US3522366A - Electrical insulators - Google Patents

Description

y 1-970 P. J. LAMBETH ELECTRICAL INSULATORS 7 Filed April 1, 1969 3 Sheets-Sheet 1 July 28,- 1970 P. J. LAMBETH 3,522,366

' ELECTRICAL INSULATORS Filed April 1, 1969 :5 Shets-Sheet z y 23, 1970" P. J.. LAMBETH $522,366

ELECTRICAL INSULATORS Filed April 1, 1959 3 Sheets-Sheet 5 United States Patent ()ffice 3,522,366 Patented July 28, 1970 3,522,366 ELECTRICAL INSULATORS Peter John Lambeth, Brookham, Surrey, England, assignor to Transmission Developments Limited, Gloucester, Gloucestershire, England, a British company Filed Apr. 1, 1969, Ser. No. 811,764 Claims priority, application Great Britain, Apr. 1, 1968, 15,576/68; Dec. 19, 1968, 60,467/68 Int. Cl. H01]: 17/56 US. Cl. 174-212 13 Claims ABSTRACT OF THE DISCLOSURE diameter, size 0, size 1, size 2 and the positions in the set as position 1, position 2 upwards from the lowest shed, the shed size number for each position in a set is given by the highest power of 2 that is exactly divisible into the position number. The length of the creepage path L between the lowest point on the rim of any shed and the point immediately below it on the next shed of the same or larger diameter is related to the shortest distance X between the two points in such a way that the maximum value of X/L does not exceed the minimum value by more than 50% and the total creepage path between the ends of the insulating material is at least 4 times and typically 10 times the axial length.

BACKGROUND OF THE INVENTION Field of the invention This invention relates to shedding electrical insulators for high voltages and is directed more particularly but not exclusively to rod-type insulators for suspending the conductors of overhead power transmission lines.

Description of the prior art It is well known that the performance of an insulator for an E.H.V. transmission line in conditions ofsevere atmospheric pollution is a function of creepage path length of the insulator. The longer the creepage path, the less is the risk of flashover. One way of producing a long creepage path in a given overall length for an insulator is to employ a large number of plain sheds of large diameter. It is also known however that, if the air gap between insulator sheds is reduced, then the risk of discharges in the air between the sheds at their outer edges or between a water droplet on an upper shed and a shed beneath it is increased.

SUMMARY OF THE INVENTION It is an object of the invention to provide an insulator that, for a given axial length, has very high resistance to flashover by power frequency and switching surge voltages under severe conditions of wet pollution and under heavy rain, or a short axial length for a given flashover strength without the need for unduly. large diameter sheds.

According to the present invention, an electrical insulator has a plurality of sheds with adjacent sheds of different diameters along the whole or at least the greater part of the length of the insulator, the total length of the creepage path between the ends of the insulating material being at least 4 times and preferably at least 8 times the axial length of the insulating material.

With this construction, water droplets which collect on the periphery of a larger diameter shed will not be immediately above the outer extremity of the next shed below. Moreover the length of the air gap path between the extremities of adjacent sheds which are of two different diameters is greater than the vertical spacing of these two sheds. It has been found in practice that an insulator having sheds alternately of larger and smaller diameter gives a better performance than an insulator of similar length having uniform diameter sheds. As will be explained hereinafter, it is preferred however to use more than two different diameters for the sheds.

Since the creepage path length is the most important single parameter used in determining the performance of a polluated insulator, provided that the design is such as to permit effective utilisation of the creepage path (that is to say the risk of shorting out substantial sections of creepage by water droplets and/ or air discharges between sheds is minimised) it is preferred to adopt a high ratio of about 10 for the total length of the creepage path to the axial length of the insulation of a rod type insulator.

It is preferred in the insulator of the present invention that the length of the creepage path (L) between the lowest point on the rim of any shed and the point immediately below it on the next shed of the same or larger diameter, when the axis of the insulator is vertical, is related to the shortest distance (X) between the two points, in such a way that the maximum value of X /L for any two such points on the insulator or on a major part of the length of the insulator shall not exceed the minimum value of X/L by more than 50%.

As previously mentioned, it there are two shed sizes, the larger and smaller diameter sheds are arranged alternately. If there are more than two shed sizes they are preferably arranged in a repeating pattern. This pattern is preferably such that if there are N different shed sizes then the pattern will be repeated after 2 sheds, and the order in which the sheds are arranged is determined by the following procedure:

Let the sheds be size 0, size 1, size 2 Size (N-l) where the shed diameters increase (as but not necessarily in proportion with) the size numbers, and let the position of the sheds in a set be position 1, position 2 position 2 where position 1 is the lowermost shed of the set; then the shed size number is given by the highest power of 2 that is exactly divisible into the position number. Thus with four shed sizes, for all odd position numbers the size number is 0 for positions 2 and 6 the size number is 1, for position 4 the size number is 2 and for position 8 the size number is 3. 1

Although N may be equal to 2 we prefer to make it or more, so that the pattern repeats after a minimum of four sheds.

A complete insulator will normally include a number of repeating patterns of sheds as described above. The uppermost shed of the insulator is preferably of the largest size (i.e. the pattern sequence is completed at the top), but the lowest shed need not necessarily complete a pattern.

For rod type suspension insulators for overhead power lines the shed diameters are preferably so chosen that drips from the rim of any shed, whether or not affected by electrical stress and/or wind or deflection of the insulator from the vertical when in normal use will not regularly fall onto the smaller shed below it. This will be effectively achieved by ensuring that a line drawn touching the outer extremity of a larger shed at an angle of 30 to the axis of the insulator does not intercept one of the smaller sheds beneath it before intercepting a shed of the same or larger size.

The shortest distance between the lowest part of any shed when the insulator is vertical (upright) and the surface of any other shed is preferably not less than 8 mm.

A line joining a point on any shed furthest from the insulator axis and the midpoint of the shed root where it meets the insulator stem in the same plane as the first point and the insulator axis should preferably but not essentially make an angle of between 50 and 65 with the axis of the insulator. For any single insulator the angle of each shed will usually be substantially the same.

The sheds are preferably of thin walled conical shape with their upper and lower surfaces each generated by rotation about the insulator axis of a substantially straight line passing through the axis. Instead of having a straight line generator, the sheds may be of generally conical form with upper and lower surfaces generated by the rotation of smoothly curved generator lines of a shape such that the maximum deviation from a straight line is not greater than the average thickness of the shed. Either the upper or lower surface of a shed, or both may he stepped or corrugated, provided that the maximum depth of any step or corrugation does not exceed 40% of the average thickness of the shed.

Each shed may be of substantially uniform thickness throughout or tapering with a taper angle up to 15.

The preferred conical shape of the sheds described has the advantage that, if the insulator is to be greased with a hydrocarbon or other grease, the grease is easily applied by spraying and easily removed by a simple scraper.

The invention may be applied to a porcelain insulator with end caps. In this case the body of the insulator may be formed integrally with the sheds as a single porcelain element, end caps being secured to the ends of the porcelain element in the known way. The invention however may equally be applied to rod type insulators constructed in other ways for example formed from a plurality of units assembled together, e.g. moulded resin insulators on a resin-bonded glass-fibre core. This type of material permits of a high ratio of shed overhang to thickness, as is required if more than two diameters of shed are used.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a side elevation, partly in section, of a rodtype insulator;

FIG. 2 is a side elevation, partly in section, of another construction of insulator; and

FIG. 3 is a view similar to FIG. 2 of a slightly different construction of insulator.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1, there is shown a rod-type insulator for suspending a conductor for an E.H.V. overhead transmission line. The insulator has a main body portion formed of porcelain and having integral sheds which are alternately of larger and smaller diameter. In this particular embodiment the insulator has fourteen larger diameter sheds 11 and thirteen smaller diameter sheds 12. Each shed is formed with a concave or conical undersurface and a generally convex upper surface, these surfaces merging smoothly into the main body portion of the insulator, the upper and lower shed surfaces both sloping downwardly and outwardly so that rain tends to run off the sheds or forming droplets collecting at the lowest point part on a shed, which part is at or near the outer periphery. The ends of the insulator are secured in metal end caps 13, 14 in the known way.

FIG. 2 illustrates part of a moulded resin insulator. The insulator is conveniently moulded from a synthetic resin and may be secured around a resin-bonded glassfibre core. The moulding is preferably carried out by vacuum casting using an epoxy resin. Because of the high vantage of such an arrangement over that with two shed sizes is that, for a given overall diameter the ratio of creepage paths to gap widths can be increased, and the pollution performance of the insulator improved.

In FIG. 2, there are shown two of the largest size sheds, 20, 21, one intermediate size shed 22 and two of the smallest sheds 23, 24. The gap widths are shown as the dimension X between sheds 20, 21, the dimension X between sheds 22 and 21, the dimension X between sheds 23 and 22 and the dimension X between sheds 24 and 21. Associated with the gaps X, X, X" and X are creepage path lengths L, L, L" and L respectively, these creepage path lengths being the shortest lengths (in a radial plane through the axis of the insulator) along the surface of the sheds between the two ends of the respective gaps. In FIG. 2 the values of X/L X/ L" etc. vary from 0.0758 to 0.0936, the shed taper is 5,

the shed angle is 63, and the ratio of the length of creepage path to the length of the insulation of the insulator is 10.

In FIG. 3, which illustrates a modification of the insulator of FIG. 2, the corresponding values are 0.103 to 0.136, 5, 63 and 6.9. In both figures N=3 and consequently the pattern repeats every four sheds.

It will be noted that, in all the constructions illustrated to prevent drips from the rim of any shed, whether or not affected by electrical stress and/or wind or deflection of the insulator from the vertical, from falling onto the smaller shed below it, there is suitable difference of diameters of the sheds. More particularly a line drawn touching the outer extremity of a larger shed at an angle of 30 to the axis of the insulator does not intercept one of the smaller sheds beneath it before intercepting a shed of the same or larger size.

In all the constructions illustrated, the shortest distance between the lowest part of any shed when the insulator is vertical (upright) and the surface of any other shed is made greater than 8 mm.

I claim:

1. An electrical insulator having a plurality of sheds along at least the greater part of the length of the insulator, the sheds being of N different diameters, N being a whole number equal to or greater than 3, the shed sizes being of size 0, size 1, size 2 size (N1), the shed diameters increasing in the order of the size numbers, and the sheds being arranged in a repeating pattern with 2 sheds in each set of the repeating pattern, the positioning of the sheds in the pattern being defined as position 1, position 2 position 2 in numerical order upwards from the lowest shed and wherein the shed size number is given by the highest power of 2 that is exactly divisible into the position number, the total length of the creepage path between the ends of the insulating material being at least 4 times the axial length of the insulating material.

2. An electrical insulator as claimed in claim 1 wherein the total length of the creepage path between the ends of the insulating material is at least 8 times the axial length of the insulating material.

3. An electrical insulator having a plurality of sheds along at least the greater part of the length of the insulator, the sheds being of N different diameters, N being equal to or greater than 3, the sheds of different sizes being arranged in a repeating pattern of 2(Nr1) sheds in each set of the pattern, the shed sizes being of size 0, size 1, size 2 size (N-l), the shed diameters increasing in the order of the size numbers, and the positioning of the sheds in the pattern being defined as position 1, position 2 position 2 in numerical order upwards from the lowest shed and wherein the shed size number for each position in a set is given by the highest power of 2 that is exactly divisible into the position number and wherein the length of the creepage path L between the lowest point on the rim of any shed and the point immediately below it on the next shed of the same or larger diameter, when the axis of the insulator is vertical, is related to the shortest distance (X) between the two points in such a Way that the maximum value of X/L for any two such points on the insulation or on a major part of the length of the insulator does not exceed the minimum value of X /L by more than 50% and wherein the total length of the creepage path between the ends of the insulating material is at least 4 times the axial length of the insulating material.

4. An electrical insulator as claimed in claim 3 wherein the uppermost shed is a shed of the largest diameter.

5. An electrical insulator as claimed in claim 3 wherein the sheds are arranged so that a line touching the outer extremity of a larger shed at an angle of 30 and extending downwardly through the axis of the insulator does not intercept one of the smaller sheds beneath it before intercepting a shed of the same or larger diameter.

6. An electrical insulator as claimed in claim 3 wherein the shortest distance between the lowest part of any shed when the insulator is vertical and the surface of any other shed is not less than 8 mm.

7. An electrical insulator as claimed in claim 3 wherein a line joining a point on any shed furthest from the in sulator axis and the midpoint of the shed root where it meets the insulator stem in the same radial plane as the first point and the insulator axis makes an angle of between 50 and 65 with the axis of the insulator.

8. An electrical insulator as claimed in claim 7 wherein the aforesaid angle for each of the sheds is substantially the same.

9. An electrical insulator as claimed in claim 3 wherein the sheds are of conical shape with upper and lower surfaces each generated by rotation of a straight line generator passing through the axis.

10. An electrical insulator as claimed in claim 3 wherein the sheds are of generally conical form with upper and lower surfaces generated by the rotation of smoothly curved generator lines of a shape such that the maximum deviation from a straight line is not greater than the average thickness of the shed.

11. An electrical insulator as claimed in claim 3 and having the upper and/or lower surface of one or more sheds stepped or corrugated, the maximum depth of any step or corrugation not exceeding 40% of the average thickness of the shed.

12. An electrical insulator as claimed in claim 3 wherein each shed is of substantially uniform thickness.

'13. An electrical insulator as claimed in claim 3 wherein each shed isof tapered section in a radial plane through the axis of the insulator, the taper angle not exceeding 15.

References Cited UNITED STATES PATENTS 1,768,948 7/1930 Baum 1742l2 X FOREIGN PATENTS 871,851 1/1942 France. 940,400 10/ 1963 Great Britain. 978,501 12/ 1964 Great Britain. 1,010,362 11/1965 Great Britain.

OTHER REFERENCES Suberkrub: German printed application No. 1,005,554, published Apr. 4, 1957.

LARAMIE E. ASKIN, Primary Examiner US. Cl. X.R.

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