US2597590A - Combined grading and corona shield - Google Patents

Description

May 20, 1952 R. MCCOY 2,597,590

, COMBINED GRADING AND CORONA SHIELD Filed June 3, 1948 2 SHEETS-SHEET l May 20, 1952 R. L. mocoy 2,597,590

COMBINED GRADING AND CORONA SHIELD Filed June 5, 1948 2 SHEETS--SHEET 2 H/5/7770/7/VEY the transmission line conductor.

Patented May 20, 1952 COMBINED GRADING AND CORONA SHIELD Robert L. McCoy, Baltimore, Md., assignor to 1 General Electric Company, a corporation of New York Application June 3, 1948, Serial No. 30,933

Claims.

This invention relates to combined grading and corona shields and, more particularly, to shields of this type for use in high voltage transmission lines to provide flash over protection and improve voltage distribution on the insulator strings and to suppress corona on articles of line hardware such as the suspension clamps and strain clamps used to support the transmission line conductors.

The practical necessity of providing flash over protection for long insulator strings and uniform voltage distribution along the interconnected units of these strings has been met, in the past, by providing a grading ring disposed adjacent the lowermost insulator of the insulator string. The grading ring is normally a closed loop member supported approximately level with the skirt of the lowermost insulator and spaced from the insulator string to draw flash over away from the insulators. With the advent of ultra-high transmission line voltages, those in the range from 350-500 kv., it is found that corona eifects, particularly on line hardware, present problems not adequately met by the conventional grading ring structure. Unsuppressed corona is of course undesirable since it represents a power loss and also causes troublesome radio interference.

In order to overcome this deficiency of structures known to the art, I have invented combined grading and corona shields which effect flash over protection and improved voltage distribution on the interconnected insulator units and, at the same time, provide adequate corona suppression, even for line hardware, at operational voltages in the 350-500 kv. range. The structure which I provide includes a grading ring component which is supported by a mechanical arrangement extending above, below and beyond either end of, for example, a suspension clamp used to hold up In this manner, the suspension clamp is effectively enveloped by the mechanical support and I find that corona discharge from sharp edged portions of the clamp, such as nut and bolt edges, is effectively suppressed. The mechanical support for the grading ring is attached to the clamp support in such a manner that the inertia of the combined grading and corona shield is not added to that of the clamp. This enables the clamp to follow conductor vibrations more faithfully. Furthermore. it is not necessary to disconnect my combined grading and corona shield structure in order to remove the transmission line conductor from its supporting clamp. These features are advantageous since the combination ;shi elcl while offering electrical advantages, does ;not interfere with clamp movement nor does it complicate the removal of the transmission line conductor from the clamp.

It is therefore an object of my invention to provide new and improved combined grading and corona shields for use in transmission line installations and particularly those operating at ultra-high voltages, such as voltages in the 350-500 kv. range.

It is another object of my invention to provide such a combination shield which includes a grading ring in combination with a mechanical arrangement which supports the grading ring and also suppresses corona on an article of transmission line hardware, such as a suspension clamp.

It is a further object of my invention to provide such a combination shield wherein an article of transmission line hardware, such as a suspension clamp, is effectively enveloped by corona suppressing mechanical members.

It is still another object of my invention to provide such a combination shield having means to connect the shield to the support for the article of line hardware, such as a suspension clamp, Without adding the inertia of the shield to that of the clamp.

It is still another object of my invention to provide such a combination shield wherein the corona suppressing mechanical members while extending above, below and beyond either end of an article of line hardware, such as a suspension clamp, do not have to be removed in order to disconnect the transmission line conductor from the clamp.

These and other objects and advantages of my invention will become apparent from the following detailed description taken in conjunction with the attached drawings in which Fig. 1 is a pictorial illustration, viewed slightly from above, of a combined grading and corona shield constructed in accordance with the teaching of my invention and of the type adapted for use with a suspension clamp and associated insulator string; Figure '2 is a side elevation view of the assembly shown in Figure 1; Figure 3 is an end elevation view of the assembly shown in Figure 1; Figure 4 is a sectional view taken on line 44 of Figure 2; Figure 5 is a pictorial view, slightly from below, of a combination grading and corona shield constructed in accordance with the teaching of my invention and of the type adapted for use wi th a strain clamp-assembly and associated insulator strings:- Figure 6 is an end elevationview of the assemblygshown'in Figure 5} and Figure 7 is a side elevation view of the assembly shown in Figure 5. In the drawings the supporting clamps and insulator assemblies are shown in dotted outline in order that the shield structure can be made to stand out more clearly.

Referring first to that embodiment of my invention illustrated in Figures 1-4 of the drawings, it is observed that the combination grading and corona shield effectively surrounds and envelopes the clamp which supports the transmission line conductor. This combination shield includes, as its main components, a grading ring In, parallel corona shielding members I I and l2 and a support member l3. As can be observed readily from Figures 1-4, the parallel members H and [2 extend downward from each oppositely disposed portion [4 and I5 of the grading ring 10 and, furthermore, the parallel members H and I2 include substantially horizontal parallel portions l5 and I1. All of the members I043 preferably consist of steel tubing of all welded construction and joints, or rather connections, between the individual members are formed by weld fillets such as is illustrated at (8 in Figure 1.

The construction of the combination shield illustrated in Figures 1 and 4 is such that the shield is entirely symmetrical about a central axis and this axis coincides with the longitudinal axis of the supporting insulators when the shield is correctly positioned with respect to its associated clamp and insulator string. Referring to Figure 2 it can be seen that the center line this being the center line through the axis of the lowermost insulator 19 of an insulator string 20, also forms what can be described as a central axis about which the grading ring [0 is disposed. The oppositely disposed end portions H and 15 of the grading ring are spaced equidistant from the central axis 17; in other words the distance a equals the distance a. Also, the end portions l4 and 15 are dished-up, this being done in order that flash over will strike the grading ring farthest away from the insulator assembly. The extent to which end portions 14 and are dished-up is such that they are approximately level with the bottom of the skirt of the lower insulator l9.

Ihe conditions of symmetry mentioned in connection with end portions 14 and I5 also apply to the parallel corona shielding members H and I2. For example, referring to Fig. 3, it will be observed that the members H and I2 are spaced equi-distant from the central axis 9:. In other words, the distance b equals the distance 22.

Support for the grading ring l0 and the corona shielding members I i and I2 is achieved by means of the supporting member l3 which extends upward as a tubular member from each of the parallel portions l6 and ll and which is flattened along the top portion 2 i, as shown in Fig. 4. This flattened portion 2i is cut-a-way at 22 to receive the support lug 23, said lug being attached to the lowermost insulator 9. A weld connection indicated at 2-"; connects the supporting member I3 to the lug 23. In this manner the entire shield assembly carried by the supporting lug attached to the lowermost insulator Id. The suspension clamp, indicated generally by the reference numeral 25, is also supported by the support lug 23 but the connection of the clamp to the support lug is pivotal connection, indicated at 25 in i, and virtue of this connection the .ciamp E2 is ire-e to pivot unaffected by the inertia of the shield This is an advantage since in normal useage conductor vibrations cause some oscillation of the supporting clamp and, in order to avoid excessive beating of the conductor against the clamp, it is desirable to have an arrangement wherein the clamp can faithfully follow the conductor vibrations.

The symmetrical arrangement of the grading ring IS with respect to the central axis x is found to provide adequate flash over protection and to improve voltage distribution on the insulators of the insulator string in a manner known to the art. In addition, the parallel corona shielding members I l and 12, together with the supporting member [3, provide corona suppression for the suspension clamp since these members effectively envelope the clamp. From viewing Figures 1-4 it can be seen that the members i I and I2 extend beyond each end of the clamp 25, see Figure 2, and also extend below each side of the clamp, see Figure 3. The support member l3 extends from below each side of the clamp over and above the clamp. For example, the flattened portion 2| of support member l3 lies above the clamp 25. Also, referring to -Fig. 2 where the grading ring 40 is shown in horizontal position with the dished-up portions l4 and I5 uppermost, it is seen that the flattened portion 2! projects above the grading ring 19 whereas the corona shielding members II and i2 do not project above the grading ring but, instead, extend below it.

As stated before, the transmission line conductor and its supporting clamp can be removed from beneath the insulator string without disturbing the shield. Reference to Figures 3 and 4 of the drawings will readily confirm this. As shown there, the space beneath the clamp is unobstructed so that it is merely necessary to break the pivotal connection at 26 in order to remove the clamp and conductor 21 from beneath the insulator string 28.

The parallel portions l6 and ll of corona g. shielding members H and I2 have their respective longitudinal axes parallel also and these axes are generally parallel to the longitudinal axis of the conductor 21. Referring to Fig. 4, the respective axes of portions l6 and H are designated at 28 and 29 and the axis of the conductor 21 is indicated at 30. Portions 3i and 32 of support member 13 extend at right angles generally upward from the horizontal parallel portions 16 and H of the corona shielding members H and 12. More precisely described, a plane including the center line of support member [3 is perpendicular to a plane including the center lines 28 and 29 of the horizontal portions l6 and Il. This is a further illustration of the symmetry of the shield assembly with respect to the insulator and clamp assembly.

The modification of my invention illustrated in Figures 5-7 is particularly adapted for use with strain clamp assemblies used at dead end towers. As is customary at dead end locations, double insulator strings, designated generally by the reference numeral 33, are provided and the lowermost insulators of the double string are connected to a support yoke 3i. This support yoke, in turn, supports a strain clamp arrangement 35, said clamp having connection to a main conductor 35 and a jumper loop 31.

The combination shield used in conjunction with the assembly just described includes a grading ring 38, parallel sets of corona shielding members 39 and All and parallel corona shielding loops 4! and 42. Grading ring 38 is symmetrically disposed about a central axis 1 see Figures 6 and 7, and oppositely disposed portions .43 and 44 of the grading ring 38 are spaced equi-dlstant from the central axis 11. In other words, the distance equals the distance 0. Portions 43 and 44 are dished-up in order to attract flash over, should it occur, to a point outermost from the insulator assembly. The dished-up portions 43 and 44 are approximately level with the bottom of the lowermost insulators of the insulator strings 33.

The parallel corona shielding members 39 extend slightly down and away from portion 43 of grading ring 38 and are provided with flattened upturned ends 45. These ends are positioned opposite to similarly upturned ends 46 on the parallel corona shielding members 40, the latter of course extending slightly down and away from the dished-up end portion 44 of grading ring 38 in manner similar to the members 39. The oppositely disposed upturned ends 45 and 46 are connected together, with the yoke support 34 between them, by means such as the connecting bolts 41. There are a corresponding number of members 39 and 40 with the result that there is a corresponding number of opposite ends 45 and 46 and these ends are connected in the manner just described.

The parallel loop members 4| and 42 also have flattened end portions, indicated at 48 and 49 in Fig. 7, and these end portions are connected to corresponding end portions of the shielding members 39 and 40. For example, the flattened end portion 48 of loop member 42 is connected to the flattened end portion 45 of shielding member 39. Means such as bolt assembly 50 provide the connection. Similarly, the flattened end portion 49 of loop 42 is connected to the flattened end portion 46 of shielding member 40. Of course, loop 4| is silmilarly supported from the other shielding members 39 and 40. Referring to Fig. 6 it can be seen that loop members 4| and 42 are spaced equi-distant from and symmetrically disposed with respect to the central axis 11. In other words, the distance d equals the distance d.

As has been described with reference to the construction shown in Figures 1-4, the construction shown in Figures 5-7 likewise includes the features of not adding inertia to the clamp and making it easy to disconnect the conductor without interfering with the shield assembly. As can be seen from Fig. 6 the conductor can be removed from beneath the shield, there being no obstruction in its way. Also, since the clamp 35 is supported beneath the yoke 34 and the shield assembly is attached to the yoke, the inertia of 'the shield is not added to that of the clamp.

Steel tubing of all welded construction is suitable for the members 38-42 and wherever the tubing members are connected, not being flattened out. welding is suitably employed.

Shield arrangements of the type I have described are found to effect considerable improvement in corona suppression besides maintaining adequate flash over protection and voltage distribution on the insulator string. Referring to the construction shown in Figures 1-4, for an installation employing a 30 unit insulator string,

individual insulators being of approximately 10" diameter; a 1.65" diameter conductor; and a suspension clamp approximately 21 /2" long, it is considered that 2" diameter tubing is satisfactory for the parts |0|3 and that the grading ring It! should be approximately 30" wide and 42" "longazOther dimensions of parts can be gaugeddn: proportion to the dimensions'just gglvenr It :is important that the diameter-of the tubing making up the shield assembly be slightly greater than that of the conductor. The oval construction of the grading ring I0 is preferably used in order to secure necessary clearance from the tower support; otherwise a circular grading ring would be quite adequate.

With the example just given, and using a simulated tower arrangement, it is found that the shield assembly; besides providing adequate flash over protection and improved voltage distribution on the insulator string, also suppresses corona rather well. For example, in a test run under dry conditions, corona. was observed at kv. in the absence of the shield whereas with the shield no corona was observed until 325 kv. line to ground, which is approximately 562 kv. line to line. Distinct benefit was noticed from the presence of the enveloping members |||3. In other words, corona from sharp edges of clamp members was effectively suppressed.

While, in describing my invention thus far, I have referred to its application to suspended insulator units, the invention is capable of application to equipment having other types of interconnected insulator units. For example, the embodiment illustrated in Figures 1-4 is capable of application to high voltage switch and bus insulator stacks wherein the shield support is arranged to remove electric stress from the bus clamp and cap of the insulator on the line end. As will be readily apparent to one skilled in the art, in this type of application the combined grading and corona shield would appear inverted as compared to its showing in Figures 1-4 since switch and bus insulator stacks extend'upward from a fixed support rather than being suspended from above. An idea of the inverted appearance may be gained from viewing Figures 1-4 upside down.

From the foregoing, it can be seen that improved combined grading and corona shield arrangements have been described which offer distinct electrical advantages and, in addition, retain mechanical and maintenance features deemed desirable. The shield construction is symmetrically disposed with respect to the associated insulator and clamp assembly, thereby effecting more uniform voltage distribution and improved corona suppression. The assembly is rugged and has relatively few parts. It is easily attached to the clamp support and the clamp and conductor can be readily detached from the support without interference of the shield.

Accordingly, while I have described and illustrated specific embodiments of my invention, I wish it'to be understood that I do not wish to be restricted solely thereto but that I do intend to cover all modifications thereof which will be readily apparent to one skilled in the art and which come within the spirit and scope of my invention.

Having thus described my invention, what is claimed as new and desired to be secured by Letters Patent is:

1. A combined grading and corona shield comprising a closed loop grading ring disposed about a central axis, said ring having oppositely disposed dished-up portions, parallel corona shielding members extending without discontinuity between and connected to said oppositely disposed dished-up portions, said shielding members be ing symmetrically disposed with respect to said central axis and having parallel straight length portions and said'straight length portions having parallel axes," and a support member extending between and connected to said straight length portions, said support member having a center line lying in one plane and said plane being perpendicular to a plane including the axes of the parallel straight length portions of said corona shielding members.

2. A combined grading and corona shield comprising a closed loop grading ring disposed about a central axis, said ring having oppositely disposed dished-up portions, parallel corona shielding members extending without discontinuity between and connected to said dished-up portions, said shielding members being symmetrically disposed with respect to said central axis and being located below the grading ring when said ring is positioned horizontally and the dished-up portions of said ring are uppermost, and a support member extending between and connected to said corona shielding members intermediate the ends of said members, a portion of said support member extending above said grading ring when said ring is in said horizontal position.

3. A combined grading and corona shield comprising a closed loop grading ring disposed about a central axis, said grading ring having similar oppositely disposed portions, parallel corona shielding members extending from each of said oppositely disposed portions, ther being a corresponding number of corona shieldin members extending from each oppositely disposed portion of said grading ring and the end of one of said shielding members extending from one of said portions being positioned opposite the end of a corresponding one of said shielding members extending from another of said oppositely disposed portions, parallel loop shaped corona shielding members, and means connecting said loop shaped corona shielding members to the oppositely disposed ends of the corona shielding members extending from oppositely disposed portions of said grading ring.

4. A combined grading and corona shield comprising a closed loop grading ring having oppositely disposed portions, a corresponding number of parallel corona shielding members extending from each of said oppositely disposed portions, an end of one or" the corona shielding members ex tending from one of said grading ring portions being positioned opposite the end of a corresponding corona shielding member extending from the opposite portion of said gradin ring, said oppositely disposed ends being flattened, parallel loop shaped corona shielding members, said members having oppositely disposed flattened ends, means connecting the flattened ends of said loop shaped corona shielding members to oppositely disposed flattened ends of said first mentioned corona shielding members, and means connecting the oppositely disposed ends of said first mentioned shielding members to a common support member.

5. The combination with a transmission line conductor, of a clamp therefor, means for connecting said clamp to and beneath one of a series of overhead connected insulators, and a combined grading and corona shield comprising a grading ring surrounding said clamp connecting means, spaced corona shielding members connected to said grading ring, said shielding members having correspondingly shaped portions, said clamp bein positioned between said shielding members and said members extending above, below and beyond each end of said clamp, a portion or one f i o n h ld ng m e ein ara-1.

8 to a corresponding portion of another oi said members, a support member connecting said corona shielding members and extending from below said clamp on each side of said clamp to above said clamp, and means connecting said support member to said clamp connecting means.

6. The combination with a transmission line conductor, of a clamp therefor, means for connecting said clamp to and beneath one of a series of overhead connected insulators, and a combined grading and corona shield comprising a grading ring surrounding said clamp connecting means, said grading ring having oppositely disposed dished-up ends, parallel corona shielding members extending between and connected to said dished-up ends, said clamp being positioned between said shielding members and said members having horizontal portions positioned below said clamp, a support member extending from one of said horizontal portions to another of said horizontal portions, and means connecting said support member to said clamp connecting means.

7. The combination with a transmission line conductor, of a clamp therefor, means for connecting said clamp to one of a series of connected insulators, and a combined grading and corona shield comprising a grading ring surrounding said clamp connecting means, said grading ring having oppositely disposed portions, a corresponding number of parallel corona shielding members extending from each of said oppositely disposed portions, the ends of corresponding ones of said shielding members bein oppositely disposed, parallel loop shaped corona shielding members, connected to said first mentioned corona shielding members, and means connecting said first mentioned corona shielding members to said clamp connecting means.

8. The combination with a transmission line conductor, of a clamp therefor, a supporting yoke for connecting said clamp to a plurality of series of connected insulators, and a combined grading and corona shield comprising a grading ring surrounding said yoke, said grading ring having oppositely dispos d portions, a corresponding number of corona shielding members extendin from each of said oppositely disposed portions, said shielding members having matched ends disposed opposite each other, means connecting said ends to said yoke, parallel loop shaped corona shielding members, and means connecting said loop shaped corona shielding members to said first mentioned corona shielding members.

9. A combined grading and corona shield comprising a closed loop grading ring disposed about a central axis, said ring having oppositely disposed dished-up portions, spaced apart parallel corona shielding members extending downward from said dished-up portions when said grading ring is in horizontal position and said dished-up portions are uppermost, said shielding members being connected to said dished-up portions and being symmetrically disposed with respect to said central axis, and a support member extending between and connected to said shieldin members, said support member being substantially perpendicular to said shielding members and extending from below the grading ring to above the grading ring when said rin is in said horizontal position, and said support member being located midway of the distance between the dishedup portions of said grading ring.

it). The combination with a transmission line conductor, of a clamp therefor, means for connecting said clamp to one end of an insulator cs- 9. sembiy, and a combined gradin and corona shield comprising a grading ring surrounding said clamp connecting means, said grading ring having oppositely disposed dished-up portions, spaced apart corona shielding members extending below said clamp and connected to said dishedup portions, said clamp being located between said shielding members and the space below said clamp being unobstructed by said shieldin members whereby said clamp can be withdrawn downward from between said shielding members without interference from said members, and means connecting said shielding members to said clamp connecting means.

ROBERT L. MCCOY.

REFERENCES crrnn The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,611,871 Austin Dec. 28, 1926 1,695,892 Fortescue Dec. 18, 1928 1,741,333 Peek Dec. 31, 1929 1,799,977 Fortescue Apr. 7, 1931 1,802,743 Wahlberg Apr. 28, 1931 1,893,318 Austin Jan. 3, 1933 2,232,665 Ruofl Feb. 18, 1941 2,539,641 Sheadel Jan. 30, 1951

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