US2316808A - Electric cable - Google Patents

Description

April 20, 1943. T. F. PETERSON 2,316,808

ELEGTRI C CABLE Filed NOV. 28, 1939 INVENTOR Patented lApi'. 20, 1943` UNITED STATES PATENT GFFICE Claims.

The present invention relates to electric cable and its production wherein the insulation is oil or oil impregnated paper and the oil is maintained under pressure. Generally this is accomplished by means of channels along the length of the cable with reservoirs connected to them at the cable joints or terminals; and in this manner it is proposed to provide for expansion of the oil created by rise in temperature and to keep the consequent pressure rise below that causing distortion oi the cable sheath. Because of the viscosity of the oil compound and its sluggishness in circulation, it has been further proposed to cushion against undue pressure rise within the cable by distribut ing inert gas particles through the insulation during the cable manufacture; such cable cannot be operated under high externally applied pressure, for only small amounts of gas can be tolerated in the insulation under stress and'gas solubility in the oil increases with pressure. Another form of cushioning provides gas-containing ilexible metal containers within the cable structure to compensate for expansion and contraction; these are subject to fatigue and eventual disruption. A more recent development in pressure cables is the type wherein the oil-impregnated insulation is enclosed by an impermeable membrane and pressure is applied through a gas or liquid medium outside of the membrane. ous types the oil is either degassed or has less gas in solution than it can hold at 25 C. and atmospheric pressure.

It is generally known that increase in pressure on an insulating oil medium raises the permissible operating voltage because of the insurance by the pressure of the elimination of voids. There is, however, no substantial increase in intrinsic dielectric strength with .pressure whether theoil is degassed or saturated with gas at 25 C. and atmospheric pressure when placed in the cable; actually an oil saturated at atmosphericpressure is only one-half saturated at two atmospheres pressure if no additional gas is provided and the break-down strength is then not much greater than at atmospheric pressure. The insulating strength of an oil, however, goes up with pressure when created by a gas and the. gas is-allowed to go into solution with the oil in amounts proportionate to the pressure increase. Placing gas under pressure over oil in reservoirs at cable joints does not serve to accomplish this result, for gas migration into or throughv the oil is extremely slow and the oil does not receive the amount of gas necessary for the increase in dielectric In these Variin contact with the body of oil or compound at strength obtained by satisfying the increased solubility.

According to the present invention pockets or reservoirs of trapped gas (nitrogen, carbon dioxide or other suitable gas) are disposed along the cable length and out of the iield of electrical stress, as at the sheath or the ller space portions or in the conductor strands, these pockets preferably providing means for insuring gas saturation in a liquid insulating material at the initial operating pressure; they are useful, however, in compensating for pressure changes when the oil is not saturated or does not contain appreciable amounts of dissolved gas. The reservoirs are so formed that the gas is held by gas-tight walls intervals suicient to insure pressure compensation throughout the body and without leakage or migration along the length of the cable from the reservoirs under variations in temperature and pressure. In one embodiment the reservoirs are formed by capillary tubes or tubes of such a size that the gas cannot pass out throughthe oil sealing the openings` into the tubes except as it is absorbed or dissolved; in another embodiment the tubes are so formed and the openings into them so disposed as to provide oil seals for gas trapped in the tubes regardless of the position of the cable. To insure saturation at the initial operating pressure and pressure compensation at all times gas reservoirs are distributed along the cable to supply gas in amounts in excess of that requiredfor saturation at the highest pressures applied or apt to be caused by temperature rise.

The pressure at which the cabie is operated is transmitted along the cable by the insulating body of oil or compound. In the case of continucus tubes an initial pressure may be provided by the gas in the tube but the iinal pressure is transmitted along the cable by the insulating body of oil or compound.

' In the drawing:

Fig. 1 is an assembly view of a Acable and incorporating one embodiment of the invention with terminal connections for supplying pressure;

Fig. 2 is an enlarged portion of Fig. 1;

Fig. 3 is a cross section of Fig. 2;

Fig. 4 is a modification in longitudinal section shown as applied to a single conductor cable;

Figs. 5 and 6 are longitudinal views illustratingV openings into the tubes;

Figs. 7 and 8 illustrate modifications. In general the invention applies to any type of single or multiple conductor cable of the oilv-iilled type; the oil in the preferred form of the invention carries in solution its full capacity of a gas. For maintaining gas saturation under all operating conditions the embodiment illustrated in Figs. l, 2 and 3 consists of a continuous tube ii laid helically along the length in a valley i2 of a three conductor cable i8 with each conductor i5 surrounded by oil-impregnated paper insulation i and a grounded permeable metal tape iti; the conductors are enclosed by the duid-tight sheath il! and the valleys maybe filled with jute or other material. `Thetube ii can be of flexible material such as copper and of circular cross section as shown. The cross-section need not be circular but it canbe triangular as shown in Figure "l, part iii; or it can be made of a bundle of capillary sized tubes iii, as shown in Figure 8.

Openings or perforations are provided at intervals into the helical tube ii (or the equivalent forms iS and it). These openings are located so as to result in anticline domes or gas traps. Where it is convenient to mark thecable on the outside to show the position of the tube, the perforations are located at the bottom side of the installed cable. Otherwise the perforation can be at alternate turns of the tube ii thus insuring asiasoa al cable, the trap volume is 20% of oil volume, nitrogen pressure will vary less than 1 atmosphere from an initial pressure of 5 atmospheres when the temperature varies from 20 C. to 80 C. By this means relatively large volumes of gas can be trapped along the cable with the prevention of a horizontal movement of gas into the insulation and of a replacement by gas of the oil in upper horizontal portions of the insulation. Likewise migration uphill Ain sloping cables is checked by this construction.

domes or traps regardless of the cable position.

With tubes of capillary sizei. e. a size in which gas will not pass oil, the perforations can be at any desired intervals.

The helical tube ii can be extended into and through joints 2i, 22 connecting thecable section with other sections so as to have its terminals 23, 24 within and extending above the oil levels in oil reservoirs 25, 26 located at the joints. The reservoirs can be of any suitable construction that withstands the pressures; for instance they can be of the telescoping or diving bell type in which the volume of gas is sealed within the bell above the oil surface.

In theoperation of the foregoing embodiment the tube can be filled with gas or with oil at atvrnospheric pressure before the cable is installed.

When the cable has been jointed and connections made to the reservoirs, gas is passed through theA tube Il and the pressure within the tube raised to some value preferably above that at which the. cable will operate, say three to ve atmospheres so that the gas is introduced in suicient quantity to drive out any oil in the tube and preferably to saturate all theoil within the' sheath at operating pressures. The oil within the sheath is placed under pressure by the pressure of the gas on the surface of the oil in the reservoirs; the oil pressure can be separately transmitted along the cable through a tube similar to Stube Il f instead of dependingon the transmission of pres- --sure along the cable insulation; for example, the

hollow conductor I4', as shown in Fig. 3vwitha core/of oil, provides for such pressure transmission/ through the contacts with the oil of the f insulation existing between the conductor strands. As the oil pressure is transmitted to the oil inthe cable, some oil enters perforations D20 and rises in the tube sections. Some of .the

gasv in the tube traps may likewise be dissolved /by the entering oil,"but the surplus gas' serves as a pressure stabilizer and compensates for changes in oil solubility with the changes in pressure occurring during operation. The ratio of "trapvolume to oil volume per unit length of cable may vary from cable to cable, depending on temperature cycles. desired range W solubility characteristics "f ifiina convention- Turning to the embodiment shown in Fig. d of the drawing there is illustrated a helical tube 30 within a single hollow conductor 3i to provide gas traps. The construction is otherwise similar to lthat shown in Fig. lwith inlet openings for the entry of oil.

Figs. 5 and 6 illustrate forms of inlet openings with which the tubes can be provided. Fig. 5 shows a square cut opening Eli, while Fig. 6 shows a'V notched opening 6i. These apply to the different forms of tubes including those of capillary sizes. f

It is not essential that the tubes be continuous from joint to joint. A number of closed tubes can be ,arranged in series or long tubes can be provided with partitions 52 as shown in Fig. 5. Gas can be sealed in the tubes before installation and openings created after installation or after the cable is fabricated. For instance, openings can be sealed with a fusible soft metal or the like and the seals melted by heating the cable or the oil; or pressure can be applied to force the seals. Moreover check valves' can be provided at the openings or within the tubes to prevent iiow as shown at 62 in Fig. 6.

What is claimed is:

l.. Cable comprising in combination a conductor, insulation about the conductor including an oil, a fluid-tight sheath about the insulation, and means .within the sheath providing a gas cushion in restricted contact with-the oil, said vmeans comprising a gas reservoir of anticline -means within the sheath providing a gas cushion in restricted contact with the oil, said means comprising a plurality of gas reservoirs of antioline configuration having impermeable `walls providing domes for trapping the gas, and openings in limbs of the reservoirs permitting contact between the trapped gas' and the oil external to the reservoirs.

3. Cable comprising in combination a conduc-l tor, insulation about the conductor including an oil, a fluid-tight sheath about the insulation, an

impermeable tube of helical conformation within the sheathproviding a dome containing trapped gas. and an opening into the tubepermitting contact between the trapped ga's and the oil externa] to the tube. said opening being so positioned that lthe trapped .gas .is retained in the dome in the tube.

4. Cable comprising in combination a conductor.- insulation about the conductor including an oil. a uid-,tight sheath about the insulation, an impermeabletube of helical conformation within the sheath providing domes at intervals along the length of the cable containingtrapped gn,

and openings into the tube permitting contact between the trapped gas and the oil external to the tube, said openings being so positioned that the trapped gas is retained in the domes in the tube.

5. Cable comprising in combination a conductor, insulation about the conductor including an on, a. uuid-tight sheath about the insulation, an impermeable helical tube within the sheath providing a reservoir containing trapped gas, an v10 openingito said tube permitting contact of the gas with the oil external to the tube and so positioned that the trapped gas is retained in the reservoir, and a second tube within the sheath containing oil for the transmission of pressure along the length of the cable, said second tube being perforated for contact of the oil therein with the oil of the insulation.

THOMAS F. PETERSON.

Images