US2373843A - Insulated electrical conductor terminal structure - Google Patents

Description

April 17, 1945 J. H. NICHOLAS 2,373,843

INSULATED ELECTRICAL CONDUCTOR TERMINAL STRUCTURE Filed Oct. 28, 1942 Patented Apr. 17, 1945 UNITED STATES PATENT OFFICEA INSULATED ELECTRICAL CONDUCTOR TERMINAL STRUCTURE James H. Nicholas, Chicago, Ill., assignor to G & W Electric Specialty Company, Chicago, Ill., a corporation' of Illinois Application October 28, 1942, Serial No. 463,582,4

6V Claims. (Cl. 174-73) This invention has to do with terminal devices for insulated electrical conductors of the type enclosed within a, metallic sheath, and relates more'Y particularly to an electrostatic ux controlling structure *associatedy with an end portion ofv said sheath to direct therefrom, externally of the device, flux lines that would otherwisev extend from said sheath end portion interiorly ot the device to the conductor.

The general object' of the invention is to provide an improved and more eifective means for dispersing` the iiux lines traversing a medium between an end of the conductor sheath and a conductor portion of different electrical potential projecting axiallyfrom such sheath, whereby the potentialv difference between the conductor and its sheath can obtain at higher values without incurring excessive ionization of said medium.

The prevalent practice in the insulation of electrical conductors from:v their conductor sheaths, as in cables employed in the commercial transmission of high voltage electricity, is to supplementl the` solid insulation between these conductors and their sheaths with oil; to iill any voids that-occurinth'ei solid insulation or between such insulation and the conductor or the sheath and thereby provide an insulation; having greater dielectric strength to withstandI stresses resulting from line potential surges, Insulated conductors of this type generally require accessory equipment for supplying oil tov them under pressure to prevent the development' of an oil ldeficiency at any point. However, the original and maintenance cost of such equipment is suicient that for installations in the lower part of the voltage range in which the oil-lled insulated conductors or cables have been used heretofore, the practice of using cables filled withcompressed gas is finding favor. A cable containing nitrogen, commonly maintained at about iifteen pounds per square inchI gauge pressure or higher, has proven satisfactory and has certain advantages not pertinent to the present disclosure.

The terminal devices usedV with these cables arev communicative therewith so that an oil-lled terminal device, such asa pothead, is used with an oil-lled cable, whereas a gas-lilled terminal devicey is used with agas-lled cable. Hence there arises another general object of this invention, namely,. the' provision of an electrostatic flux dispersing structure employable in such a manner with a gas-filled terminal device that excessive stressy upon the gas portion. of the dielectric therein. will be avoided without materially increasing the sine of the device.

lll

A more specific object is the provision of an electrical conducting shield about an exterior portion of a terminal devicev insulating tube and connectable with an electrical conducting sheath of an insulatedl conductor extending into the tube', to create an electrostatic flux pattern between said external shield and the conductor in parallel with the electrostatic flux pattern betweenv the sheath terminus and the conductor interiorly of the tube as an expedient for decreasing the concentration of the' latter pattern.

These and other desirable objects inherent to and encompassed by the invention will be better understood from the ensuing description together with the annexedv drawing, wherein:

. Fig. l is a View partly in side elevation and partly insection, illustrating the invention in its application tov a pothead device associated with a dressed end= portion of a lead sheathedy conductor; and

Fig. 2 is an enlarged fragmentary sectional view presenting greater detail of parts shown in Fig. l, and also illustrating flux patterns and dielectric breakdown paths for certain construction stages.

The invention is illustrated in itsv application to a pothead terminalfor a lead sheathed cable inasmuch as it is particularly adapted for such an installation .although conceivably the invention is. capable of utility in` other types of structures.

Referring now to. the drawing, a sheathed. insulated. conductor oftthe cable type there indicated at I has a dressed end` portion I I which is initially prepared by removing therefrom a` portion of a lead4 sheath I2 from Aa point beyond an end I3 of this sheath.v Removal of this portion of the sheath I2 exposes a length of conductor insulation Ill which is` disposed between the stranded conductor proper l5 and said. sheath. Adjacently tothe end ofthe conductor I5 the insulation I4 is stripped. away, exposing that part of the conductor end portion beyond the terminus I6 of the insulation layer. An electrical conductor ierrule member I-l. is connected to this exposed end portion of the conductor I5. In the fully assembled device this ferrule member I'I is telescopedV into an electrical conductor cap I8 of which a skirtportion I9 is seal-connected to the upper end of a tubular porcelain insulator 20. The iit between the ferrule II'I' and the cap I8 establishes an electrical conductor relation between these parts wherefore said cap is electrically connected with the conductor I5 and is adaptedr to have connected therewith other electrical conductor connecting devices (not shown) lapped winding of dry varnished cambric, thatl is, each turn of the lwinding is lapped halfway over the preceding turn. The layer 23 is coextensive with the cable insulation projecting from the end of the -lead sheath I2. added onto the layer 23 a shorter layer 24 which There is begins at a point 25 spaced axially from the end of the sheath I2 and tapers with increasing radius to a point 216 at which the taper is reversed, decreasing in diameter to the end of sad layer 24 at a'pont 21. Insulation layer 24 is also made of varnished cambric together with a suitable dielectric compound interposed between successive layers to prevent the trapping of air in the dielectric mass. A suitable compound, well known in the trade by the notation G. E. 219 Com pound is obtainable from the General Electric Company at Schenectady, New York.f An electrostatic stress relief cone is formed by applying an electrical conducting medium 28 upon the tapered surface of the layer 24 nearest to the insulated conductor sheath I 2, this member 2B being in electrical conducting relation with the sheath and in the present instance is formed of varnished cambric between which a dielectric 7 compound as the aforesaid is applied. The layer 29 begins at a point 30 spaced slightly from the lend of the sheath I2 and tapers axially of the structure by increasing in diameter until a point 3I is reached. The outer periphery f the dielectric layer 29 is cylindrical between the point 3| and a point 32 at which a reversely tapered portion begins, decreasing in diameter to the end of said layer 29 at a point 33.

The function of this flux dispersing structure 22 is to deconcentrate or disperse the electrostatic flux pattern formed between the end of the conductor sheath I2 and the conductor I5 when a difference in'electrical potential exists between these parts. When in use the sheath I2 will usually be grounded whereas the conductor I5 will be potentialized with respect to ground. If no part of this-structure 22 were employed the flux lines caused to exist between the conductor and the sheath would form a pattern somewhat of the nature illustrated by the dotted lines A. It will be noted that these lines form a pattern in which the lines reaching farthest from the sheath end approach parallelism with the conductor and this is because the dielectric constant (specific inductive capacitance) of the cable insulation is greater than that of the surrounding medium, so that the flux tends to pass through the cable insulation and follow the surface of such cable in the medium of the 'lower dielectric r Vby iiux creepage occurs as explained above.

constant to the grounded sheath electrode. This surface creepage may be suiicient when increased pursuant to line voltage surges to which the conductor I5 may be subjected, to so increase and concentrate the flux lines within a medium about the cable insulation layer I4 above the sheath end I3 that excessive ionization of the medium will there occur, precipitating its electrical failure and flashover between the sheath end I3 and the end cf the conductor exposed above the insulation layer I4. When the insulation layer 24 and the member 28 are assembled with the sheath I2 in the manner shown (assuming the layer 29 not present) the flux pattern will change to one somewhat as illustrated by the short dash lines B. It will be observed that these flux lines are more nearly perpendicular to the insulation wrappings than are the flux lines A. However, when the difference in the potential of the conductor I5 and the sheath I 2 is raised sufficiently, the fiux lines B will be projected vmore distantly from the end of said cone than illustrated in the drawing, causing excessive ionization along the cable surface from member 28 and ultimate electrical flashover over the surface of the insulation layer 24 along a path such as that illustrated by the long dash line C. This is due to the relatively large dielectric constant of the insulation where- By addition of the solid dielectric layer 29, which has greater dielectric strength (in contrast to dielectric constant or conductivity of electrostatic iiux) than a gas at low pressure, ionization at the cone member 28 can be suppressed. When the potential between the conductor and the sheath is increased to a certain value, ionization y will occur along the surface of the layer 29 beginning from a zone in radial registry with the terminus of the stress cone member `28. Under these circumstances the lashover will occur at a higher voltage than noted above but between theV sheath end I3 and the upper end of the conductor, along a path somewhat as that illustrated by the dot-dash line D. l

While the embedding of the conical stress relief member 28 in the dielectric layers 24 and 29 results in the disposal of additional dielectric material in the path of the electrostatic flux lines bridging the flared or larger diameter end of said member and the conductor I5, and thus correspondingly increases the difference in potential that may exist between the said conductor and its sheath I2 without causing ionization of the dielectric along the surface of the conductor insulation within the tubular insulator 20 and ultimate ashover, it has been found that the potential difference between the conductor I5 and its sheath I2 can be still greater without resulting in ashover by providing an velectrical conductor shield 35 about the outer periphery of said tubular insulator at a portion thereof adjacently to its end that is' disposed in proximity to the end of the sheath I2. This shield 35, which is an important feature of the present invention, may be applied to the insulator 20 by firing a conductive coating on the insulator at the surface area desired to be covered. Another method of ailixing the layer 35 is to spray molten metal onto the desired surface area and allowing this sprayed metal to solidify in the form of a thin electrical conductor sheet. The shield 35 may consist of a preformed metal body, or, it may be incorporated into the metallic body or mounting brackets of the device. Said shield 35 may completely circumscribe the tubular insulator 20, the upper end thereof reaching to and curving radially outwardly and downwardly a short distance upon the under side of the -lower` petticoat 36 of the bushing. Preferably the shield 35 is a solderable material such as copper anda portion 3T thereof extends downwardly over the outer periphery of a shoulder 38 extending about the base of the insulator to facilitate the lonnection of a flared upper edge portion 39 of a wiping sleeve 4U to the insulator through a solder layer 4I. Priorto attaching the wiping sleeve 40 to the insulator a ring 42 of heat resisting material as felted asbestos is interposed between the wiping sleeve and the insulator to provide adam onto which molten solder forming the ring 4I can be poured into the space between the flared portion 39 of. the wiping sleeve andend vof the stress relief cone member 28. This places the shield member 35 in radial registry with the upper portion of the dielectric layers 24 and 28. Inasmuch as the shield member 35;is electrically connected with the sheath I2, the potential difference between the shield 35 and the conductor I5 will be the same as that between the sheath and said conductor. It follows that there will be electrostatic flux lines bridging the space between said shield 35 and the conductor l5, these flux lines forming a pattern such as that illustrated Iby the dotted lines E. Particularly it should be noted that the electrostatic flux lines E traverse the solid insulation or dielectric material to reach that part of the conductor I5 which is also linked with the stress relief member 28 by flux lines, In this manner the shield 35 is operable to supplement the stress relief member 28 as a terminus for ilux lines reaching to the projecting portion of the conductor and thus diminish the number of flux lines linked with said member 28 for any given potential existing between the conductor I5 and its sheath I2. In fact with a flux dispersing structure and tubular insulator 29 of the proportions illustrated in the drawing, addition of the shield member 35 so diminishes the flux concentration at the stress relief cone 28 that upon increasing the potential between the conductor I5 and its sheath I2 to the point of excessive ionization, the ashover actually takes place exteriorly of the tubular insulator, between said shield 35 and the electrical conductor parts connected with the conductor I5 at the upper end of the tubular insulator, when air at atmospheric pressure is contained in the tubular insulator about the solid insulation. The insulation layer 29 is instrumental in obtaining this external flashover. If it were omitted from the shown structure the shield 35 would not prevent internal flashover, which would occur along the zone indicated |by the line C. Since the layer 29 has greater dielectric strength than air it prevents the formation of conductive corona streamers issuing axially from the end of the stress relief cone member 28 at potentials suiiiciently great for the flux pattern at E to attain proportions signincantly relieving the flux stress at said cone.

By incorporating the dielectric layer 29 in the flux dispersing structure 22, described above, the potential attainable between the conductor I5 and the sheath I2 ybefore incurring flashover, along the path indicated by the dot-dash line D, is 30% to 49% greater than the potential attainable between said conductor and its sheath when the layer 29 is omitted and when the ashover occurs along a path such as that indicated by the long dash line C. When the layer 29 is present as well as the shield 35 and its connections with the sheath I2, the aforesaid flashover exteriorly of the porcelain tube has been obtained at a potential between the conductor l5 and its sheath as much as 65% greater than that incurring flashover under similar circumstances when the shield 35 is not used, and approximately greater than the potential attainable betweensaid conductor and its shield when the stress relief memiber 28 is exposed exteriorly of the dielectric layer 24 by the absence of the layer 29 and when the bushing shield 35 is not used. These percentages are based upon experiments employing a tubular insulator approaching the maximum length at which the flashover occurs exteriorly instead of interiorly thereof.

This relatively compact flux controlling structure including the tubular insulator shield 35 so increases the opposition to flashover that a gaslled terminal device equipped therewith need be no larger for use upon a cable of a given potential than an oil-lled device having only a conventional stress relief cone.

While I have herein shown and described but a single preferred embodiment, thought to be fully illustrative of the invention, it should be understood that the invention. extends to other forms, arrangements, structures, details and degree of improvement falling within the scope and spirit thereof and not sacricing rall of its material advantages.

I claim:

1. In a terminal structure for an insulated electrical conductor having a portion projecting outwardly from an electrical conductive casing therefor, the combination of a flux-dispersing structure including an electrical conducting member spaced radially from and projecting axially of and about said projecting insulated conductor portion in electrical conducting relation with said casing, a tubular insulater about a projected end portion of said electrical conducting member and, also about the projecting conductor portion and projecting axially thereof a greater distance from said casing than the iiuX-dispersing structure, an electrical conducting shield about the outer periphery of said tubular insulator within a zone extending toward the end of the projecting conductor portion to an axial limit more distantly from said casing than the portion of said electrical conducting member most distant from said casing, and electrical conducting means extending between said shield and said casing.

2. In a terminal Structure for an insulated electrical conducto-r having a portion projecting from an end of an electrically conductive sheath, a solid dielectric material about a section of the projecting conductor portion adjacently to said sheath end, an electrical conductive member about the projecting conductor portion, said member being embedded in said dielectric material and being internally flared radially and axially away from said sheath end, a dielectric housing about the dielectric material and also about a projected end portion of said electrical conductive member, an electrical conductive shield disposed exteriorly of the vhousing with at least a portion thereof a greater axial distance from the sheath end than the flared end of said embedded member, and means for maintaining the shield at substantially the potential of said sheath.

3. In a gas-filled terminal structure for an insulated electrical conductor having a portion projecting from an end of an electrically conductive sheath, a mass of substantially gas-freel solid dielectric material about a section of the projecting conductor portion adjacently to said sheath end, an electrical conductive member about the projecting conductor portion, said member being embedded in said dielectric material andI being internally ared radially and axially away from said sheath end, a dielectric housing about the dielectric material and also about the flared end of said embedded member, an electrical conductive shield disposed exteriorly of the housing with at least a portion thereof a. greater axial distance from the sheath end than the ared end of said embedded member, and means for maintaining the shield at substantially the potenti-al of said 'y sheath.

4. The combination set forth in claim 2, Wherein the last named means is an electrical conductor bridge between the shield and sheath and includ- A e projecting conductor portion to sustain a pat.n

tern of ilux lines extending generally axially of the conductor and oppositely from saidsheatb end' into intersecting yrelation with a section'of said projecting conductor portion, a dielectric housing aboutsaid projecting conductor portion exteriorly of the flux dispersing structure, and

. 6. In a terminal structure for an insulated electrical conductor having a portion projecting from an end of an electrical conductive sheath therefor, the combination of a tubular insulator disposed about said projecting conductor portion in radial spaced relation therewith and having an end portion in proximity to said sheath end, a body of solid dielectric material substantially lilling the space between said bushing end portion and apart of the conductor substantially radially opposed thereto, a sleeve-like electrical conducting member having an inner periphery of graduated diameter increasing toward. an end thereof, said sleeve-like member being disposed about said conductor portion with the larger diameter end thereof embedded in said dielectric material and projecting int'o said tubular insulator and with thev other end'thereof conductively connected to said sheath end, an electrical conducting shield about the outer periphery of said tubular insulator within a zone extending axially a limited distance from said insulator end, and a sleeve-like electrical conducting member interconnecting said shield and said sheath.

JAMES H, NICHOLAS.

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