US2190017A - Electric cable - Google Patents

Description

Feb. v13, 1940. w. A. DEL MYAR 2,190,017

ELECTRIC CABLE Filed June 16, 1938 i-"N'PAPeR l iO-CoNDucToR 12-5' PAPER 14- S HEATH m f D q, v Q U fg au .L0 *n s CY L? "2U cu e gn Volfaqe only {or 16 hrs. 3* Tefal 'Z4hrsf1comblc7`e cycle OTS O -l O 2O 30 40 50 LIFE 1N CYCLES Conducfor Maxxmum Operafnq Tamb. =85C. (approx) Shea D 60"C (approx.) Room Tamb Z5 C. (approx.) 1.z=

; l m TYPE/NTYPES L di C. 0.7n i 1:1,.5. 0 v

G o nn O 0.25 I aNvENToR q/ W///am A. 13e/Mal" l o l o Zooo Aooo @cmo sooo 10,000 mona wooo E Pound Per Sq. in.

Patented Feb. i3, 194@ UNITED STATES PATENT OFFICE ELECTRIC oABLE Application June 16, 1938, Serial No. 214,022

4 Claims.

My invention relates to electric cables for high voltages, more particularly to an impregnated paper cable for use under such conditions of service that ionizable voids cannot be formed as a 5 result of cyclical expansion and contraction of the oil and other components of the cable.

In appearance it is scarcely distinguishable from an ordinary cable of this type but it differs therefrom in the use and disposition of two types of paper which may be obtained as larticles of commerce on specification from makers of cable paper. These types of paper will be distinguished herein as types N and S. The two types of paper differ in the slopes of their stress-strain curves, as

illustrated in diagram shown as Figure 3.

The object of this invention is to prevent void formation in impregnated paper insulated power cables such as ordinarily result from cyclic loads ing. It is therefore pertinent to consider what happens in 'acable underl such conditions and this is most readily accomplished by considering a specific example.

For this purpose we study a 500,000 circular mil cable with 750 mils of insulation. The distribution of materials under the sheath at 20 C., will be as shown in columns 1 and 2 of the following table. When heated to 70 C., the approximate distribution of materials will be as shown in columns 3 and 4. The expansion of the copper,

paper ilbers and lead are neglected as inconsequential to our present purpose.

If the viscosity of the oil isgreat enough to prevent the oil being forced out, the increase of oil volume of 1.5% will cause a linear stretch of the tapes of about 0.75%. l

Referring to diagram, Figure 3, it will be seen that type N paper will have a stress of 8500 lbs/sq. in. while type S paper will have a stress .of 10,800 lbs/sq. in.. Furthermore, at all elongations the stress in type S paper is greater than in type N paper. If therefore, the insulation be made in two layers, one of type S and one of type N paper and if the type S paper be applied over the type N paper, the type S paper will keep the type N paper in compression and so prevent the 5 formation of voids between tape faces, at all temperatures, both in the rising and falling temperature periods of the load cycle, until the cable returns to its original condition.

lt is relativelyxfar more important to prevent '10 void formation in the inner layers of paper than in the outer, as the electric stresses which cause ionization in the voids are greater at the inner layers. Hence the importance of keeping the inner layers tight, especially in the cooling pe- 15 riods of the load cycle, by having them held in an elastic grip by the outer layers whenever they have been stretched from their original condition.

It is, of course, possible to use three or more grades of paper differing in the slopes of their l) stress-strain curves, always applying the papers in the order of increasing tensile stress for a given elongation, from the conductor outward. In this way each layer will tighten the one Within it. Ordinarily, however, two types of paper are 25 suflicient.

Two types of paper were used having stressstrain characteristics as shown in diagram, Figure 3. It was found that a cable made with 60 tapes of type S paper over 160 tapes of type N 30 paper, lasted through at least twice as many cycles of accelerated load cycle aging tests a`s cable made in the way now customary'in the industry. This result was obtained in test after test and, some 16 cables of this type were sub- 35 jected to accelerated load cycle aging test.

This result was surprising as type S paper is generally denser than typel N paper and the way now customary in the industry is to make cables with the inner layers of denser paper than the 4o outer layers.

Comparative load cycleelife tests were made of cables with different kinds and combinations of insulating papers, and the results diagrammed as shown in Figure 4.

The rise in power factor in the course of these cycles of load was taken as the measure of deterioration of thecable.

Curve A refers to a cable made entirely of type N paper; curve B to a cable made entirely of type S paper; curve C to a cable with type S paper adjacent to the conductor and type N paper outside. Curve D refers to a cable made in accordance with the present invention, i. e., with type N paper inside and type S paper outside.

It will be noted that not only does curve D lie below the other curves, indicating a longer life before it attains a given power factor, but the curve is smoother than the others and tends to become horizontal.

Both the smoothness and the ilattening of the curve are indications of retention of uniformity during load cycle testing. l

The foregoingand other features of my invention will now be described in connection with the accompanying drawing forming part of this specication in which I have shown my cable in its preferred form, after which I shall point out more particularly in 4the claims those features which I believe to be new and of my own invention.

In the drawing:

Figure 1 is a cross section through a single conductor cable.-

Figure 2 is a cross section through a preferred form of a multi-conductor cable.

Figure 3 is a diagram showing the comparative y slopes of the stress-strain curves of the papers which I propose to use in my insulation.

Figure 4 is a diagram showing the curves indicating' comparative load cycle life tests made with cables with different kinds of insulation.

In the carrying out of my invention I employ a conductor I0 over which I wrap a plurality of layers of paper tape l I of the type N paper heretofore referred to and over the layers of the type I 4in usually something like one-quarter of the initial value. Furthermore. applying the outer paper more tightly than the innertends to wrinkle the latter; indeed it is not unusual to apply the outer layers at a lower .tension than the inner, in order to avoid wrinkling. v

I wish it distinctly understood that my electric cable herein described and illustrated is in the form in which I desire to construct it and that changes or variations may be made as may be convenient or desirable without departing from the salient features of my invention and I therefore intend the following claims to cover suoliI modifications as naturally fall within the lines of invention.

I claim:

1. An electric cable comprising a conductor, a sheath and impregnated paper insulation interposed therebetween composed of two or more types of paper differing in ratio of elongation to tensile stress, the types of paper being applied in decreasing order of this ratio from the conductor outward.

2. An electric cable comprising a conductor, shield and sheath and between the conductor and shield, insulation of impregnated paper, composed of two or more types of paper differing in ratio of elongation to tensile stress, the types of paper being applied in decreasing order of this ratio from the conductor outward.

3. An electric cable comprising a conductor having an insulation of impregnated paper wrapped in layers about the conductor, thelinner layers held in elastic grip by the outer layers, whenever they may be stretched from their original condition due to variation in load cycle.

4. The cable of claim 3 having a shield over the insulation and a mechanical protection

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