US2160660A - High potential terminal - Google Patents

Description

May 3o, 1939.

H. M. HOBART HIGH POTENTIAL TERMINAL Filed Sept lnverwtor:

Hen ry M .Hobart His tto They.

Patented May 30, 1939 UNITED STATES PATENT OFFICE General ElectriCom pany, a corporation of Application September 23, 1937, Serial No. 165,327

7Claims.

Terminals'or bushings 'as now constructed for high potential -electrical apparatus, such as cables and transformers, are very large and ex'- pensive, especially where large amounts of power are involved. 'Ihls is chiey because of the size of the insulator which is usually made of porcelain with numerous ribs on the periphery to increase the length of the surfacecreepage path. The higher the potential, other things being equal, the greater is the cost. For some installations, the cost of a single terminal is several thousand dollars.

'Ihe object of my invention is the provision oi.' an improved terminal orbushing `for high tension apparatus, and specifically one `which is substantially smailerin size for the same potential than those now in use and one which is substantially cheaper to construct.

In carrying out my invention, a substantial departure is made from the established practice. Instead of insulating the center conductor ot the terminal by wrappings of paper im- Dregnated with oil, the said conductor is left bare and the insulation thereof eilected by high pressure gas. This pressure will ordinarily'be diil'erent for diierent voltages but for many installations the gas pressure desirably may be of the order of 600 pounds per square inch. The transformer or other tank associated with the terminal may also contain insulating gas but at a much lower pressure, say oi the order of 300 .pounds per square inch. The purpose in using a lower pressure in the tank which is relatively large, than in the terminal which is relatively small, is to avoid havingto make the tank unduly expensive in virtue of the thick,walls required. While Iconsider it preferable to employ compressed gas as insulating media in both terminal and tank, my improved terminal may advantageously bel used with apparatus that is submerged mju sued tanks. Another advantage of my improved construction arises from the fact that the insulator is small and hence the gas chamberwithin it is likewise of small volume. This permits the use ot gases of the highest insulating qualities which are the more expensive. The con of such gases for use in large tanks would ordinarily be prohibitive on account oi' the-quantity required.

In the drawing is illustrated in vertical section a terminal or bushing embodying my invention.

I indicates e. part of a metal tank or casing containing high potential electrical apparatus such, for example, as a 'transformer which it is desired to connect to some outside conductor. From one point of view, the casing can be re- Earded asa metal wall through which itis'desired to extend a high voltage conductor. The particular apparatus within the tank is not material and for 'that reason hasnotv been illustrated. The part o! the casing not occupied by the apparatus is filled with compressed gas which serves in part to insulate the conductors within it. rBecause the casing is or may be large, it is desirable to limit the pressure o1' the gas to a-value .which can safely be confined without making the wallsunduly thick and heavy. The pressure may desirably be of the order of 300 pounds per square inch. The upright or neck portion of the tank is provided with a. cover 2 which is securely bolted thereto with a sealing gasket between the adjacent surfaces. On the cover is supported the insulator forthe center conductor 3, the lower end of which is con-` nectedby a connector to the electrical apparatus within the tank in the ordinary way and the upper end bya connector to an outside conductor leading to or from other electrical apparatus.- 'I'he chamber t within the insulator is illled with a selected insulating gas under pressure which is substantially higher than that in the tank, Vysay of the order or 600 pounds'per square inch. Because oi the high pressure, the insulator must be of suilicient strength to withstand said pressure and is of special construction. This means that the diameter of the insulator should be small aswell as highly resistant to internal pressure. Decreasing the diameter introduces the problem of properlyinsulating' the center conductor. `The terminal comprises a vertical tube 5 made of brous laminated material, as wrapped paper for example, the layers of which are united by a binder' such as a condensation product, the latter being cured under heat and pressure to form a rmstrong body. The lower end oi' the tube is internally screw threaded to receive a cap'i which may be of the same material as the tube, said cap having a recessl through which the conductor I extends. In the cap is located a well packed screw threaded plug 8 that forms a seal for the lower end of the conductor 3. It is desirable that the material of the vertical tube shall have a high dielectric constant because the stress in a material is inversely as its dielectric constant, and it is desired to` keep it low in the solid material and high in the gas.

At a suitablev portion of its length, the tube is somewhat enlarged to form two shoulders 9.

Surrounding the enlarged portion is a metal sleeve I which is sealed to or forms a part of the cover 2. The sleeve is externally screw threaded at its ends to receive gland nuts II and I2, said nuts having internal iianges which flrmly-'f'press packings I3 against the ends of the sleeve to form pressure tight joints. `Surrounding the tube above the nut I2 is a -body of insulation It of any suitable character. The upL per end of the tube is provided with a series of ring shaped barriers I5 made of insulating material and desirably of the same kind as the tube itself. Between the barriers are small spacing rings I6. The barriers materially increase the resistance to passage of current from the exposed end of the center conductor to the metallic tank or casing, and also serve to reinforce' the tube.

'Ihe upper end of the tube 5 is internally threaded to receive the threaded end of an insulator I1 which may be made of strong glass,

porcelain or equivalent insulating material. The

insulator iskprovided with parallel ribs I8 which not only increase the length of the current creepage surface thereof but also strengthen it as well. Because of' the small surface of the wall of the bore of the insulator, it is capable of withstanding high internal pressure.

Where the center conductor passes out through the insulator, a gland nut I9 is provided and also a compressible packing 20. 'I'hese parts may be taken as illustrative of the parts at the lower end of center conductor. Since the upper end projects out into the surrounding atmosphere which may be humid or dirty or both, the distance between the conductor emerging at I9 and` the cover 2 of the metal tank must be great for high voltages in order to avoid surface leakage.

' The distance between the lower gland nut I I and 5, cap 6 and insulator I1 may be regarded as a the bottom plug 8 must be proportioned so as not to exceed the volts per mil of disruptive strength of the material of the tube 5. Gas is admitted to the small chamber 4 within the terminal by a supply pipe 2I subject to the control of a valve 22, said pipe being threaded into the metal sleeve I0.

In the broader aspect of my invention, the tube completely closed vessel irrespective of the material of which it is composed, having the center conductor permanently sealed at the'points where it passes'through the ends of the vessel. Also that with effective seals to prevent leakage, the vessel may be charged once and for all. Under this condition, the inlet pipe 2i may not be required.

Various gas and mixtures thereof having high dielectric strength are available for insulating the centerconductor I within the chamber ofthe terminal.' For example, I may use carbon tetrachloride gas, CC14, either by itself or mixed with air or other gas. With a gas such as CCM at atmospheric pressure,'i'f maintained above itsv liquefying temperature of I7 deg.C. a gap or space between bodies. of 1.5 millimeters is equal to a gap of 30 millimeters between such bodies underl atmospheric air conditions. As the pressure of the gas is increased and ifl its temperature is also-in-A creased by the amount necessary to prevent its liuuefying at the increased pressure. its value as a dielectric very greatly increases. Several gases which are especially suitable furthe present purposes tend when cold to become liquid under the high pressures employed. Nitrogen gas under suitsblepressure may also be used as a filling for the chamber of the terminal but will not provide such great dielectric strength as CC14 gas. I do not by the foregoing intend to be understood as exhausting the subject of suitable gases but merely refer to them as examples. Also it is to be understood that a mixture oi gas may be employed, provided they have a sufliciently high dielectric strength for the voltage to which the center conductor is subjected.

Certain of the gases 'which are available for the present purpose and which are characterized by a particularly high dielectric strength have a relatively high liquefaction temperature, that of CCli being 77 C. at atmospheric pressure. At 400 pounds absolute, it is 190 C. In order to maintain the insulating material in s. gaseous state while under relatively high pressure, a heating means is employed which may take various forms. In some cases, some or all of the necessary heat may be provided by the heat generated by the electric current in its passage through the center conductor, or the heat generated may in whole or in part be due to external means. For example, the center conductor may have its resistance suitably increased or it may be composed wholly or in part of magnetic material. Surrounding the terminal at a. suitably selected point and supported in any desired manner is a coil of insulated wire 23 which is energized by a low potential source of alternating current, such as is available in transformer containing power stations. The current passing through the coil acting by induction causes lines oi force to pass through the iron of the center conductor and induce eddy currents and hysteresis therein which result in raising its'temperature, and this elevated temperature may be made sufficient to maintain the insulating fluid within the chamber 4 in a gaseous condition. The effect can be intensifed by using in coil 23 a current of high periodicity instead of the usually available 60 'cycle current. But forv moderate heating, the convenient and always available 60 cycle current will suillce.

According to present practice, the center conductor of terminals is made of low resistance material so asLto reduce losses, but for the purposes of the present invention, I depart from said practice to the extent necessary to heat the gas contained in the conductor enclosing vessel. Where a considerable amount of heat is required to prevent liquefaction of the gas under the determined pressure, I may make the whole or a portion of the conductor of high resistance material such as that commonly referred to as nichrome. In some cases, the vessel may contain a surplus of liquid C014 and the amount which evaporates will be dependent upon the temperature and pressure.` An increase of pressure-increases the dielectric strength of the gas. As previouslyjndicated, the heat generated by the center conductor may form the sole heating means for the gas, or the heat for the gas may be generated by means external to the vessel or both internal and external means may be utilized for the purpose.

While certain materials and gases Ihave been indicated or suggested in the foregoing as suitable for the purposes hereof, it is to be understood that in their selection care should be exer- 1d conductor bare, it follows that its diameter can in the chamber ofthe insulator i'or insulatingy the conductor that at the operating temperature- 'the diameter of the conductor permits the radial depth of the gas space which in turn permits decreasing the overall diameter oi the .vOlts and upwaidf It 'will be noted that there is no insulation, such as the usual paper wrapping, applied to the center conductor. By leaving the be small and hence that of the enclosing insulator. In referring to the fact that the center conductor is uninsulated, I mean that it has no external covering which of itself ailords the necessary insulation or a covering which contributes to any substantial extent in insulating the conductor. The tube l constructed as described great strength and one well adapted to withstand the heavy pressureto which it is subjected. By arranging the lower end oi' the porcelain or glass insulator I'l in telescopic relation with the tube, said insulatoriswell reinforced at thepoint of connection. By decreasing the diameter of the upper part oi thechamber or bore within the insulator I'Isoastoreducetheexposedarea,the eii'ectivepressuretendingtobursttheinsulator is reduced to a safe value.

What Iciaim as new and desire to secure by Letters Patent o! the United Btatesis:

1. Aterminal i'or high-potential electrical apparatus an uninsulated conductor op- Ierating at approximately 100,000 volts and upwards, a sealed vessel through which the conductor extends and projects therefrom at both ends, said conductor being sealed at the points where it'enters and emergesi'rom the vessel, a nllingofinsulatinggasiortheveeimaintained undersuchhighpressurethatittendstobecome liquid when cold, and means supplying suiiicient heattothegaswhen current isilowingthrough the conductor to gasiiy said liquid and maintain it in a gaseous state.

2. A terminal for high potential electrical apparatus comprising an uninsulated conductor. a chambcred insulator enclosing the conductor and closedatbothofitsendasealingmeansforthe partsoi'theconductorpamingthroughsaidciosed ends, aiillingotgaslmdersuchhilhm ottheconductoritisliquimandmeansfcrimpartingsumcientheattothebodyoi'gasto maintainitina-gaseousstatelmderoperating conditions while currentisiiowingthroughthe conductor a sealed vessel through which the conductor extends and projects at both ends, said conductor being sealed at the points where it emerges from the vessel, and a iilling of insulating gas of low thermal conductivity under high pressure for the vessel which tends to become liquid under such high pressure when cold, the heat from the conductor when current is ilowing through it being suiilcient to vaporize said liquid.

4. A terminal for high potential electrical apparatus comprising an uninsulated conductor, a. sealed vessel through which the conductor extends and projects at both ends. said conductor being sealed at the points where it emerges from the vessel, a iilling of insulating gas for the vessel under high pressure which tends to liqueiy when Jccld, and means external to the vessel for heating and vaporizing the liquid within the vessel when the terminal is in service.

5. A terminal for high potential electrical apparatus comprising an uninsulated conductor, a sealed vessel through which the conductor extends and projects at both ends, said conductor being sealed'at the points where it emerges from the vessel, a filling of insulating gas for the vessel under high pressure which tends to liqueiy when cold, and means both inside and outside oi .the vessel for heating and vaporizing liquid within the vessel when the terminal is in service.

6. A terminal for high potential electrical apparatus comprising an uninsulated conductor of magnetic material, a chambered insulator enclosing the conductor and closed at both ends, said conductor extending through the insulator at said ends, sealing means for the parts of the conductor passing through said closed ends, a illling ci insulating gas under high pressure in the chamber oi the insulator, said gas tending to yliquefy under the pressure to which it ris subA jected, and a coil oi wire surrounding the insulator through which current in causes heating by induction of the central conductor to prevent liqueiaction of the gas under operating conditions.

'1. A terminal for high DOtential electric conductors comprising an insulator having a central chamber, one end ofthe insulator being exposed to a cooling atmosphere, a conductor in the chamber extendingaxially o! the insulator, seallng means for the conductor where it extends throughthewallsoitheinsulatonaiiliingoi high pressureL insulating gas tor the chamber which tends to liquefy when its temperature is reducedbysaidatmosphere,andaheatingmeans iormaintaining the gas above its liqueiying temperature during normal operating conditions oi HENRY Il. H OBART.

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