US1884311A - Laminate insulation - Google Patents

Description

Oct. 25, 1932. s g s 1,884,311

LAMINATE INSULATlON Filed Jan. 9, 1929 mama 0a. 25, 1932 UNITED STATES PATENT OFFICE DONALD M. SIMMONS, OF OSBORNE, PENNSYLVANIA, ASSIGNOR TO GENERAL CABLE CORPORATION, OF NEW YORK, N. Y., A CORPORATION OF NEW EBSEY LAMINATE INSULATION Application filed January 9, 1929. Serial No; 331,172.

My invention relates to improvements in insulation for electric conductors, and particularly to insulation which includes laminae of paper, cloth, or other porous material, filled with insulating substance. The insulating substance when applied is in liquld condition. A typical case is cable insulation formed of paper tape wrapped to place and filled with insulating compound. The object of my improvements is to strengthen such a body of insulation against breakdown.

The accompanying drawing is a fragmentary view of a cable, showing in elevatlon the conductor itself, and diagrammatically and in axial section a body of surrounding insulation built in embodiment of my invention. The paper or other porous material which forms the immobile portion of such an insulating envelope as I have indicated is ordinarily applied in the form of a ribbon or tape of relatively great width and small thickness, and in the building up of the laminze one upon another spaces of relatively small width are commonly formed between the successive turns of tape in any given lay er; for, ordinarily, in the laying of the tape the adjacent edges of successive turns are spaced apart at a slight interval. In the building of cables particularly, the tape is laid spirally around the conductor, and the spacing apart of the edges of successive turns 1s a feature of value and importance, since it afiords in the finished cable superior flexibility. The cable may be bent without undesired wrinkling of the paper of which the body of insulation is fundamentally composed. ln the building of layer upon layer the spaces between succeeding turns in an underlying layer are covered by the continuous web of the next overlying layer. That is to say, thesuccessive layers of paper or its equivalent are so superposed that the spaces in one layer are offset with respect to those of an adjacent layer.

This is illustrated in the drawing. The cable conductor is indicated at 1. Upon it the tape 2 is spirally Wrapped, and between the turns of the spiral spaces (a, c, a l, e, are left. The spaces 2') of the second layer are offset with respect to spaces a of the first layer; and the spaces 0 of the third layer are offset with respect to the spaces 6 of the second; and so on.

These spaces, indeed, are of small size. Even so, under practical conditions of cable building, they are of sufficient size to present a problem and a difliculty-which by my invention is solved and overcome.

The wrapped-on body of paper or the like, after application, is filled with insulating material in fluid condition. All the pores are occupied, and the spaces a, b, 0, etc. as well.

Generally speaking, insulation so built is the best that the art knows for the purpose.

In the transmission of power at high voltage every weakness is searched out, and it becomes a matter of engineering to design a cable of minimum dimensions and cost to carry, through long periods of time, current of given maximum power.

s between a body of paper or equivalent fibrous material filled with oil or other fluid or viscid insulation and a free body of such fluid or viscid insulation of like dimensions, the former is, dielectrically considered, much the stronger. And, with attention confined to free bodies of liquid or viscid insulation, the unit dielectric strength is much greater, relatively speaking, in a thin layer than in a layer of greater thickness. In structures such as that under consideration, all bodies of compound which fill spaces otherwise void are weaker than the bodies of compoundfilled paper, and the bodies of compound which fill the spaces a, b, 0, etc.. since they are of much greater radial depth than any other bodies of free compound within the structure, are by far the weakest portions of the whole insulating envelope.

As cables such as that under consideration continue in use, there is apt to be diminution in compound-whether because of i and more complete absorption by the rous bodies, or in consequence of flow tc other regionsand consequent voids are apt to appear. voids, wherever they occur, are, beyond ali else, weaker.

lie the transmission of alternating the stress upon the insulation is dist,

in inverse proportion to relative specific inductive capacity. That is to say, at points where the specific inductive capacity of the insulating wall is less, the strain tending to break-down is greater.

Coming to conditions of actual practice, the body of compound-filled porous paper may have a specific inductive capacity of about 3.5; whereas free bodies of liquid or viscid insulation if present within the spaces a, b, 0, etc. may have a specific inductive capaclty of 2 or slightly more than 2; and voids, if voids are found, have a specific inductive capacity of 1.

Therefore the tendency to break-down throu h the oil-filled s aces a, b, 0, etc. (if, indee ,these spaces be lled with oil), which exists because of the inferior strength of free bodies of oil to resist break-down, is mtensified, because of the concentration of stress upon these regions of low specific inductive capacity. And in the use of cables whose insulation is of the character described, whether the current carried be direct or alternating, it is found that the cable tends to break down recisely at the compoundfilled spaces. It is there that, under excessive stress, ionization occurs and charring begins, accompanied sometimes and perhaps facilitated by chemical change in the insulating compound itself.

My invention consists in a modification of structure, whereby break-down strength at the weakest points is increased and the efl'ect of differences in specific inductive capacity of materials is diminished.

In reference made above to the drawing, I have for purposes of discussion, assumed that in the completed cable the spaces a, b, 0, etc. were filled with insulating compound alone. Such is not, in fact, the case. In the wrapping of the paper tape I wrap at the same time a strand of fibrous material, placing it between the spaced-apart edges of the tape, so that in the completely wrapped structure it occupies the spaces a, I), 0, etc. The advantages of the spaces left in the course of wrapping between the edges of successive turns of the tape still remain. When the body of insulation is completed by impregnation with insulating compound of the immobile portion of the envelope, the bodies of compound which otherwise would wholly occupy the spaces (1,7), 0, etc. are no longer free bodies. Their continuity as free bodies of compound is interrupted by the fibres of the strands there introduced. By such provision the breakdown strength of the structure as a whole is substantially increased.

The specific inductive capacity of such introduced strand of fibrous material is in any.

case greater than that of the insulating compound (and much greater than that of any void which might at that point develop), and accordingly, the concentration of dielectric stress is by the presence of the introduced strands diminished.

The introduced strand may be a twine of hemp or other vegetable fibre, impregnated with insulating compound; The specific inductive capacity of such twine is approximately that of paper; and, since the twine by its presence will effect the breaking up of the continuity'of the bodies of compound within the spaces, the electrical strength of the structure will be greatly increased. In

any case it'will be perceived that, whereas the wrapped on strips of paper which the art knows are practically incompressible longitudinally, the strand applied in the manner described and consisting of material such as I have indicated is compressible, to the end that, while achieving the result which makes this invention useful, it still allows the cable to be flexed.

The strength of the-structure as a whole may be increased still more by the selection of the material for the introduced strand. If, for example, twine of hem or other vegetable fibre be used, impregnate with a phenol condensation product, whose specific inductive capacity is high, the tendency to localized concentration of stress will be in larger measure corrected; or again, if the strand be a strand of metal wool (the sepcific inductive capacity of metal being infinite in value) the limiting case is found so far as concerns range of available materials) and the tendency to concentration of stress will be substantially eliminated. A structure which includes metal wool so introduced is specifically claimed in a companion application filed by me on even date herewith, Serial No. 331,171, upon which application Letters Patent No. 1,775,072 were issued September 2, 1930.

And, as has already been said and repeated, the breaking up of the continuity of the bodies of compound by fibres of solid material in any case increases the strength of the structure to resist break-down.

I claim as my invention:

1. In an electrical installation each layer of which is laminate insulation formed of strips of fibrous material of relativel great width and small thickness laid side y side with spaces of relatively small width between the edges of adjacent strips, the spaces in one layer being ofiset relatively to the spaces in an adjacent layer, and strands of fibrous material laid in the spaces between adjacent strips, the whole being filled with insulating compound.

2. In an electrical installation laminate insulation formed of strips of fibrous material laid side by side with spaces between the edges of adjacent strips, strands of fibrous material impregnated with a material of a high s cific inductive capacity laid in the spaces etween adjacent strips, the whole being filled with material of lower specific inductive capacity than said first material.

3. In an electric cable structure a conductor, a body of wrapped-on porous tape of relatively great width and small thickness enveloping said conductor, the turns of ta pc spaced apart edge from edge by spaces of relatively small width a strand of fibrous material laid in the space between adjacent turns of tape, the whole being filled with insulating compound.

4. An electric cable structure including a conductor and an envelope of insulation surrounding the conductor, said envelope consisting of a. wrapped-on body of oil-filled porous material, adjacent turns of the Wrapped-0n body being spaced apart and the intervals between such spaced apart turns being occupied with insulation of specific inductive capacity greater than that of the remaining portions of the envelope.

5. In an electric cable structure a conductor, a body of insulation comprising overlying layers of spirally wrapped strip material, the edges of adjacent turns in said layers being separated by relatively narrow spaces, and strands of fibrous material laid in the spaces between adjacent turns of a layer.

6. In an electrical installation laminate insulation formed of strips of fibrous material relatively incompressible in the direction of the length of the structure laid side by side with spaces between the edges of adjacent strips, and compressible strands of fibrous material laid in the spaces between adjacent strips, the whole being filled with insulating compound.

In testimony whereof I have hereunto set my hand.

DONALD M. SIMMONS.

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