US3312774A - Semi-insulating shielding for cables and the like and comprising discrete "floating"patches of semi-conductive material - Google Patents

Description

April 4, 1967 "r. F. PETERSON 3,312,774 SEMIINSULATING SHIELDING FOR CABLES AND THE LIKE AND COMPRISING DISCRETE "FLOATING" PATCHES OF SEMICONDUCTIVE MATERIAL Filed Feb. 10, 1965 2 Sheets-Sheet 1 l 5 m M m PM W A W A M MM 1 L p w m M A w F u 1 4 5 P N r ,A n .01 w m N I & n m .1 l m f M 2 w (ONDUCTOE I0 SC (AVE/9 United States Patent O SEMI INSULATING SHIELDING FOR CABLES AND THE LEKE AND COMPRISING DISCRETE FLOATING PATCHES OF SEMI-CONDUCTIVE MATERIAL Thomas F. Peterson, deceased, late of Shaker Heights, Ohio, by John D. Drinko, 1956 Union Commerce Bldg. 44114, and Central National Bank 'of Cleveland, 123 Prospect Ave. SW. 44101, both of Cleveland, Ohio Filed Feb. 10, 1965, Ser. No. 431,736 6 Claims. (Cl. 174-120) This application is a continuation-in-part of patent application Ser. No. 187,649, now abandoned, filed Apr. 16, 1962, and relates to capacitative shielding and stress control useful for high voltage electric cables, condenser bushings, and the like. 1

Examples of conductive shielding (e.g., with a layer of aluminum foil) for conductors and/or their insulation are well known to the art, as are examples of semi-conductive shielding (tag, as described and claimed in Patents 2,322,702 and 2,446,387).

With either conducting or semi-conducting shielding with prior art attempts to place such shielding interleaved between radially separate portionsof a body of insulation, a difiiculty has been that the shielding serves as a path for short circuits. Thus, whenever corona or over-voltage causes adjacent insulation breakdown the shield serves as a path, for example, along a length of cable to find its next weakest point.

It is an object of the present invention to provide simple means for overcoming the above mentioned difliculty.

Other objects and advantages will become apparent and the invention may be better understood from consideration of the following description taken in connection with the accompanyingdrawing, in which:

FIG. 1 is an elevation view of a single conductor cable;

FIG. 2 is a radial cross section of the cable of FIG. 1 with patch thicknesses exaggerated for clarity;

FIG. 3 is a patch thickness exaggerated view to show patch layers on both sides of an insulating material tape; FIG. 4 shows a modification;

FIG. 5 is a graphical representation of a portion of the electric field between patches according to one aspect of the present invention, as viewed in longitudinal section;

FIG. 6 is a graphical representation as in FIG. 5 except viewed transversely;

FIGS. 7 and 8 are illustrations of modifications in which semi-conductive patch portions comprise conductive material embedded in an insulating material tape.

FIG. 9 shows another modification.

In the manufacture of cable, insulating tapes, or semiconductive tapes, or insulating layers rendered conductive or semi-conductive on one or both sides, may be applied dry and ultimately the cable subjected to high temperature heating and vacuum to drive off gases and moisture, and then an impregnating oil introduced to saturate the entire mass.

According to one aspect of the present invention, a single conductor cable, as illustrated in FIGS. 1 and 2, may have a central conductor 10. While the present invention relates to a-discontinuous or patch configuration of semi-conductive material, it is by no means essential or even desirable that every semi-conductive layer be so characterized, and in FIGS. 1 and 2 about the conductor 10 is shown a more or less conventional continuous semi-conductive coating 11 which may be of insulating lacquer or paint carrying conductive carbon or metal particles. 7

For purpose of illustration, let it be assumed that over the semi-conducting coating 11 is a cable insulation layer 12 of paper (although for some applications it could be of another material; rubber, varnished cambric, or the like). In accordance with the invention, at some point, or points, that is with one hundred layers, it might be at fourth points or at 25th, 50th and th layers, but in the illustrated embodiment at the very next layer (for simplicity of showing), is a so-called patch layer 13 which may be applied helically, or otherwise, and which utilizes small patches, e.g., 1" x 1" staggered and spaced, e.g., 1.0" in one direction and 0.2" more or less transverse (and in a direction of otherwise possible fault current) so as to form a discontinuous layer of semi-conductive material assumed affixed to or incorporated in an insulating material 131'.

As shown in FIG. 1, next above the patch carrying layer 13 is a wrapping of, let it be assumed, paper tape 12', over which is a layer 14 having semi-conducting patches, over which is another paper layer 12", over which is another patch layer 14, over which is another paper layer 12", over which is another conventional semi-conductive 11, over which there may be a usual metal armor tape 15, and an outside sheath 17, .e.g., of lead.

The present invention is directed specifically to a combination of a conventional dielectric (that is, paper tape insulation or rubber, or whatever) with at least one patch layer interleaved within the body of the dielectric, said layer being also of insulation but having in or on at least one face or in or on opposite faces thereof spaced patches of semi-conducting material. The patch layer which may either comprise tape helically wound, so that the patches may look like those shown in layer 13 in FIG. 1, or may comprise separate radial windings or sleeves so that the patches have their edges transverse to the axis of the cable as shown at 14 and 14'.

In either event, and as perhaps most clearly seen in FIGS. 3 and 4, there is, with respect to the longitudinal axis of the cable, a longitudinal (as well as circumferential to helical) limitation of the extent of each patch. Those in the art will understand that the drawings, for clarity, are not to scale, that actually that which I call the patch layer may have semi-conductive material patches only 1 or 2 mils thick or the patches may be impregnated in the surface of the insulating material tape 13i so that each patch 13P outer surface is coplanar with portions of insulation 13i between patches (see FIG. 7). Even if they are not embedded the separations between discrete patches will be filled, due to the resilience of the insulation and/or the presence of insulating oil.

In FIG. 8, the patch layer shown comprises semi-conductive patch portions 13F having conductive particles impregnated therethrough.

In FIG. 9, a patch layer is formed by a tape wound helically, for example with adjacent edges. Patch portions 13P in this tape are spaced from each other and from the tape edges.

In FIGS. 3, 4, 7, 8 and 9 the actual thicknesses of the patches and of the insulation 131' on or in which they 7 are supported are exaggerated for purposes of clarity but it will be seen that there may be both top of tape patches 13T and bottom of tape patches 13B staggered with respect to each other. The insulation 13i may be that of an ordinary rlubber tape, and the patches 13T and 13B may be put thereon by spraying (as through a mask), or by printing, or according to various other obvious practices. Alternatively, the arrangement may be as in FIG. 4 where in no direction are the patches on any one side separated .by an extent equal to their own widths and whereby upper surface patches and lower surface patches overlap one another; or if desired there may be patches only on one side of the tape as is the assumed condition in FIG. 1. Insulating material tape carrying such patches resistivity. where between the value for insulation to 10 ohms 3 may be formed wound with either abutting or slightly overlapping edges, or spaced.

Many modifications are possible and the patch configuration may be applied to any insulating material, e.g., to some of the paper tapes of so-called paper insulated cable, and thus give the insulation improved characteristics. This is so because the semi-conductive patches constitute electrical stress equalizing barriers which, unlike metal layers and/or continuous semi-conducting strips, do not have the disadvantage of providing continuous conductive or electro-potential paths. If at one point in the length of a cable or condenser bushing, transformer bushing, or whatever, there is a weakening of the insulation or a failure from a conductor to a first intermediate layer of conventional conductive or continuous semi-conducting shielding, the electrical stress on the total cable, etc., length is increased across the other layers. This often immediately causes a subsequent failure of the next layer or layers perhaps several hundred feet on down the cable (or several inches or feet on down the bushing) Again there may be introduced a failure of a third layer up or down the length. However, with the patch arrangement of the present invention, these difficulties are obviated, because of the fact that continuity of conductivity is not essential for stress equalization, because capacity effect or lines of force allow stress equalization to be obtained without the difficulties described. Further, production of the patch strips can be accomplished simply and economically by printing with a conductive material such as carbon ink, or by dusting a fine conducting material onto the surface of insulating material, or by rubbing it into the surface, in patch configuration, or by using other methods to apply, for example, a carbonaceous or metal powder (with or without a dispersion or insulating medium) to, or impregnate in, insulating material in the form of a tape, or sleeve, or whatever.

If any data is needed as to make-up or numerical resistivity values of conventional semi-conductive shielding materials it can be found in the above-mentioned patents. As stated therein the surface resistivity of semiconducting material may be millions of times that of metal foil.

Volume resistivity is usually somewhat higher than surface resistivity (due to difliculty of making currents divide equally through a solid, because of skin effect) and it is usually more convenient to talk of surface it would be short circuited, and not next to the sheath, where it would be short circuited, but at a mid-point or mid-points (and at less mid-points than if conducting materials were to be used either as continuous layers or as patches) is a semi-insulating layer. To talk merely of conductivity or resistivity might be somewhat misleading because they are one thing for D.C. and usually something quite different for the A.C. condition which is conventional when one talks of high voltages in the United States. But, in general, electrical conductors (copper, aluminum, steel, etc.) have very small values of resistivity, e.g., 10- ohm per cm. per cm. (often abbreviated as ohm-cm?) Far different is the resistivity of ordinary insulating materials (rubber, paper, oil) for which the values of resistivity are on the order of 10 ohm-cm.

Within this vast spread, from one end of the spectrum to the other, are the semi-conductors (words first used for shielding, at least as early as 1929, later, e.g., in 1948, used for transistors). For the semi-conductors as used incable shielding heretofore, the ranges of resistivity have been, for example, from several ohms to 10 ohm-cm? Now, in accordance with the present invention, e.g., considering 1 square patches spaced 0.2 apart per tape (and with contiguous tape windings also spaced 0.2 apart) and with staggered patches (as in FIG. 3) then, even considering all the parallel paths (e.g., directly longitudinal, plus helically, plus obliquely across patch corners, plus obliquely from patch above to nearest patch below in the same layer, as the word layer is used herein and in the claims hereafter), the average resistivity of the patch layer is going to fall closer to the 10 value .for full insulativity than to the (e.g;, 10 value for the semi-conducting material of which the patches are made,

and the patch layers average resistivity might well be 3x10 ohm-cmF.

of patches of solid metal foil, an even greater difference is noted in such things as heating (due to eddy current and other useless losses, which lead to inefficient heating, at least, and sometimes to complete ruin, by blow up, of the cable), power factor, specific inductive capacity, and the like. Many of these things have heretofore been aggravated in direct proportion to an increase in the protection afforded. A tabulation may be made as shown in the following table.

Resistivity. S.I.C.

Ohm 0111. Heating Pl. (Dielectric Constant) Conductors 10- High Semi-Conductors E.g. 3 to 10 8 Low 100 Semi-Insulating Layers of 8.0. Patches-Preference. E.g. 3X10 Very low 2 11-. 20 3 1042 Insulation Eng. 10 12 to 10 Very low .2 to 5 2 to 2. 6 Dry Air Insufficient data 0 0 1 1 Not. ap licable. 2 (More I; ran .10.) 3 (More than 06.)

5 Before breakdown.

Actually semi-conductivity may lie anyper cm. per cm.) and the value for conductors ('10- ohms per cm. per em.) but as the words are used hereafter the semi-conductive range is narrower (e.g., 3 to 100,000,000 ohm-cm?) The present invention (of patchwork) makes a quite different average resistivity range available because the patches are discontinuous, so that even though it is somewhat preferred to have the resistivity of the individual patches fall in the semi-conducting range, the average resistivity, considering a typical (repeated) length of patch layer, will be higher.

It is desired to be understood that what has been introduced, and preferably not next to the conductor, where Operation of the invention which provides neither conductivity nor seinhconductivity from one good electrical contact point to another, may be understood from reference to FIGS. 5 and 6. Here patches 13' are shown, for simplicity without showing the insulation on which they are mounted or the other insulation layers (e.g., 12)

. between which they are interleaved. It is axiomatic that when a body (e.g., the central conduct-or 10 in FIG. 1, not shown in FIGS. 5 and 6) is electrically charged (e.g., with respect to the metallic sheath 17 of FIG. 1, also assumed but not shown in FIGS. 5 and 6) an electric field is formed which is represented by a group of lines beginning at the region of positive charge and ending at the region of negative charge. Portions 18 of such lines are shown in FIGS.

While this will be different than. if, by way of contrast, the patch layer were made up 5' and 6, where it is seen that the lines are collected by the SC. patches 13 in any semi-insulative layer. It is this which prevents'the gaps between adjacent patches of each patch layer from being weak points in the cable. It will be apparent that the present invention is bottomed upon the fact that it is impossible for an electric held to exist tangent to a conductor. This dictates a relatively small space between adjacent patch edges. Preferably this space is less than the radial distance (a) from one patch layer to another (considering any opposite sides patches as shown in FIGS. 3 and 4 as belonging to the same layer.), or (b) from one patch layer to the next layer of contin uous conducting or continuous semi-conducting material. Otherwise thevalue of the invention may be lost since some lines of force might penetrate the gaps between patches of anyone layer and the patch layer would then no longer be an equipotential layer, the capacitative gap between patches (per layer) being greater than that between layers.

The invention is believed to be equally amplicable to any insulated conductor electrical device, such as a condenser bushing, although, for simplicity, only embodiments in connection with cable have been mainly described. If a lone central conductor is cylindrical as shown, then the patches as well as any more conventional semi-conducting layers are in the form of essentially cylindrical equipotential surfaces positioned at varying radial distances from the center of the conductor and concentric therewith but there is at least one layer (a patch layer) characterized by the fact that a patch or patches extend axially along only a portion of the other equipotential surfaces comprising conventional central conductor outer surface, outer metal inner surface, conventional semi-conducting layers, if any, etc. The device may not be cylindrical and may have some other shape such as oval in which case the equipotential surfaces may also be oval in shape. In the claims hereafter the words generally cylindrical are intended to cover conventional conductors whether round or oval or sector shaped in cross section, but any intermediate equipotential surfaces in the insulation need not exactly follow the contour of the conductor or conductors but could follow the contour of the outer sheath instead.

The word patch as used in the claims need not he considered as synonomous with one of plural patches (e.g., for any one layer). There might be but one patch (per layer, or there might be but one patch layer), but in any event it is contemplated that the semi-conducting patch (i.e., portion), which is generally concentric to the conductor, will have a length but a fraction of the total length of the conductor (by which is meant less than that portion of the conductor which, excluding joint connections, is surrounded by the principal insulation, i.e., the high voltage insulating dielectric).

Generally though, the patches are relatively small. As a specific example, consider a cable having a central conductor (or conductors) and about 1" of oil impregnated paper tapes which might be strips of paper ranging from .003 to .008 in thickness and with widths of approximately 1", with open butt (openings between turns) of, for example, In intermediate layers (or layer) such a tape has relatively small semi-conducting patches each extending, for example, over the 1" width of the tape and for a distance of l to several inches along the length of the tape, with each patch separated from the next patch along the length of the tape by a distance of ,5 the same distance as that of the opening between turns.

If there is but one equipotential surface appearing midway between the conductor and the generally grounded outer sheath then, if the conductor has, for example, a peak potential of 12,000 volts, it might be expected that the equipotential surface at the mid-point would have a peak of 6000 volts but that, of course, will vary depending upon the kind of insulation, form or shape of the 1. In a high tension cable having an inner conductor,

an outer metal sheath, a dielectric insulation, semiconducting shielding on an inner surface of said dielectric insulation adjacent said conductor, and semi-conducting shielding on an outer surface of said dielectric insulation adjacent said sheath, the improvement consisting of a semi-insulating shielding layer which is embedded in said dielectric insulation intermediate of and electrically insulated from each of said semi-conductive surface shieldings, said semi-insulating embedded shielding comprising a plurality of spaced patches each having a conductivity which is at least in the semi-conducting range, each of said patches being spaced and thus electrically insulated in all directions from every other patch whereby to preclude short circuits through, along, and around said embedded. shielding.

2. An electrical device having a generally cylindrical conductor operable at a high electrical potential to produce an electric field emanating at right angles from the surface of the conductor, at least two layers of dielectric material comprising a radially inner layer concentric with and surrounding said conductor and a second layer concentric with and surrounding the inner layer, and at least one patch of semi-conducting material located intermedi ate the two dielectric layers while having an effective length, measured axially of the conductor, which is but a fraction of thelength of each of said dielectric layers, and having an effective circumferential extent which, measured in degrees, is but a fraction of the circumference of each of said dielectric layers, whereby to provide a layer of semi-insulating characteristic.

3. In a high tension cable having an inner conductor, an outer metal sheath, a dielectric insulation, semi-conducting shielding on an inner surface of said dielectric insulation adjacent said conductor, and semi-conducting shielding on an outer surface of said dielectric insulation adjacent said sheath, the improvement consisting of a semi-insulating shielding layer which is embedded in said dielectric insulation intermediate of and electrically insulated from each of said semi-conductive surface shieldings, said semi-insulating embedded shielding comprising a plurality of spaced patches each of semi-conducting material, each of said patches having no dimension greater than one inch, and each of said patches being spaced and. thus electrically insulated in all directions from every other patch whereby to preclude short circuits through, along, and around said embedded shielding.

4.'An electrical device as in claim 2 further characterized by there being plural patches intermediate the dielectric layers, all of said patches lying on a face of an insulating material tape.

5. An electrical device as in claim 2 further characterized by there being a third dielectric material layer intermediate the inner layer and the second layer, and by semi-conductivity being resident in patches along one face of said third dielectric material layer while the opposite face thereof carries semi-conductivity in patches which span the intervals between patches on the first mentioned face.

6. In an electrical device having a longitudinal axis,

(a) a conductor element extending along said axis for carrying current at high potential,

(b) a coaxial first solid insulating dielectric arranged immediately about the conductor element and except 7 for end terminal regions axially coextensive therewith,

(c) at least one semiinsula'ting layer which is characterized by having material which is semi-conductive and which material is in discrete portions and thus discontinuous both longitudinally and circumferentially to heli-cally whi-le otherwise arranged immediately about and coaxial with said first dielectric to be operable at midpotential due to capacity effect despite the discontinuities,

(d) second solid insulating dielectric Occupying the spaces extending longitudinally and circu mferentially to helically between the said portions of material which is semi-conductive,

(e) a coaxial third solid insulating dielectric arranged immediately about said second dielectric and about said material which is operable at rnidpotential, and (f) a coaxial outer conductive layer operable at ground potential while surrounding said conductor, surrounding all of the insulating solid dielectric and surrounding said material operable at midpotential.

References Cited by the Examiner UNITED STATES PATENTS 2,081,517 5/1937 Van Hoffen l74-127 X 2,260,845 10/1941 Urnston 174-105.1 2,453,313 11/1948 Gordon 174-1021 3,088,995 5/1963 Baldwin 174l27 3,090,825 5/1963 VOlk 174-36 15 LEWIS H. MYERS, Primary Examiner.

E. GOLDBERG, Assistant Examiner.

Claims (1)

1. IN A HIGH TENSION CABLE HAVING AN INNER CONDUCTOR, AN OUTER METAL SHEATH, A DIELECTRIC INSULATION, SEMI-CONDUCTING SHIELDING ON AN INNER SURFACE OF SAID DIELECTRIC INSULATION ADJACENT SAID CONDUCTOR, AND SEMI-CONDUCTING SHIELDING ON AN OUTER SURFACE OF SAID DIELECTRIC INSULATION ADJACENT SAID SHEATH, THE IMPROVEMENT CONSISTING OF A SEMI-INSULATING SHIELDING LAYER WHICH IS EMBEDDED IN SAID DIELECTRIC INSULATION INTERMEDIATE AND OF AND ELECTRICALLY INSULATED FROM EACH OF SAID OF SAID SEMI-CONDUCTIVE SURFACE SHIELDINGS, SAID SEMI-INSULATING EMBEDDED SHIELDING COMPRISING A PLURALITY OF SPACED PATCHES EACH HAVING A CONDUCTIVITY WHICH IS AT LEAST IN THE SEMI-CONDUCTING RANGE, EACH OF SAID PATCHES BEING SPACED AND THUS ELECTRICALLY INSULATED IN ALL DIRECTIONS FROM EVERY OTHER PATCH WHEREBY TO PRECLUDE SHORT CIRCUITS THROUGH, ALONG, AND AROUND SAID EMBEDDED SHIELDING.

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