US2745897A

US2745897A - High voltage electric termdinator

Info

Publication number: US2745897A
Application number: US269365A
Authority: US
Inventors: James H Nicholas
Original assignee: G&W Electric Specialty Co
Current assignee: G&W Electric Specialty Co
Priority date: 1952-02-01
Filing date: 1952-02-01
Publication date: 1956-05-15
Anticipated expiration: 1973-05-15

Description

May 15, 1956 J. H. NICHOLAS 2,7

HIGH VOLTAGE ELECTRIC TERMINATOR Filed Feb. 1, 1952 4 Sheets-Sheet l COND UC TION GLAZ E INVENTOR. James H. niche las May 15, 1956 J. H. NICHOLAS HIGH VOLTAGE ELECTRIC TERMINATOR 4 Sheets-Sheet 2 Filed Feb. 1, 1952 CONDUCTION GLAZE INVENTOR. ames H. mchalas y 15, 9 J. H. NICHOLAS 2,745,897

HIGH VOLTAGE ELECTRIC TERMINATOR Filed Feb. 1, 1952 4 Sheets-Sheet 3 CONOUCTION 01. TH 6E I CONDUCTOR T0 GROUND V IN PERCGNT CON TROL TUBE S. I. C.= I00 OIL AND PAPER S.I.C.=4-

VOLTAGE DISTRIBUTION BETWEEN CONDUCTOR 8 EXTERNAL GROUNDED SURFACE OF HIGH SIC. STRESS CONTROL TUBE,

INVENTOR. James H. Thcholas May 15, 1956 J- H. NICHOLAS HIGH VOLTAGE ELECTRIC TERMINATOR Filed Feb. 1, 1952 4 Sheets-Sheet 4 GLAZE IN V EN TOR. James H. The ho las United States Patent 2,745,897 HIGH VQLTAGE ELECTRIC TERMINATOR James H. Nicholas, Chicago, IlL, assignor to G 8; W Electric Specialty Company, Chicago, Ill., a corporation of Illinois Applicationl ehrnary l, 1952, Serial No. 269,365 8 Claims. (Cl. 174--73) This invention relates to high voltage electrical pot heads such as are used for terminating the end of an insulated conductor, particularly a cable conductor, or for terminating a conductor constituting one terminal of a high voltage transformer or circuit breaker. The present invention is particularly useful in connection with potheads or cable terminations at voltages above the 160 kv. class.

It is one of the objects of the present invention to provide means for controlling the potential gradient in the outer insulator of a pothead or terminator structure to maintain as far as possible a uniform axial potential gradient in the transition from the radial electric field to the axial or longitudinal field.

The use, on extra high voltages, of potential gradient control means such as have heretofore been employed in potheads up to 160 lrv. would generally result in an extremely large porcelain tube or tubes, which would be very costly and inefficient. A considerably greater length and larger bore diameter would generally be required. Therefore, the ability of the porcelain tube to withstand the high internal hydraulic or gas pressures employed on pipe type cable systems would be reduced. The rupture stresses might be so great as to make the design infeasible.

It is one of the objects of the present invention to provide means for controlling the external surface potential gradients in such a manner as to keep them at a more uniform value for the entire overall length of the porcelain of the pothead, thus using all of the surface with an equal and therefore high efficiency. In accordance with the principles of the present invention there is employed a ceramic tube of fairly rugged wall section and of material having an extremely high dielectric constant, that is, a specific inductive capacity of 100 to 200. By properly applying a conducting media on the external surface of such a tube or cylinder a very effective radial stress control tube can be obtained. This is so because the radial voltage division between two coaxial electrodes insulated by two or more dielectric materials is not only a function of the various radii of the electrodes but also of the dielectric constant or specific inductive capacity (S. I. C.), and the thickness of the various dielectric materials. The higher the S. l. C. of one material used compared to the S. l. C. of the other material the less voltage the higher S. I. C. material has developed across it. Therefore, if the ratio of the S. I. C. of the two materials is made quite high, say 50 or 100 to one, then the high S. l. C. material will have a very small potential drop across it.

Thus, even though the outside surface of the high S. i. C. stress control tube is metallized, the effect of the voltage division is such that the electrode potential would appear to exist at the inner surface of the tube because the S. l. C. of the cable and built-up dielectric is roughly 3, compared to 100 to 200 of the new control tube.

The proximity shielding effect is, therefore, not limited by manufacturing difficulties and fragile construction but is dependent only on how close radially the stress crime 2 shielding braid can be brought to the inner wall (bore) of the high 8. I. C. material.

One method of obtaining uniform surface voltage division on alternating current equipment is by capacity division. By employing several capacitor units in series, and controlling the capacitance of each unit to a certain ratio of the total, a definite voltage division can be obtained across each capacitor. Ideally, ten capacitors of identical value connected in series would each have 10% of the total voltage applied across each unit. This would then be a perfectly uniform voltage division. However, the capacitance division, as applied to potheads and bushing, is complicated by the fact that a capacitance exists between each unit capacitor and the insulated conductor which passes within the unit capacitor. The capacitor units near the grounded end of the structure would be required to carry more capacitance current than those near the conductor or line potential end. This increased current passing through the units nearer the grounded end would produce a greater potential drop across these units than across the unit near the line end. Therefore, the uniform potential gradient would not be obtained.

It is an object of the present invention to use capacitor units of diminishing values connected in series between the grounded end and the live end and properly graded so that a uniform voltage division is obtained.

For simplicity in manufacture, capacity adjustment, and production dielectric testing, it is desirable that individual capacitor sections be employed. However, a onepiece construction which results in electrically equivalent condensers in series with the unit could be used.

It is a still further object of the present invention to provide a condenser for controlling the potential gradient in the outer insulator of a high voltage terminator, which condenser is of an annular shape so that it can be slipped over the end of the high voltage conductor in assembling the terminator and wherein the condenser has a body of insulation constituting the dielectric of the condenser, which body surrounds the conductor and extends radially towards the outer insulator. A still further object of the present invention is to provide a simple and economical arrangement for making electrical connections be tween parts of adjacent gradient controlling capacitor units. This is accomplished, in the preferred construction, by using circumferentially extending garter springs that surround each capacitor unit, each spring being connected to the appropriate adjacent unit by a flexible jumper lead.

It is a still further object of the present invention to provide a combination of capacitor units for controlling the stress distribution throughout the body of the outer insulator and a radial stress control unit adjacent the grounded end of the terminator, so arranged as to afford the necessary controls wherever the dielectric stress would otherwise become excessive. It is a still further object of the present invention to provide such an assembly of capacitor potential gradient control units and a radial stress control unit wherein the capacitor units and the radial stress unit are all held in compression by a restraining spring at one end of the terminator or pothead. The restraining force is temporarily placed upon the cable during installation and is relieved by the outer porcelain tube base and cap assembly as soon as the outer porcelain unit is installed over the internal construction and the hood is compressed against the barrel by the aerial lug.

For most successful operation of the unit of the present invention the total axial capacity (the equivalent capacity of the group of unit capacitors in series) must be such that the capacity current controlling the voltage division is not appreciably affected by stray capacities and leakage currents. This is accomplished by having an axial capacity current, of the order of one to five milliamperes, de-

pending upon the surface conditions and the voltage of the conductor.

While the high voltage design of the present invention is primarily intended for use on cable systems which depend upon high internal hydraulic or gas pressures for their satisfactory operation, the principles of the present invention are also applicable to potheads for low pressure systems.

The attainment of the above and further objects of the present invention will be apparent from the following specification taken in conjunction with the accompanying drawings forming a part thereof.

In the drawings:

Figures 1 and 2, when placed end to end in axial alignment, are a longitudinal sectional view of a terminator embodying the present invention;

Figure 3 is a half elevational view and a half longitudinal sectional view of a capacitor unit of the present invention;

Figure 4 is a plan view of the capacitor unit;

Figure 5 is an enlarged cross section through the conductor and grading tube and shows the potential gradient therethrough;

Figure 6 is a diagrammatic view showing the approximate distribution of the equipotential lines in a terminator of the present invention; and

Figure 7 is a view similar to Figure 3 but showing an alternate capacitor grading tube.

Reference may now be had more particularly to the drawings wherein like reference numerals designate like parts throughout.

In the high voltage cable terminator of Figures 1 and 2 a cable 1 is mechanically connected to the outer porcelain insulator of a pothead or terminator 2 in the same manner as shown and more fully described in my pending application Serial No. 219,294, filed April 4, 1951, to which reference may be had. The means for sealing the end of a cable that enters the pothead may be the same as that shown and described in my application above referred to.

The cable conductor is indicated at 3. It is covered with the conventional wrapped paper insulation and surrounded by a conventional jacket 5 of insulation, of the type known as polyethylene. In preparing the end of the cable for connection within a cable terminator the jacket end of the cable is removed in the usual manner to terminate at 6, and the cable shielding braid 7 is removed to a point slightly above the end 6. Thereafter a stress cone insulator 9 is formed around the wrapped paper cable insulation 8. The stress cone insulator may 7 be a preformed Wrapping, known in the art, or may be formed in situ, as is also known in the art. In the case of the preformed stress cone it consists of an impregnated wrapped paper tube tightened on the cable during installation. The ground connection of the cable shielding braid 7 is continued by a wrapping of metal braid 16 which continues up to and slightly beyond the point of maximum diameter of the stress cone 9. The metal shielding braid 1% is then covered by a wrapping of cover insulation 11, all as described in the above referred to pending application. The cover insulation 11 is preferably a wrapping that can be compressed considerably without permanently distorting the same. One suitable material is a spongy crepe paper.

A stainless steel conical body 20 having a bottom flange 29a is secured at its lower end to the terminator mounting plate and suitably gasketed to provide a liquid-tight seal, and is grounded. A metal ring 21 is welded within the body 29 for supporting the stress control structure to be presently set forth. The ring 21 has a series of supporting bolts 22 threaded thereinto and secured in place by lock nuts. The supporting bolts 22 are uniformly spaced from one another, there being any suitable number of such bolts, six, eight or more. The supporting bolts 22 are of metal and support at their upper ends a base or seating ring 23 of metal, on which seating ring 23 the capacitor grading equipment, to be presently described, rests. The body 2d has a metal ring 24 welded to the top thereof to facilitate securing the outer porcelain insulator 27 in place, as by six, eight, or more bolts 28 that thread into a one-piece ring 29 that is cemented to the bottom of the insulator for drawing the insulator firmly against a frusto conical sealinggasket 29, all as shown in my application Serial No. 219,294, to which reference may be had. 7

The capacitor grading equipment of the present invention comprises a radial stress control unit 39 that rests on the seating ring 23 and in turn supports a series, in this instance ten, of capacitor potential gradient control units 32. The radial stress unit 30 is a hollow circular tube or sleeve of fairly rugged wall section, having lower and upper surrounding flanges 36-37. The material of which the tube is made is preferably ceramic, and of an extremely high dielectric constant (specific inductive capacity between and 200). One suitable material, by way of example, is titanium dioxide. A conductive glaze or coating 39 is formed on the outer cylindrical portion of the unit 30, covering substantially the entire cylindrical surface of the inner sides of the

peripheral flanges

36 and 37 where the metallic glaze terminates. The stress control tube E-ll surrounds the upper portion of the shielding braid 7 of the stress control cone 9 and reduces the axial voltage gradients in a manner similar to the stress control electrode 25 in my copending application Serial No. 219,214, filed April 4, 1951. The top and bottom of the unit are formed as perfectly flat surfaces parallel to one another and at right angles to the longitudinal axis of the unit.

The

flanges

36 and 37 merge with the body of the unit along smooth curves, free of sharp edges, so that the metal glaze on the outside of the unit is also free of sharp edges where it extends from the cylindrical portion to the flanged portion of the unit.

Each capacitor potential gradient control unit 32 comprises a cylindrical body 49 of the same material as that of the radial stress unit 30 and also has upper and lower peripheral flanges, indicated at 4Zl-41, that extend from the cylindrical body 45 along smooth curves free of sharp edges. Midway between the short flanges ll-41 there is a rather wide continuous peripherally extending flange 42 which is an integral part of the rest of the unit 32 and surrounds the cylindrical body 4C thereof, and terminates at its outer edge in a peripheral rim 44 appreciably thicker than the flange 42. The rim 44 joins the flange 42 along smooth curves, free of sharp edges, and the flange 42 likewise joins with the cylindrical body 4% along smooth curves. A conductive glaze or coating 46, of metal or other suitable material, is formed on the outer cylindrical surface of the body 40 below the flange 42. The conductive glaze covers the entire cylindrical portion between the flange 41 and the flange 42 and extends at 47 to cover the entire annular surface of the flange 42. This conducting glaze terminates at 48. The rim 44 extends a slight distance beyond the end of the conductive coating. At its lower end the conducting glaze terminates at 49 radially inwardly of the outer end of the flange 41, so that the flange extends beyond the end 49 of the glaze around the entire body 32. The upper half of the unit 32 has a similar metallic glaze 46' formed therein, identical in extent with the glaze 46, so that the upper and lower halves of the unit 32 are identical. The top and bottom surfaces of the unit 32 are fiat, parallel planes so that similar units can be placed one upon another with substantially no spaces between them.

It is apparent from the above description that the two metallic coatings 46-46 and the capacitor grading unit 32 constitute plates of a condenser wherein the flange 42 constitutes the dielectric, and that the size and material of the flange determines the capacity of the condenser. In addition, the cylinder surfaces 46-46 are capacity coupled with the cable conductor 3, the dielectric comprising the cylindrical body of the

capacitor grading tube

30 or 32 and the mass of insulation between the

unit

30 or 32 and the cable conductor.

In the pothead illustrated in Figures 1 and 2 there are ten capacitor grading units 32 stacked one upon another, the lowermost one resting on the grounded radial stress unit 30. The respective units make a snug fit around the insulation 11 which, due to its compressible character, provides a suitable medium to take care of any radial expansion due to heating of the cable proper. Each axial condenser unit 32 is connected electrically in series with its adjacent units. This is accomplished by providing each grading tube 32 with two separate helically coiled metal garter springs 56 each formed as an endless ring and embracing the metal glaze on the cylindrical portion of the grading tube, one below and one above the flange 42. Each spring is stretched and therefore tensioned by the cylindrical body of the capacitor grading tube, so that each spring remains in place and in electric contact with the conductive coating on the tube. Connection between adjacent condensers is formed by short braided copper or bronze jumper leads 58 each of which is soldered or otherwise electrically secured at its opposite ends to springs on adjacent capacitor grading tubes, as may be seen from Figures 1 and 2. The lowermost grading tube 32 is connected at its bottom half by a jumper lead 58, to a spring 60, identical with the spring '6, that surrounds and is tensioned around the conducting coating 39 on the lowermost radial stress unit 30, said spring being also connected by a similar jumper lead 62 to one of the grounded bolts 22. The uppermost spring of the series of capacitor grading to es is connected by a jumper lead 66 to a metal yoke 68 that rests upon the upper flange of the uppermost grading control unit 32, and is electrically connected to the cable conductor. It is thus apparent that the capacitor grading tubes are connected in series between ground potential at their lower ends and the conductor potential at their upper ends. The bight portion 69 of the yoke 68 has a centrally located hole therethrough through which extends a metal connector stud 70 that is mechanically and electrically secured to the end of the cable conductor 3. The yoke 68 is pressed downwardly by a coiled spring 72 that bears at its lower end at the top of the yoke and at the upper end is received in a cap 74 that is held in position by a nut 76 threaded on the stud 70. The spring 72 acting through the yoke 68 presses against the top of the uppermost potential grading tube 32 and maintains all of the grading tubes and the lowermost stress unit 30 in engagement on the seat of the seating ring 23 during assembly of the pothead. This stress is taken over by the insulator 27 and the base and cap assembly when the pothead has been assembled.

For mechanical simplicity all of the capacitor grading units 32 may be of identical construction. However, as pointed out previously, there is an electrical advantage in making these control units of different capacities ranging from a maximum capacity of the lowermost unit 32 and a minimum capacity of the uppermost unit 32. The variation in capacity is obtained by making the flanges 42 of the respective units of different thicknesses, the flange 42 of the lowermost unit being of minimum thickness and the corresponding flange of the uppermost unit being of maximum thickness, with the flanges of the intervening units of thicknesses grading between the upper and lower values. In one preferred construction, which is the one here described, the radial capacity for the unit 30 and for each one of the units 32 was 11.5 micromicrofarads, and the axial capacity between the conductor surfaces 4747 of the lowermost unit 32 was 2400 rnicromicrofarads, and the capacity between the conductive glazed surfaces 6 47--47 of the uppermost or line voltage control unit 32 was 200 micromicrofarads, and the capacities between the surfaces 47-47 of the intervening units graded between those two values.

The upper portion of the pothead is sealed in any conventional manner, for instance, as shown in my pending application Serial No. 219,294 above referred to, to which reference may be had. This seal consists of a cap assembly which includes a metal hood 86 having a closed thin metal tube 88 in which the stud makes a sliding fit, the tube being then compressed on the stud to establish proper electrical and mechanical connections. The upper end of the stud has longitudinal slots therein to permit fluid to flow past the stud within the tube 88, as may be required during formation of the pothead. The hood 86 rests on and is sealed over the upper end of the insulator 27, a suitable frusto conical sealing gasket 89 being interposed to facilitate the sealing action. A unitary ring 90, which is suitably secured to the bottom of the hood, provides means for bolting the hood to a unitary ring 92 that is cemented around the top neck of the insulator 27, all as shown in my aforesaid application. The usual metal corona shield 94 is provided. The hood has a tapped boss 95 for receiving fittings used during installation.

Reference may now be had more particularly to Figure 5 which shows an enlarged cross sectional view through the cable conductor and the grading tube 30 or the lower talf of the bottom unit 32, and shows the voltage gradient, in percentage, from the outer surface of the cable conductor, through the cable insulation and through the grading tube to the potential on the glaze 46 of the tube 32, which is ground potential in the case of the lowermost tube 3 if we assume that the insulation between the cable conductor and the grading tube 32 has a specific inductive capacity of 4 and that the specific inductive capacity of the insulation of the tube 32 is 100, then the voltage distribution starting at cable conductor voltage at the periphery of the cable conductor 3, as indicated by the ordinate line 101, is indicated by the curve 100, progressing downwardly along the curve until at the inner periphery of the tube 32 the voltage, indicated at 102, is 0.5% of the total voltage between the cable conductor and the metallic glaze 46 on the outside of the tube 32. From the point 102, which indicates a voltage of 0.5% on the inner surface of the body of insulation 40 of the tube 32, the voltage gradient then follows along the line 104 through the thickness of the insulation 40 to the potential of the conducting glaze 46. It is thus apparent from the curve of Figure 5 that the potential on the inner surface of the respective grading tubes 32 is only 0.5% of the voltage on the outside of the potential grading tube, so that within the pothead insulator 27 the potential on the outer surface of the wrapped insulation around the cable conductor is, at each point of the axial length thereof, at a value substantially equal to that of the adjacent outer condenser plate 46 or 46'. Since the capacitor grading tubes 32 are arranged to provide a uniform stepped voltage stress distribution from the line voltage at the top of the pothead to ground voltage at the bottom thereof, it is thus apparent that substantially the same voltage distribution is obtained along the axial length of the cable conductor covering insulation.

Reference may now be had more particularly to Fig ure 6 which shows the approximate distribution of the equipotential lines in the pothead above described. In this pothead the ten capacitor grading tubes 32 are of identical heights and the flanges 42 of the respective ones thereof are of different thicknesses to give an axial capacity of 2400 micromicrofarads for the bottommost tube 32 and 200 micromicrofarads for the uppermost tube 32, as set forth above. The total axial capacitance, namely, the equivalent capacity of the group of capacitance 32 in series are such that the capacity current controlling the voltage division is not appreciably affected by external stray capacities and leakage currents. The axial capacity current is made to be of the order of one to five milli-amperes, depending upon surface conditions and voltage rating. The equipotential lines 111 to 12.9 are lines from to 95% of the line to ground voltage in uniform steps of between successive ones of the lines 111 to 120. These lines show the relative uniform axial distribution of the voltage both in the internal dielectrics as well as in the external dielectric.

In considering the flow of capacity current through the connected condensers of the respective capacitor grad ing tubes 32 there are a number of factors that must be borne in mind. One is the usual current flow through the condensers that are connected in series from the line potential to the ground potential as determined by what might be called the axial capacity of the respective units. Another is the fact that capacitance exists between each unit capacitor and the insulated cable conductor that passes within it. This may be referred to as the radial capacity as distinguished from the axial capacity. It is apparent that all of the units are required to carry the capacitance current resulting from the axial capacity and that the capacitor units 32 near the grounded end of the structure would be required to carry more of the radial capacitance current than is required of those units 32 that are nearer the conductor or line potential end, since the radial capacitance current at any point of the cable flows to ground only through those units 32 between that point and ground. This increased current passing through the lower units would normally tend to produce a greater potential drop across the lower units 3 than the potential drop across the the units 32 nearer to the top of the pothead. Therefore, a uniform potential gradient would not be obtained with uniform axial capacity in the respective capacitor grading tubes 32. By having the capacitor units of progressively increasing values between the line end and the grounded end, and properly graded, a uniform voltage division between the respective grading tubes 32 at the flange 42 of each such unit is obtained.

It is desirable that individual capacitor sections 32 be employed because this simplifies manufacturing problems and permits production dielectric testing of the respective units and capacity adjustment in assembling units of different capacities. However, a one-piece construction of two or any other number, up to all, of the units 32, with or without the unit 30, may be provided in which the equivalent condensers formed at the respective flanges 42 of that unit are electrically in series the same as the different units 32. This is illustrated in a fragmentary manner in Figure 7, wherein the unit, indicated at 32a, comprises two or more, up to all, of the units 32 of Figures 1 and 2. The parts of Figure 7 that are the same as corresponding parts of the unit of Figure 3 have been designated by the same reference numerals with the subscript a added to designate this alternate structure.

The high voltage pothead described above is primarily adapted for use on cable systems which are of the high internal hydraulic or gas pressure type. The principles can, however, be applied to potheads for low pressure systems. In the design of such systems there is much greater leeway as to the outer porcelain bore diameters, since these diameters are not limited by the porcelain rupture stresses involved on the high pressure systems.

In compliance with the requirements of the patent statutes I have here shown and described a preferred embodiment of my invention. It is, however, to be understood that the invention is not limited to the precise construction here shown, the same being merely illustrative of the principles of the invention. What I consider new and desire to secure by Letters Patent is:

l. A terminator for a high voltage conductor comprising an outer hollow insulator into which the conductor extends and in which the cable conductor terminates, covering insulation around the conductor within the insulator, grounded means at the end of the hollow insulator into which said conductor extends, and means for controlling the potential gradient of the covering insulation and the outer insulator in an axial direction comprising a sleeve of solid insulation surrounding the covering insulation and within the hollow insulator, axially spaced discs of solid insulation integral with the sleeve and projecting radi lly therefrom, conductor means forming condenser electrodes covering the radially extending surfaces of the discs and constituting each disc as the dielectric of a separate condenser, means connecting said condensers in series, and means for connecting the series connected condensers wi in the insulator in circuit between ground and the end of the conductor.

2. A terminator for a high voltage conductor comprising outer hollow insulator into which the conductor extends and in which the cable conductor terminates, covering insulation around the conductor within the insuiat r, grounded means at the end of the hollow insulator into which said conductor extends, and means for controlling the potential gradient of the covering insulation in axial direction comprising a sleeve of solid insulation surrounding the covering insulation and within the hollow insulator, axially spaced discs of solid insulation integral with the sleeve and projecting radially therefrom, conductor rneans forming condenser electrodes covering the radially extending surfaces of the discs and constituting each disc as the dielectric of a separate condenser, means connecting said condensers in series, means for connecting the series connected condensers within the insulator in circuit between ground the end of the conductor, and separate conducting sleeves spaced axially on the sleeve of insulation and insulated from one another and capacity coupled With said high voltage conductor and connected to the respective condenser electrodes.

3. A terminator for a high voltage conductor coincJising an outer hollow insulator into which the conductor extends and in which the cable conductor terminates, covering insulation around the conductor within the insulator, grounded means at one end of the hollow insulator into which said conductor extends, and means for controlling the potential gradient of the covering insulation and the outer insulator in an axial direction com rising a sleeve of solid insulation surrounding the covering insulation and within the hollow insulator, axially spaced discs of solid insulation integral with the sleeve and projecting radially therefrom, conductor means forming condenser electrodes covering the radially extending surfaces of the o cs and constituting each disc as the dielectric of a separate condenser, means connecting said condensers in series, in ans for connecting the series connected condensers within the insulator in circuit between ground and the end of the conductor, and separate conducting slceves spaced axially on the sleeve of insulation and insulated from one another and capacity coupled with said high voltage conductor and connected to the respective con-denser electrodes, the series connected condensers being of differing capacities progressively graded, with the condenser which is of maximum capacity located at the grounded end of the series connected condenser circuit.

4. A terminator for a high voltage conductor comprising an outer hollow insulator into which the condoctor extends and in which the conductor terminate covering insulation around the conductor within the insulator, grounded mounting means for the insulator, means for controlling the potential gradient of the covering insulation and the outer insulator comprising a sleeve of solid insulation surrounding the covering insulation and Within the hollow insulator, said sleeve having a specific inductive capacity at least ten times that of the covering insulation, and means for fixing the potential gradient on the sleeve between conductor potential at the end of the sleeve closest to the end of the conductor and a fixed lower potential at the opposite end of the sleeve, said last named means comprising a number of series connected condensers within the insulator and in circuit between ground and the end of the conductor and spaced from one another in a direction axially of the insulator, each of said condensers comprising a disc of solid insulation surrounding the cable conductor and extending radially towards the inner wall of the outer hollow insulator and having separate conducting glazes on each of two opposite surfaces thereof constituting condenser electrodes separated by the dielectric of the disc, said discs being integral with said insulation sleeve and spaced from one another axially thereof.

5. A terminator for a high voltage conductor comprising an outer hollow insulator into which the conductor extends and in which the conductor terminates, covering insulation around the conductor within the insulator, grounded mounting means for the insulator, means for 1 controlling the potential gradient of the outer insulator comprising a sleeve of solid insulation surrounding the covering insulation and within the hollow insulator, said sleeve having a specific inductive capacity at least ten times that of the covering insulation, and means for fixing the potential gradient on the sleeve between conductor potential at the end of the sleeve closest to the end of the conductor and a fixed lower potential at the opposite end of the sleeve, said last named means comprising a number of series connected condensers within the insulator and in circuit between ground and the end of the conductor and spaced from one another in a direction axially of the insulator, said sleeve comprising a number of separate sleeve members stacked one upon another and each member having one of said condensers integral therewith.

6. A terminator for a high voltage conductor comprising an outer hollow insulating body into which the conductor extends and in which it terminates, and means for controlling the potential gradient of the outer insulating body comprising series connected condensers within the body and in circuit between ground and the end of the conductor and spaced from one another in a direction axially of the insulator, each of said condensers comprising a disc of solid insulation surrounding the cable conductor and extending towards the inner wall of the outer hollow body and having separate conducting glazes on each of two opposite surfaces thereof constituting condenser electrodes separated by the dielectric of the disc.

7. A terminator for a high voltage conductor comprising an outer hollor insulating body into which the conductor extends and in which it terminates, covering insulation around the conductor within the body, and means for controlling the potential gradient of the outer insulating body comprising series connected condensers within the body and in circuit between ground and the end of the conductor and spaced from one another in a. direction axially of the insulator, each of said condensers comprising a disc of solid insulation surrounding the cable conductor and extending towards the inner wall of the outer hollow body and having separate conducting glazes on each of two opposite surfaces thereof constituting condenser electrodes separated by the dielectric of the disc, and conducting members around the covering insulation and capacity coupled with the conductor and spaced from one another axially of the covering insulation, the respective conducting members being electrically connected with the adjacent condenser electrodes.

8. In a terminator for a conductor of a high voltage cable having a casing sealing the end of the cable, an outer insulator into which the cable conductor extends and a stress control cone having a tapered conductive layer at or near ground potential surrounding the cable within said outer insulator, the improvement comprising an auxiliary stress control electrode at or near ground potential and surrounding points adjacent to the wide end of the stress control cone, and a sleeve of solid insulation between said auxiliary stress control electrode and said conductive layer of said stress control cone, said sleeve of insulation having a specific inductive capacity at least ten times that of the average specific inductive capacity of the insulation between said sleeve and said high voltage conductor.

References Cited in the file of this patent UNITED STATES PATENTS 1,797,878 Palm Mar. 24, 1931 1,810,385 Austin June 16, 1931 1,868,962 Atkinson July 26, 1932 1,950,608 Hanson Mar. 13, 1934 2,068,624 Atkinson Jan. 19, 1937 2,219,910 Webb et al. Oct. 29, 1940 2,386,185 Beaver et a1. Oct. 9, 1945 FOREIGN PATENTS 583,628 Great Britain Dec. 23, 1946 457,070 Canada May 31, 1949