DESC:FIELD OF THE INVENTION

[001] The present invention relates generally to electrical insulators, and more particularly to a composite insulator with polymeric sheds.

BACKGROUND OF THE INVENTION

[002] Electrical insulators used in power transmission and distribution, typically comprise an elongate body of electrically insulating material and a metal fitting at each end for respective electrical connection to a source of high potential, for example a power conductor, and a point at another potential. The insulator body may be made of ceramic, for example porcelain or glass, usually having sheds on its outer surface, or a glass fiber core within a sheded housing of polymeric material. The sheds are provided to increase the electrical performance, by decreasing the leakage current that may flow along the surface, by increasing the leakage path length of the insulator and to prevent the formation of a continuous moisture path from one end of the insulator to the other.

[003] High-voltage insulators for overhead lines have long been produced from electrically insulating materials such as ceramic, porcelain or glass. The ceramic and porcelain insulators however, are heavy and bulky. They require specialized assembly fixtures or processes and are awkward and difficult to handle and ship. In addition to that, the ceramic insulators are brittle and susceptible to chipping and fracture.

[004] Recently, few electrical utilities have begun accepting polymer composite materials for some types of insulators. These are typically composed of a central rod made of fiber reinforced polymer and an outer weather shed made of silicone rubber or ethylene propylene diene monomer rubber (EPDM) and other rubbers and epoxy resin based compounds. Composite insulators are, lighter in weight, and have excellent hydrophobicity and other technical advantages. This combination makes them ideal for service in polluted areas.

[005] The composite insulator is a common insulating component wildly used for the electrical equipment such as the bushing for the transformers, the circuit breakers, the instrument transformers, the power transformers, the cable terminations, the surge arresters, the gas insulated switchgear (GIS) and so on. The composite insulator can be designed for high voltage application both in outdoor service and indoor service.

[006] For HV applications, conductors are usually supported on the transmission towers by means of disc insulators in the form of strings. The string of disc insulators provides a maximum leakage distance and prevents flashovers across the insulator.

[007] With the use of composite material for the manufacture of insulators, the string insulators are being increasingly replaced with long rod insulators. Long rod insulators appear similar to the string insulators. However, they are manufactured in a single piece. The insulator consists of a "long rod" usually of Fiber reinforced Plastic (FRP) to bear the insulator load. This rod is designed to have high tensile strength. The housing of the insulator is usually made of silicone rubber or similar material. The end fittings of the insulator are directly crimped on to the FRP rod. Long rod insulators are puncture proof and have high arc resistibility. Long Rod insulators are lighter than strings of disc insulators of a similar voltage rating. Long Rod Insulators are used in both HVAC and HVDC applications.

[008] A conventional composite insulator 100 available in prior art is illustrated in FIG. 1. The conventional composite insulator comprises an insulation rod (not visible in Fig) for bearing mechanical loads, and polymeric spiral sheds. The polymeric portion over the rod between two consecutive sheds which is mounted against the rod is also referred to as sheath. Additionally, the composite insulator also comprises metal end fittings (not shown) at the top and bottom ends for connecting to other devices. The insulation rod is solid, however in other embodiment of the composite insulator a hollow insulator rod may also be used. Normally, the insulation rod is made of fiber-reinforced plastic (FRP), the sheds are made of polymeric material such as silicone rubber, and the metal end fittings are made of, primarily, galvanized cast iron or other metals viz. aluminum.

[009] In comparison to conventional insulators of glass or porcelain, composite insulators have the advantage that they have excellent insulating properties when used in areas with highly polluted atmosphere, since they are largely dirt repellant and to some extent also encapsulate contaminants in an insulating manner. Therefore, composite insulators with shielding sheaths of silicone rubber are being used increasingly to upgrade existing overhead lines with electrical insulation problems resulting from atmospheric impurities, in that the conventional insulators of porcelain or glass are exchanged for composite insulators with a shielding sheath of silicone rubber.

[0010] Contaminants in the air and in many indoor environments may accumulate on electrical insulators, reducing the dielectric strength and electrical insulating ability, leading to electrical breakdown. Breakdown, generally caused by flashover along the length of the insulator, is considered to be a serious problem. Contaminants and dust that accumulate on the insulator sheds reduce the electrical resistance of the insulators, thereby increasing the probability of electrical current flow known as flashover. These contaminants may be generated by salt water, road salt, salt flats, desert dusts, petrochemical industries, etc., and may be transported to the surface of the insulator by gravity, electrostatic attraction, migration of high-permittivity particles into regions of large electric fields, evaporation of solutions and wind. When these materials are deposited on the insulator surface, they behave as a highly variable resistor that tends to be unstable in the presence of electric fields. The leakage current through this resistance causes heat, electrochemical reactions, discharges and flashover.

[0011] Electrical insulator housings for outdoor use require certain properties in order to function efficiently. For instance, such housings must provide an electrical creepage path greater than overall housing length in order to reduce leakage of electricity to the ground. The higher the voltage across the conductor to be insulated, the longer the creepage path must be in order to prevent flashover (short circuiting).

[0012] Presently the most common method used for fabricating composite insulators is injection molding. The main drawback of manufacturing discrete shed insulators by molding, is primarily the need to use increasingly complex and expensive molds as the dimensions of the shed changes and also the production difficulties linked to the increase in volume for long rod insulators. Other difficulties derive from the need to produce small number of insulators with small design changes or just for making prototypes with a given mold. This is due to the fact, that the mold is an element linked with highly precise shapes and dimensions, and given the complexity of the mold cavities it cannot be easily modified, which makes the mold hardly adaptable for different needs of the users.

[0013] Furthermore, the creepage path required for a particular insulator varies according to the atmospheric pollution of the area of installation, and this may require the presence of a different number of sheds for a given product and a given profile, hence the need for different molds to produce the number of sheds required. In other cases, the shed profile may vary to satisfy specific user needs, necessitating, as above, the use of a new mold capable of molding the new profile requested.

[0014] In addition, as the service voltage requirement of insulator increases, the dimensions of the insulator increase. It is clear that with varying dimensions it becomes rather difficult to produce the shed by molding it in a single cycle. The molding should thus be executed in a number of operations, causing further technical problems.

[0015] Furthermore, the use of injection-molding methods limits the selection of polymer material basically to compounds with moderate or low viscosity which can be easily injected into the molds but which often present a compromise regarding the resistance to the tracking effect. From this standpoint, for example, the compounds with liquid silicon rubber bases are the easiest to inject but offer only modest resistance to the tracking effect and particularly high costs.

[0016] A possible alternative to injection molding process is extrusion which is a relatively simple and continuous process not involving expensive molds.

[0017] Several attempts were made to manufacture the composite insulator with spiral shaped shed of polymer using extrusion process. However, tests showed that the shed profile of the spiral shed composite insulator made using extrusion process did not match the performance of composite discrete shed insulators. This was due to the fact that they didn’t conform to recommended design standards for insulators.

[0018] In view of the limitations inherent in the available composite insulators with spiral shed of polymeric material, there exists a need for an improved composite insulator which overcomes the disadvantages of the prior art and which can be manufactured by extrusion in a cost effective, reliable, secure and environmental friendly manner.

[0019] The present invention fulfils this need and provides further advantages as described in the following summary.

SUMMARY OF THE INVENTION

[0020] In view of the foregoing disadvantages inherent in the prior arts, the general purpose of the present invention is to provide an improved combination of convenience and utility, to include the advantages of the prior art, and to overcome the drawbacks inherent therein.

[0021] A primary objective of the present invention is to provide a composite electrical insulator which is simple and cost effective.

[0022] In one aspect, the present invention provides a composite electrical insulator having a top and a bottom portion which comprises an insulation rod, a polymeric sheath on the said rod and a plurality of polymeric sheds mounted over the insulation rod. Each of the shed, comprises a helical portion, a point of profile change and a shed land portion formed as a curve having a predefined radius of curvature.

[0023] In another aspect of the present invention, the helical portion is defined by a shed angle which is an angle formed by the helical portion with an axis perpendicular to the axis of the composite insulator.

[0024] In another aspect of the present invention, the thickness of the shed varies from a maximum near the insulation rod, and decreasing along the radial direction to a minimum at an outer tip of the shed.

[0025] In yet another aspect of the present invention, the point of profile change is on the upper edge of the shed towards the top portion of the insulator.

[0026] In another aspect of the present invention, the curvature of the shed land portion is upwards facing towards the top portion of the insulator.

[0027] In yet another aspect of the present invention, the shed land portion is a curve which starts from the point of profile change and ends on the outer diameter of the shed.

[0028] In another aspect of the present invention, the shed land portion is defined as a curve of predefined curve length which starts from the point of profile change and ends on the outer diameter of the shed.

[0029] In another aspect of the present invention, the predefined curve length is derived from various parameters of the insulator including a power rating of the insulator, an outer shed diameter, an overhang length, a thickness of the sheath, a material of shed or a combination thereof.

[0030] In a further aspect of the present invention, the point of profile change is located at a predefined radial distance of an overhang length of the shed from inner side of the shed.

[0031] In one aspect of the present invention, the predefined radial distance is in a range of 30-70% of an overhang length of the shed.

[0032] In another aspect of the present invention, the predefined radial distance is in a range of 40-50% of an overhang length of the shed.

[0033] In yet another aspect of the present invention, the predefined radial distance is 46.78% of an overhang length of the shed.

[0034] In one aspect of the present invention, the predefined radius of curvature is at least 13.4 millimetres.

[0035] In another aspect of the present invention, the predefined radius of curvature is in a range of 13.4–70 millimetres.

[0036] In yet another aspect of the present invention, the predefined radius of curvature is in a range of 15–30 millimetres.

[0037] In yet another aspect of the present invention, the predefined radius of curvature is 19.8 millimetres.

[0038] In a further aspect of the present invention, the predefined radius of curvature is 23 millimetres.

[0039] In another aspect of the present invention, the predefined radius of curvature is derived from various parameters of the insulator including a power rating of the insulator, an outer shed diameter, an overhang length, a thickness of the sheath, a material of shed or a combination thereof.

[0040] In another aspect of the present invention, the insulation rod material is Fibre Reinforced Plastic and the polymeric material is silicone rubber.

[0041] These together with other aspects of the invention, along with the various features of novelty that characterize the invention, are pointed out with particularity in the claims annexed hereto and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and the specific objects attained by its uses, reference should be made to the accompanying drawings and descriptive matter in which there are illustrated exemplary embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0042] The advantages and features of the present invention will become better understood with reference to the following more detailed description taken in conjunction with the accompanying drawings in which:

[0043] FIG.1 illustrates a photograph of a composite insulator, according to prior art;

[0044] FIG. 2 illustrates a schematic diagram of a composite insulator with definition of various parameters, according to prior art;

[0045] FIG. 3 illustrates a schematic diagram of a composite insulator, according to one embodiment of the present invention; and

[0046] FIGs. 4 and 5 illustrates enlarged cross sectional view of a single shed of the composite insulator, according to one embodiment of the present invention.

[0047] Like reference numerals and names refer to like parts throughout the several views of the drawings.

DETAILED DESCRIPTION OF THE DRAWINGS

[0048] In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent, however, to one skilled in the art that the present invention may be practiced without these specific details.

[0049] As used herein, the term ‘plurality’ refers to the presence of more than one of the referenced item and the terms ‘a’, ‘an’, and ‘at least’ do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced item.

[0050] Reference herein to “one embodiment” or “another embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment can be included in at least one embodiment of the invention. The appearances of the phrase “in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments mutually exclusive of other embodiments. Further, the diagrams representing one or more embodiments of the invention do not inherently indicate any particular order nor imply any limitations in the invention.

[0051] The terms “composite insulator” or “composite electrical insulator” interchangeably used in the present description, conveys the same meaning and both refer to the same object. The term ‘insulation rod’ or ‘FRP rod’ have been interchangeably used in the present description and refers to the same component.
[0052]

[0053] Referring to FIG. 1, that illustrates a photograph of a composite insulator, according to prior art. The composite insulator comprises a FRP rod (not visible in FIG) over which the spiral shed made of polymeric material is provided. As can be seen from the picture, the spiral shed comprises of a single profile defined by a shed angle and pitch (distance between two adjacent sheds). The disadvantage with such profile is the formation of trough like structure which allows the accumulation of water and a continuous path for current from top to bottom. The leakage path is undesirable and has to be kept to minimum. The composite insulator shown is manufactured using an extrusion process.

[0054] Referring to FIG. 2 which shows a schematic diagram of a composite insulator with definition of various parameters. The shed angle also referred to as helix angle is the angle made by the shed profile with a line perpendicular to the axis of the insulator as shown. The composite insulator has to be installed in a particular orientation only. There is one top portion having top end fitting and a bottom portion with bottom end fitting. The top and bottom portion of the insulator is defined by the slant of the shed. The slant of the shed has to be downwards after installation, for the insulator to work, so the portion towards the direction of the downward slant of the shed is the bottom portion and the other portion is top portion of the insulator.

[0055] The spacing also called the pitch is the distance along the axis of the insulator between two same points on consecutive sheds. The shed overhang is the defined as the distance from the base of the shed on the insulation rod and upto the outermost point of the shed along the radial direction of the insulator. Shed overhang dimension is approximately equal to outermost radius of the insulator shed minus the insulation rod radius. There may be slight variation in the shed overhang dimension based on the thickness of the sheath on the composite rod. In such case the shed overhang is equal to outermost radius of the insulator shed minus the insulation rod radius minus the sheath thickness.

[0056] Referring to FIG. 3, that illustrates a schematic view of the composite electrical insulator 10, according to one embodiment of the present invention. The composite insulator is made up of an insulation rod 12, a polymeric sheath 14 on the said rod and a plurality of polymeric sheds 16 mounted over the insulation rod 12. Each of the shed 16 includes a helical portion 18 with its slant towards the bottom portion of the composite insulator, a point of profile change 20 and a shed land portion 22 formed as a curve having a predefined radius of curvature. In one embodiment of the present invention, the insulation rod material is Fibre Reinforced Plastic and the polymeric material is silicone rubber.

[0057] The shed 16 is wound over the insulation rod 12 like a screw in helical shape. The required shed angle for the composite insulator 10 is decided by the required outer diameter of the shed and the pitch of the composite insulator. In one embodiment of the present invention, thickness of the shed 16 varies from the maximum near the FRP rod 12, and as it goes outside, decreasing along the radial direction to a minimum at the tip of the shed.

[0058] Referring to FIGs. 4 and 5 illustrates enlarged cross sectional view of a single shed of the composite insulator, according to one embodiment of the present invention. The shed profile comprises of at least two portions a first portion also referred as helical portion 18 and a second portion also referred as shed land portion 22. In one embodiment, the helical portion 18 is defined by a shed angle which is an angle formed by the helical portion with an axis perpendicular to the axis of the composite insulator 10.

[0059] A point Pc 20 is defined as the Point of Profile Change from where onwards the helical portion 18 of the shed changes to shed land portion 22 towards the outer of the shed along radial direction of the composite insulator. In one embodiment of the present invention, the point Pc 20 is on the upper edge of the shed 16 which is towards the top portion of the composite insulator. The helical portion has curve shape which is almost a straight line and defined by the shed angle as shown in FIG. 3. In one embodiment of the present invention, the shed land portion 22 is a curve which starts from the point of profile change Pc 20 and ends on the outer diameter of the shed 16. In one preferred embodiment of the present invention, the curvature of the shed land portion 22 is upwards facing towards the top portion of the composite insulator 10.

[0060] The total curvilinear length of the shed profile LT is sum of curvilinear length of the helical portion LH and the curvilinear length of the shed land portion LSL. The shed profile is such that when the shed profile follows the profile of helical portion 18 along the shed angle beyond the Point of Profile Change 20, it would be almost a straight line with length equal to LT. However the profile changes at the point PC and it becomes a curve of radius of curvature RC having its concave portion towards the direction of top portion of the composite insulator.

[0061] The shed land portion curve length LSL is the total length LT minus the curvilinear length of the helical portion LH. In one embodiment of the present invention, the shed land portion 22 is defined as a curve of predefined curve length LSL which starts from the point of profile change 20 and ends on the outer diameter of the shed. In one embodiment of the present invention, the predefined curve length LSL is derived from various parameters of the composite insulator including a power rating of the composite insulator, an outer shed diameter, an overhang length, a thickness of the sheath, a material of shed or a combination two or more of these parameters.

[0062] The shed land portion 22 curve is defined by the radius of curvature RC with the start point as PC and length as LSL. The total length of the curve LSL is bent between the point PC with radius of curvature as RC and the other endpoint of the curve is at the outermost defined diameter of the shed 16 along the radial direction if the composite insulator. This is also the end point of the shed overhang along the radial direction of the composite insulator.

[0063] In another embodiment of the present invention, the shed land portion 22 curve is defined by the radius of curvature RC with the start point as PC 20, length as LSL and the center of the curve CLP as shown in FIG. 5. In one embodiment of the present invention, the predefined radius of curvature RC is derived from various parameters of the composite insulator including a power rating of the composite insulator, an outer shed diameter, an overhang length, a thickness of the sheath, a material of shed or a combination two or more of these parameters.

[0064] The curve with defined start point PC, defined length LLP, defined radius of curvature RC and defined center CLP when drawn with its concave portion towards the top portion of the composite insulator makes the shed land portion 22 curve. The start point is Pc and the other endpoint lies on the outermost diameter of the shed as shown in FIG.5.

[0065] In one embodiment of the present invention, the point of profile change PC is located at a predefined radial distance LC of an overhang length of the shed from inner side of the shed. In one embodiment of the present invention, the radial distance LC is in a range of 30-70% of the overhang dimension of the shed as shown in FIG.5. In another embodiment of the present invention, the radial distance LC is in a range of 40-50% of the overhang dimension of the shed. In one preferred embodiment of the present invention the radial distance LC is 46.78% of the overhang dimension of the shed.

[0066] The radius of curvature RC is the most crucial dimension and plays the most significant role in giving the composite insulator 10 its desired properties. The radius of curvature RC may also be derived from various parameters of the composite insulator including but not limited to the power rating of the composite insulator, the outer shed diameter, overhang length, thickness of sheath, material of shed, desired properties of the composite insulator or a combination or two or more of these parameters.

[0067] In one embodiment of the present invention, the radius of curvature RC is at least 13.4 mm. In another embodiment of the present invention, the radius of curvature RC is in a range of 13.4 – 70 mm. In another embodiment of the present invention, the radius of curvature RC is in a range of 15 – 30 mm. In yet another embodiment of the present invention, the radius of curvature RC is 19.8 mm. In a further embodiment of the present invention, the radius of curvature RC is 23 mm

[0068] In one preferred embodiment of the invention for the composite insulator of rating 11kV with following dimensions:

Diameter of insulation rod= 29.7
Outermost diameter of the shed = 94.9 mm
Shed Angle= 43.4°
Pitch/spacing = 39.91mm
Overhang = 32.6 mm
The distance LC = 30.1 mm and RC = 19.8 mm

[0069] In another embodiment of the present invention the RC = 23 mm.

[0070] The curvature and orientation of the shed land portion 22 as defined by the point of profile change PC and radius of curvature RC is very crucial for the composite insulator to function and pass quality tests. The better performance of the composite insulator of the present invention, compared to standard molded 11 kV composite insulators, in the wet power frequency test leads to the conclusion that no continuous stream of water forms on the spiral shed.

[0071] The present invention, uses extrusion as a method of manufacturing the composite insulator. The composite insulator of the present invention may be manufactured by extrusion or any similar state of the art process which produces the desired shape and dimension of the composite insulator 10.

[0072] Although a particular exemplary embodiment of the invention has been disclosed in detail for illustrative purposes, it will be recognized to those skilled in the art that variations or modifications of the disclosed invention, including the rearrangement in the configurations of the parts, changes in steps and their sequences may be possible. Accordingly, the invention is intended to embrace all such alternatives, modifications and variations as may fall within the spirit and scope of the present invention.

[0073] The foregoing descriptions of specific embodiments of the present invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teaching.

,CLAIMS:1. A composite electrical insulator having a top and a bottom portion, comprising:
- an insulation rod;
- a polymeric sheath on the said rod; and
- a plurality of polymeric sheds mounted over the insulation rod, each of the shed, comprising:
- a helical portion with its slant towards the bottom portion of the
insulator;
- a point of profile change; and
- a shed land portion formed as a curve having a predefined radius
of curvature.

2. The composite electrical insulator according to claim 1, wherein the helical portion is defined by a shed angle which is an angle formed by the helical portion with an axis perpendicular to the axis of the composite insulator.

3. The composite electrical insulator according to claim 1, wherein the thickness of the shed varies from a maximum near the insulation rod, and decreasing along the radial direction to a minimum at an outer tip of the shed.

4. The composite electrical insulator according to claim 1, wherein the point of profile change is on the upper edge of the shed towards the top portion of the insulator.

5. The composite electrical insulator according to claim 1, wherein the curvature of the shed land portion is upwards facing towards the top portion of the insulator.

6. The composite electrical insulator according to claim 1, wherein the shed land portion is a curve which starts from the point of profile change and ends on the outer diameter of the shed.

7. The composite electrical insulator according to claim 1, wherein the shed land portion is defined as a curve of predefined curve length which starts from the point of profile change and ends on the outer diameter of the shed.

8. The composite electrical insulator according to claim 6, wherein the predefined curve length is derived from various parameters of the insulator including a power rating of the insulator, an outer shed diameter, an overhang length, a thickness of the sheath, a material of shed or a combination thereof.

9. The composite electrical insulator according to claim 1, wherein the point of profile change is located at a predefined radial distance of an overhang length of the shed from inner side of the shed.

10. The composite electrical insulator according to claim 9, wherein the predefined radial distance is in a range of 30-70% of an overhang length of the shed.

11. The composite electrical insulator according to claim 9, wherein the predefined radial distance is in a range of 40-50% of an overhang length of the shed.

12. The composite electrical insulator according to claim 8, wherein the predefined radial distance is 46.78% of an overhang length of the shed.

13. The composite electrical insulator according to claim 1, wherein the predefined radius of curvature is at least 13.4 millimetres.

14. The composite electrical insulator according to claim 1, wherein the predefined radius of curvature is in a range of 13.4–70 millimetres.

15. The composite electrical insulator according to claim 1, wherein the predefined radius of curvature is in a range of 15–30 millimetres.

16. The composite electrical insulator according to claim 1, wherein the predefined radius of curvature is 19.8 millimetres.

17. The composite electrical insulator according to claim 1, wherein the predefined radius of curvature is 23 millimetres.

18. The composite electrical insulator according to claim 1, wherein the predefined radius of curvature is derived from various parameters of the insulator including a power rating of the insulator, an outer shed diameter, an overhang length, a thickness of the sheath, a material of shed or a combination thereof.

19. The composite electrical insulator according to claim 1, wherein the insulation rod material is Fibre Reinforced Plastic.

20. The composite electrical insulator according to claim 1, wherein the polymeric material is silicone rubber.