electric field is non-uniform at the terminals of the bushing, an increase in voltage causes a discharge in the gas surrounding the bushings at points with the highest electric field intensity, namely sharp points. This form of discharge is called a corona discharge. The existence of corona generates ozone which is highly corrosive for the bushing. Further, the combination of corona, moisture causes rapid deterioration of the electrical equipment within a short period of time. Also, the corona discharge may cause sparking and radio interference.
Generally, a corona shield is used to eliminate or at least partially reduce the corona discharge at the bushing terminals. Specifically, during testing of the ultra high voltage apparatus, the bushing terminal is connected to a high voltage and the apparatus is maintained at a ground potential. A corona shield placed surrounding the energized terminal of the bushing, which reduces the voltage gradient to a level well below the ionizing gradient of the surrounding gas.

The corona shields disclosed in the prior art are generally toroid or donut shaped. It is recognized that the corona shields, particularly in very high voltage applications, are very heavy. This is disadvantageous because the weight of such shields exceeds the compressive and bending strength of the bushing exerting a mechanical stress that leads to the breakdown of the bushing. Furthermore, the complexity of manufacturing increases with the total size of the corona shield.
Thus, there is a continuing need for an improved corona shield that has an improved structure and is lighter in weight than the existing corona shields. Further, there is also a need for a corona shield that reduces the electric stress to a level below that achieved by the conventional corona shields as disclosed in the prior art.
OBJECTS OF THE INVENTION :
An objective of the present invention is to provide an ultra high voltage corona shield having an improved structure for reducing the electric stress in the vicinity of a bushing of an ultra high voltage apparatus.
Another objective of the present invention is to provide a corona shield that reduces the electric stress to a level lower than that provided by the conventional corona shields.
Another objective of the present invention is to provide a corona shield that is lighter in weight than the conventional corona shields and which reduces the mechanical stress in the bushings of the ultra high voltage apparatus.

SUMMARY OF THE INVENTION :
The present invention provides an ultra high voltage corona shield comprising a substantially elliptical rigid hollow body, wherein the body has a substantially circular closed top end, a substantially flat open bottom end and an outwardly protruding continuous side wall; a means for handling the body located at the substantially circular closed top end of the body; and an aperture having a top end and a bottom end, wherein the aperture is disposed in at least a portion of the continuous sidewall of the body for providing a connection to a bushing terminal of an ultra high voltage apparatus, and wherein the ultra high voltage corona shield reduces electric stress to 0.95 kV/mm.
BREIF DESCRIPTION OF DRAWINGS
FIG. 1 illustrates a front view of the corona shield according to the present invention
positioned on top of the terminal of a bushing.
FIG. 2 illustrates a perspective view of the corona shield according to the present
invention positioned on top of the terminal.
FIG. 3 illustrates a top view of the corona shield according to the present invention.
FIG. 4 illustrates a back view of the corona shield according to the present invention.

DETAILED DESCRIPTION:
According to the FIG. 1, the corona shield has a substantially elliptical rigid hollow body
(1) with a closed circular top end (2) and a substantially flat bottom end (3) which is open. The elliptical body (1) further has a continuous, outwardly protruding side wall (4). The side wall (4) is curved and the angle of curvature of the side wall ranges from 25° to 75°. Further, the elliptical body (1) has a handling means (5) disposed at the circular end
(2) of the body (1). The handling means (5) aids in lifting, etc. of the corona shield.
The length of the body (1) ranges from 3 m to 3.5 m. Further, the diameter of the body (1) towards the top circular end (2) is 2.5 m to 2.8 m. The diameter of the body (1) towards the substantially flat bottom end (3) is 3 m to 3.5 m.
Further, the body (1) has an aperture (6) located on at least one portion of the sidewall (4). The aperture (6) provides a means for connecting the body (1) to the lead connection terminal (9) of the bushing (10) of an ultra high voltage apparatus. The aperture (6) has a top end (7) and a bottom end (8). The top end of the aperture (7) is disposed at a distance of 1.1m to 1.2 m from the top circular end (2) of the body (1). Further, the bottom end of the aperture (8) is disposed at a distance of 0.3 m to 0.4 m from the substantially flat bottom end (3) of the body (1). The height of the aperture ranges from 1.6 m to 1.8 m and the width of the aperture is 0.8 m to 1 m.
According to the present invention, the body (1) of the corona shield may be made from any suitable material, for example not limiting to, Aluminum.

The thickness of the body (1) maybe 3 mm to 4 mm.
Further, the weight of the body (1) of the corona shield according to the present invention may be in the range of 250 kg to 290 kg.
According to the present invention, the corona shield reduces the electric stress around the terminal of the bushing of an ultra high voltage apparatus to a range of 0.93 kV/mm to 0.95 kV/mm.
The performance of the corona shield of the present invention as compared to the conventional corona shields is illustrated in the following table.
TABLE 1 : Comparative results of corona shield

Configurations Electric stress Weight
(kV/mm) (Kg)
Toroidal Corona 0.986 556
Shield (Prior art)
Elliptical Corona 0.948 284
Shield
The term "ultra high voltage apparatus" is intended to encompass apparatus such as transformers, reactor, generator, breaker, or any apparatus finding an application in high voltage systems. Further, the term "ultra high voltage" is intended to encompass voltages above 1000 kV.

It will be appreciated by those skilled in the art that the present invention can be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The presently disclosed embodiments are therefore considered in all respects to be illustrative and not restricted. The scope of the invention is indicated by the appended claims rather than the foregoing description and all changes that come within the meaning and range and equivalence thereof are intended to be embraced therein.

We Claims,
1) An ultra high voltage corona shield comprising:
a substantially elliptical rigid hollow body, wherein the body has a substantially
circular closed top end, a substantially flat open bottom end and an outwardly
protruding continuous side wall;
a means for handling the body located at the substantially circular closed top end
of the body; and
an aperture having a top end and a bottom end, wherein the aperture is disposed
in at least a portion of the continuous sidewall of the body for providing a
connection to a bushing terminal of an ultra high voltage apparatus; and
wherein the ultra high voltage corona shield reduces electric stress to 0.93
kV/mm to .0.95 kV/mm.
2) The corona shield of claim 1, wherein the corona shield reduces the electric stress to 0.948 kV/mm.
3) The corona shield of claiml, wherein the length of the body is 3 m to 3.5 m.
4) The corona shield of claiml, wherein the diameter of the body towards the top end is 2.5 m to 2.8 m.
5) The corona shield of claiml, wherein the diameter of the body towards the bottom end is 3 m to 3.5 m.
6) The corona shield of claiml, wherein the angle of curvature of the outwardly protruding continuous side wall ranges from 25° to 75°.
7) The corona shield of claiml, wherein the top end of the aperture is disposed at a distance of 1.1 m to 1,2 m from the top end of the body.

8) The corona shield of claim 1, wherein the bottom end of the aperture is disposed at a distance of 0.8 m to 1 m from the bottom end of the body.
9) The corona shield of claim 1, wherein the height of the aperture is 1.6 m to 1.8 m.
10) The corona shield of claiml, wherein the width of the aperture is 0.8 m to 1 m.
1 l)The corona shield of claim 1, wherein the body is made from Aluminum.
12) The corona shield of claiml, wherein the thickness of the body is 3 mm to 4 mm.
13) The corona shield of claim 1, wherein the weight of the body is in the range of 250 kg to 290 kg.