US3793475A

US3793475A - Dielectric system for a high voltage power transmission cable and termination

Info

Publication number: US3793475A
Application number: US00270045A
Authority: US
Inventors: E Yonkers
Original assignee: Nuclear Chicago Corp
Current assignee: TM ANALYTIC Inc AN IL CORP; Nuclear Chicago Corp
Priority date: 1972-07-10
Filing date: 1972-07-10
Publication date: 1974-02-19
Anticipated expiration: 1991-02-19

Abstract

A high voltage power transmission system includes a power transmission cable having an improved dielectric system. The dielectric system is formed of two or more concentric sections each separated by thin conductive layers and each section carrying predetermined precentages of the total applied potential. In addition there is provided an improved termination for the high voltage power transmission cable including a resistive system selected to divide the applied voltage into the preselected voltage levels.

Description

United States Patent 3,793,475

Yonkers Feb. 19, 1974 [54] DIELECTRIC SYSTEM FOR A HIGH 3,484,679 12/1969 Hodgson et a1. 174/105 R X VOLTAGE POWER TRANSMISSION CABLE 3,711,631 l/1973 Denes 174/120 R X AND TERMINATION FOREIGN PATENTS OR APPLICATIONS Inventor: Edward H. Yonkers, Wilmette, 111. 1,206,659 8/1959 France 174/120 R 155,098 10/1904 [73] Ass1gnee. Nuclear-Chlcago Corporation, Des [568 H 901 Flames, 27,859 12 1908 Great Britain 174 R 522,273 3/1943 Great Britain l74/l20 R [22] July 101972 555,332 8/1943 Great Britain 174 120 R [21] AppL N() 270,045 512,798 2/1955 Italy 174/127 Primary Examiner-Laramie E. Askin [52] US. Cl. .1 174/73 R, 174/105 R, 174/120 R 51 Int. Cl H02g 15/02, HOlb 7/02, H01b 9/02 gg g'z w iz Kolehmamen [58] Field of Searchl74/73 R, 73 SC, 102 R, 105 R, y

174/105 SC, 105 B, R, R, 120 C, 120 PP, 120 SC, 120 AR, 120 SR, 121 R, ABSTRACT 121 121 143 A high voltage power transmission system includes a power transmission cable having an improved dielec- Referencfls Cited tric system. The dielectric system is formed of two or UNITED STATES PATENTS more concentric sections each separated by thin con- 1,129,486 2 1915 Harris 174 143 ductive layers and each Section Carrying Predetep 1,702,413 2/1929 Pfiffner 174 143 x' mined Precemages 0f the total pp Potential 1,868,962 7/ 1932 Atkinson 174/73 R addition there is provided an improved termination for 2,717,917 9/1955 Isenberg 174/121 R X the high voltage power transmission cable including a 2,782,248 2/l957 Clark 2 174/120 R X resistive system selected to divide the applied voltage 3,088,995 5/1963 Baldy/1n 1 174/127 into the preselected voltage levels 3,193,712 7/1965 Harns 174/105 R X 3,433,891 3/1969 Zysk et al. 174/121 R X 8 Claims, 7 Drawing Figures 2 r 54 540 5/40 4 55a 1 47 "550 46 4g 6 55 48 55 55d 4/ 44 54d 40 PATENTED FEB I 91974 SHEUIOFZ DIELECTRIC SYSTEM FOR A HIGH VOLTAGE POWER TRANSMISSION CABLE AND TERMINATION The present invention relates to improvements in high voltage electriccables, dielectric systems, and terminations therefore, particularly power transmission cables employing solid dielectric materials for their basic insulation, for application in the high voltage and extra high voltage range, such as 230 Kilovolts and higher, and including both AC. and DC. systems.

It is well known that the dielectric strength of insulating materials expressed in potential per unit thickness in the direction of the field is not the same for all thicknesses of a given material. For example, some of the synthetic polymer materials measure several thousand volts per mil when thin samples of the order of 0.001 inch thickness are tested. The same material may test only 500 to 700 volts per mil when the test sample is xizinch thick and the material may show still lower values when the test is applied to 1 inch thick samples. This inverse relationship between the dielectric strength and thickness is not the same for all dielectric materials and it probably results from the fact that processing its material in thin sheets provides more uniform density and a tendency toward more favorablemolecular orientations as well as avoiding mechanical irregularities such as folds, voids and the like.

On the other hand, thick bodies (of the order of inches) of dielectric materials as required in high voltage cables are more subject to density variations, folds and voids. Density and molecular orientation play a part in determining the Specific Inductive Capacity (also known as the dielectric constant, K) of a material. Local variations in these properties, including voids and folds, cause variations in local electric field conditions which can influence the dielectric strength of the material unfavorably.

There are also mechanical problems encountered in trying to form a thick wall of perfectly uniform insulating material around the central conductor in a cable. The effort to obtain near perfection is not unreasonable in cable manufacture since every inch along the axis of such a cable as it is used in an electric power system must withstand the whole potential of the system continuously, and one pin-point failure causes a system outage which at 230 KV and higher is extremely undesirable because of the large areas served by such lines.

The problems encountered in striving for the needed perfection in cable manufacture are made more difficult by the geometry of high voltage cables. In the first place, the conductor must be precisely coaxial with the outer conductive shield and the insulating material must be uniform and void free in the space between the conductor and shield. In such a structure, if the insulating wall is thick compared to the diameter of the conductive core, as it must be in a high voltage cable, the insulating material necessarily performs at low efficiency in terms of capability and-cost.

Accordingly it is an object of the present invention to provide an improved high voltage cable.

Another object of the present invention is the provision of an improved dielectric system for a high voltage cable.

Yet another object of the present invention is the provision of a dielectric system for a high voltage cable having concentric separated subsections carrying a predetermined percentage of the applied potential.

Another object of the present invention is the provision of a dielectric system wherein each element of volume or subsection of the dielectric material is exposed to substantially the same voltage stress.

Another object of the present invention is to provide improved means for terminating a high voltage power cable employing the coaxial dielectric steps and intervening conductive layers of the cable to effectively grade the electric field at the termination.

Another object of the invention is to provide a high voltage power cable with coaxial dielectric steps and intervening conductive layers to unify potential variations along its axis.

Another object of the invention is to provide a high voltage power cable and termination structure with coaxial dielectric steps and intervening conductive layers including a resistor system at each terminal which interconnects the central cable conductor and each successive conductive layer with the grounded sheath to drain off cumulative electric charges which may result from local variations in its dielectric along the cable.

and transient phenomena.

In accordance with the present invention, there is provided an improved insulation system for a high voltage power transmission cable of the type including a conductive core, a conductive shield spaced concentrically outwardly of the core, and a solid dielectric system filling the space between the core and the shield. The dielectric system is formed of two or more concentric sections separated by a thin conductive layer, the thickness and dielectric constant of each subsection being selected so as to obtain a desired distribution of the applied potential.

There is also provided termination means employing the coaxial dielectric steps and intervening conductive layers of the cable in coordination with a terminating structure which grades the potential to avoid corona at the terminal and provides a voltage-dividing resistance neutral which drains abnormal charges from the stepped coaxial conductive layers.

The invention will be more clearly understood from the following detailed description when read in connection with the accompanying drawings, in which;

FIG. 1 is a cross sectional representation of a conventional high voltage power cable illustrating the voltage gradient through the insulation thereof;

FIG. 2 is a cross sectional representation of a high voltage power cable according to one form of the present invention;

FIG. 3 is a cross sectional representation of a high voltage power cable according to another form of the present invention;

FIG. 4 is a perspective view of the high voltage power cable according to FIG. 3;

FIG. 5 is a view perspective of a cable terminal according to the present invention which utilizes the coaxial stepped structure of the cable to grade the field generated by the cable potential uniformly in the space around the terminal.

FIG. 6 is a representation of the terminal of FIG. 5 in cross section along the section line 6-6; and

FIG. 7 is a representation of the cable terminal of FIG. 5 in section through axis 7--7.

Referring now to the drawings, and particularly to FIG. 1, there is represented in cross-section a conventional high voltage power cable having a unitary, solid insulation system. As therein illustrated the cable 10 includes a central conductive core 11, an outer conductive shield 12 concentric therewith, and a unitary solid insulation system 13 filling the space between the core 11 and the shield 12.

In the illustrated embodiment the inner diameter of the conductive outer shield 12 is five times the outer diameter of the central conductive core 11; or, in other terms, the insulation wall thickness is four times the radius of the conductive core 11. Broken line

concentric circles

14, 15, 16 represent the positions of the equipotential levels dividing the applied potential E (i.e. the potential difference between the core 11 and the shield 12 into four equal parts). That is, the first level circle 16 inside the shield 12 represents the position of 1%: of the applied potential; circle the position A; of the potential; and the circle 14 the position of A of the potential. The conductor, of course, is at 100% of the applied potential.

It is seen with the aid of this diagram that the inner of the potential is carried by a small fraction of the insulating material; actually the inner A of the applied potential is carried by only 5% of the insulation. This small element dictates the dielectric strength and quality of the remaining 95% of the insulation. The outer of the applied potential on the other hand is carried by 58% of the cable insulation. Under these conditions the inner 5% of the insulation experiences almost 4 times as much electrical stress as the outer 58%.

It should be understood that the electric stress or potential gradient experienced by the dielectric material in a cable of the structure shown in FIG. 1 varies continuously along the radius of the concentric system from the outer surface of the core conductor, 1 l(R to the inner surface of the outer conductive sheath, l2(R,,) in accordance with the expression:

where E, is the potential gradient at any point in the dielectric material which is at a distance (r) from the axis of the coaxial system between R, and R,.

In FIG. I the dielectric material is continuous and uniform from the core 11 to the shield 12 and the potential E is applied across the whole dielectric 13 between the outer surface of 11 and inner surface of 12. However, as shown above the gradient near the surface of 11 is much greater than it is near the surface of the sheath 12.

In order to evaluate this variation in dielectric stress within the cable insulation it is instructive to arbritrarily divide the applied potential into smaller equal parts and locate the position of such subdivisions. For example, in FIG. 1 we have divided the applied potential E into four equal parts and have located the positions of the subdivisions in FIG. 1 where the broken line 14 represents the position of the potential %E, 15 shows the position of %E, and the broken line 16 shows the position of 415 in the dielectric 13. In a coaxial geometry such as a cable these circles represent equipotential levels and extend as cylinders along its cable axis. The positions of the equal subdivisions of E were determined by employing the expression for the capacitance between concentric cylinders:

where K, represents the dielectric constant of the insulating material; L, the length of the section along the axis, R represents the larger and R, the smaller radius of the two cylinders defining the section of capacitance C. In FIG. 1 the four sections defined by the broken lines represent four capacitors in series and if they are of equal capacitance the potential between each level will be the same, in this case V4 of the applied potential.

FIG. 2 illustrates a cable 20, similar to cable 10, but modified by placing a thin conductive sheet or cylinder at each level of the insulating system corresponding to the

circles

14, 15 and 16. As therein illustrated, there is shown the cable 20 formed of a central conductive core 21 and concentric outer conductive shield 22 with an insulation system 23 filling the space therebetween. The insulation system 23 includes a plurality of concentric

insulating sections

24, 25, 26 and 27 of the same.

dielectric constant separated by thin conductive sheets or

cylinders

28, 29, 30. There is thus provided a system of separate concentric capacitors each obeying the above expression for capacitance. Now if we select radii such that the concentric subdivisions are all equal in capacitance we have C C C C and l/C l/C l/C l/C l/C Then the applied potential E will be divided into four equal subvoltages E E, E, E, and E= E E+ E, E In order for this condition to obtain the radii R R R R, and R must be as shown in FIG. 2 where R /R R /R R /R R /R, and since all parameters controlling the electrical field and dielectric stress are in the cross sectional area, we can treat the problem as two dimensional by using unity for the axial dimension. Thus, the relative quantity of dielectric material required for each coaxial subdivision is indicated correctly by the areas A A A, and A A, represents the cross sectional area of the central conductor.

This structure advantageously permits the insulation to be applied one layer at a time with the advantages of greater uniformity, favorable orientation and increased dielectric strength due to thinner sections. The conductive layers tendto unify the potential levels and maintain them in spite of small local variations in the dielectric which are unavoidable. Another advantage in this structure is that a higher quality insulation can be applied to the small volume inner layers where the potential gradient is high and then employ lower quality less expensive dielectric materials for the large volume lower stressed outer layers of the cable insulation.

The stepped dielectric sections with intervening conductive layers provide the additional advantage in that the conductive layers may be connected to suitable resistance networks at terminals which serve to drain off abnormal charges which may accumulate due to local variations in the dielectric material and to transient pnenomena (lightning and switching surges). Such resistor drain networks must always be in step with the voltage subdivisions determined by the capacitances of the successive concentric dielectric sections. For example, in both FIG. 1 and FIG. 2 we have made the capacitance of each of the four dielectric sections the same value. Thus, for any length of cable the three conductive layers between the core and sheath of FIG. 2 would tend to float at $415, 4E and /4E respectively without any drain resistor connectors particularly when the cable is in an alternating current system.

The cable design of FIG. 2 would require a resistor drain network which would divide the applied potential in the same steps as the capacitance divisions AB, /2E, M3, the resistor being connected between the core conductor and the sheath with the respective conductive layers of the cable being connected to the %E, /zE and AB points on the resistor. Thus, if there are no abnormal conditions, only the line to ground current permitted by the resistor will be present. However, the resistor networks permit abnormal charges caused by local variations in leakage current, K, density, trapped transient charges, and the like to be drained off.

Such resistor networks would be present at all tenninals and in the case of very long sections of very high voltage cable it would be advantageous to install additional terminal stations with drain resistor networks to permit this charge equilization process to take place in the long section.

Thus, in FIG. 2 we have'a cable insulation system comprising four concentric elements of the same dielectric constant with intervening conductive layers, each element carrying V4 of the applied potential. To bring about this equal division of applied potential, the

. elements must vary in thickness as shown in FIG. 2 and hence, the dielectric material in each element is exposed to varying stress increasing from the sheath to the core as described above. This system has certain manufacturing and operating advantages described above.

FIGS. 3 and 4 show another form of the invention in which each of the concentric dielectric elements not only carry the same fraction of the applied potential but they are also of the same thickness and therefore are exposed to substantially the same stress.

As in FIG. 2 the cable insulation 43 of cable 40 comprises four concentric

dielectric elements

44, 45, 46, and 47 each separated from the next by

conductive layers

48, 49 and 50. The thickness of each of the dielectric elements is the same and in the particular design of FIGS. 3 and 4 it is equal to the length of the radius of the conductive core 41. This makes the inside radius of the conductive sheath 42 5 times the length of the radius of the core 41 as was the case in FIG. 1 and FIG. 2. This choice of specific geometry simplifies mathematical expression; however, the invention is in no way limited to such relative dimensions.

As in FIGS. 1 and 2 the conductive core 41 is shown at potential E with respect to the conductive sheath 42. Each of the

dielectric elements

44, 45, 46 and 47 is shown as being exposed to the same potential, (E/4). They are of the same thickness and thus, carry the same voltage stress. However in order for this to be true the dielectric constant, K of each element must differ from the next by a definite amount such that K K K K The quantitative determination of these steps in K may be obtained from the previously employed expression for the capacita oe of concentric cylinders:

is the dielectric constant of the insulation, L is the axial length, R,, is the radius of the smaller cylinder and R is the radius of the larger cylinder.

Now since C C C C the remaining values of K are derived as follows:

O: L=1 K=C10gw 1 R R,

ro F Solid dielectric polymer materials suitable for high voltage cable insulation are available in the range 2.2 to 6.8. For example, polyethylene, polybutylene, and polypropylene blends and copolymers cover the range 2.2 to 3.2; butyl rubber and polyvinyl chloride cover the range 3.2 to 4.5; silicones and polyurethanes cover the range 4.5 to 7. Although many other polymers may be considered in this application, the above are representative. Final adjustment of K to a specific value can be made by blending and filling. The usual factors of manufacturing characteristics, field performance and cost would apply in making the selection of materials.

Referring now to the cable and termination system illustrated in FIGS. 5, 6 and 7, there is provided a cable terminating system uniquely designed to accommodate and utilize advantageously the structural features of the high voltage cable 40 illustrated in FIGS. 3 and 4. The

cable and terminal together represent improved means for handling the difficult problems encountered in terminating high voltage cables.

As therein illustrated there is provided a cable terminal 51 including a grounded element or base 52 and a high voltage cap and terminal 52. A cylindrical dielectric housing 54 of porcelain or other suitable material separates the base 52 and terminal 53 and consists (in the illustrated embodiment) of four equal cylindrical insulating elements, individually identified as 54a, 54b, 54c, and 54d. A suitable voltagelevel member in the form of metal ring clamps 55 join the outer end surfaces of the insulating elements 54 to each other and to the base 52 and terminal 53, and are individually identified as 55a, 55b, 55c, 55d, and 55e.

To provide a resistive system dividing the voltage between the

conductive layers

48, 49, 50 and 42, there is provided a resistive coating or layer 56 on the inner surfaces of the housing elements 54, each defining a resistor of specific characteristics. The grounded base 52 fits the outer sheath of the high voltage cable 40 and connects the lowermost end of the resistive coating 56 on the lowermost housing element 54a with the lowermost ring clamp 55 to provide a neutral level electrical connection as indicated at 70, FIG. 7. Thus the outer conductive sheath 42 is terminated at and connected to the base 52 at 70. The outer insulating element 47 of the cable 40 is cut off at the top of the first housing element 54a and the conductive layer 50 is also cut off at this point. An intermediate electrical connection 66 interconnects the conductive layer 50 with the adjacent ends of the resistive coating 56 of

housing elements

54a and 54b. Likewise insulating element 46 and conductive layer 49 are cut off at the topof the second housing section 54b and a suitable electrical connection 67 electrically connects the conductive layer 49 with the third housing clamp 55c and the adjacent ends of the resistive coating 56 in the

housing elements

54b, 54c. The insulating element 45 and conductive layer 48 are cut off at the top of the third housing element 54c and an electrical connection 68 is provided for connecting the conductive layer 48 to the adjacent ends of the resistive coating 56 on the

housing elements

54c, 54d and to the fourth housing clamp 55d. The inner most insu- V lating element 44 is cut off at the top of the fourth housing element 54d and the central conductor 41 of the cable 40 is secured to the metal terminal 53. The metal terminal is, of course, at line voltage and serves as an electrical connector 69 electrically securing the central conductor with the uppermost end of the resistive coating 56 on the fourth housing element 54d and the end housing clamp 55e.

Filler elements

71, 72, 73 and 74 are provided filling the space between the outer surfaces of each stepped concentric insulating section and the surrounding housing element to exclude air or other gases from the interior of the closed housing. It should be noted that the inside surface of each of the

housing sections

54a, 54b, 54c, 54d carries the electrical resistors 56. These resistors are connected in series and to each of the conductive elements of the cable as shown at 66, 67,68, 69 and 70.

47, 46, 45, and 44 and the corresponding conductive layers which separate them 50, 49 and 48 divide the ap- V plied voltage, E into four substantially equal parts as explained in the relation to the embodiments of FIG. 2 and FIGS. 3 and 4. Now the resistor linings of the terminal housing sections are designed so as to have the same value of resistance. Thus the resistor system alone will divide the applied voltage into four equal parts which is the same as the voltage divisions produced by the cable structure ie [3/4 with zero potential at the ground clamp 55a; E/4 at the second clamp 55b; E/2 at the third clamp 55c; 3E/4 at the fourth clamp 55d; and E at the top clamp 55e and terminal 53.

Thus, the cable potential E is spread into space in four equal sections and by selecting adequate axial lengths for each section, problems arising from excessive field concentration and the resulting corona are avoided. The terminal resistor also acts as drain circuits for abnormal charge accumulation which may develop on the conductive layers in the cable as described in the cable specification.

Although'the present invention hasbeen' described by reference to several embodiments thereof, it will be apparent that numerous other modifications and embodiments may be devised by those skilled in the art which will fall within the true spirit and scope of the present invention.

What is claimed as new and desired to be secured by Letters Patent of the United States is:

1. In combination:

a high voltage cable comprising a conductive core,

a conductive shield spaced concentrically outwardly of said core,

a solid dielectric system filling the space between said core and said shield, said dielectric being formed of a plurality of concentric sections, each section being of a material having a preselected dielectric constant providing a preselected voltage stress acrosseach section to provide a preselected voltage level across each section;

non-load carrying conductive layers between the sections; and

a resistance system interconnecting the conductive core, conductive layers, and conductive shield, the resistances of said system being selected to divide the applied voltage into said preselected voltage levels.

2. The combination of claim 1, a termination on said cable including a dielectric housing formed of a plurality of insulating elements, said resistive system comprising resistive layers on the inner surface of said insulating elements.

3. The combination of claim 2 and including voltage level members defined at the ends of said elements being connected to said resistance system.

4. A high voltage cable comprising a conductive core, a conductive shield spaced concentrically'outwardly of said core, and a solid dielectric system filling the space between said core and said shield, said dielectric system being formed of a plurality of concentric sections of equal thickness and capacitance having different dielectric constants providing substantially equal voltage stresses across each section, and non-load car rying conductive layers between the sections.

5. A high voltage cable as set forth in claim 4 above wherein the capacitance across each section follows the equation:

a dielectric housing formed of a plurality of insulating elements;

a resistive layer on the inner surface of each element;

voltage level members at the outside ends of said elements for applying said preselected voltage levels across respective ones of said elements;

neutral level electrical means for connecting a conductive shield of a high voltage cable to an end one of said resistive layers and an end one of said voltage level members on an end one of said insulating elements;

intermediate electrical means for connecting conductive layers to intermediate ones of said voltage level members in stepped relation with the outermost conductive layer being connectable with the voltage level member at the other end of said end one of said insulating elements; and

line voltage electrical means for terminating a conductive core of a high voltage cable and connecting the same to the resistive layer at the remote end of the other end one of said insulating members and to the voltage level member thereof.

7. A termination as set forth in claim 6 wherein the concentric sections are of equal thickness, and the conductive layers are at equal increments of voltage, the further improvement wherein said insulating elements are of equal length.

8. In combination, a termination of the type set forth in claim 7 above with a high voltage cable of the type set forth therein, each concentric section and conductive layer at its inner surface being terminated in stepped relation to the length of adjacent insulating elements; and further including insulating filler elements filling the space between the outer surface of each stepped concentric section and the surrounding insulating element.

Claims

1. In combination: a high voltage cable comprising a conductive core, a conductive shield spaced concentrically outwardly of said core, a solid dielectric system filling the space between said core and said shield, said dielectric being formed of a plurality of concentric sections, each section being of a material having a preselected dielectric constant providing a preselected voltage stress across each section to provide a preselected voltage level across each section; non-load carrying conductive layers between the sections; and a resistance system interconnecting the conductive core, conductive layers, and conductive shield, the resistances of said system being selected to divide the applied voltage into said preselected voltage levels.

4. A high voltage cable comprising a conductive core, a conductive shield spaced concentrically outwardly of said core, and a solid dielectric system filling the space between said core and said shield, said dielectric system being formed of a plurality of concentric sections of equal thickness and capacitance having different dielectric constants providing substantially equal voltage stresses across each section, and non-load carrying conductive layers between the sections.