Description:
BACKGROUND OF THE INVENTION
The present invention relates to a high voltage electrical cable which includes insulation of optimum dielectric strength for minimized dielectric losses and to which simplified terminations and splices are possible. To decrease dielectric losses in high voltage cables, it is desirable to make the insulation thereof very thick, however, the dielectric strength of the insulation in such cables is known to decrease as its thickness increases. Two distinct causes are known for this decrease, impurities at the interfacial boundaries when a multiple layer insulation construction is utilized and the existence of voids generally within the insulation. The impurities result in electrical stress concentrations while the voids result in ionization and therefore, both cause electrical defects which are deleterious to the dielectric strength of the insulation.
An extrusion process is utilized in the fabrication of most solid dielectric high voltage cables to deposit the insulation. It is commonly known that bubbles or air pockets are inherent in such a process and cause the voids in the insulation. Many techniques have been tried to reduce the number of voids, one of which is to extrude the insulation in multiple layers and thereby dissipate most of the bubbles or air pockets from each separate layer before or during curing. Theoretically, this technique results in smaller and fewer voids than are encountered in a single layer construction and furthermore, the configuration irregularities resulting in the single layer construction from sagging is no problem with this technique. However, most insulating materials must be vulcanized to establish a bond between the separate layers and the heat of this process produces gaseous byproducts which become entrapped as voids to thereby frustrate the primary purpose of this technique. Furthermore, surface impurities are inherent in the processing of insulating materials whether or not vulcanizing is required. Therefore, cable insulation of the multiple layer constructions presently known in the art, leave much to be desired.
Electrical breakdown problems are encountered at terminations and splices with high voltage cables having a metallic shielding coaxially arranged around a load conductor with the insulation disposed therebetween. The breakdowns occur because an air gap or other auxiliary components of less dielectric strength than the insulation exists between the outer shield and the inner conductor at terminations and splices. Stress relief cones are commonly utilized to solve this breakdown problem by dispersing the voltage gradient between the conductor and shield. However, these cones increase installation costs and have a limited effectiveness.
SUMMARY OF THE INVENTION
It is, therefore, a general object of the present invention to increase the dielectric strength and reduce the dielectric losses of a high voltage cable by disposing therein an insulation which minimizes and obviates the disadvantages of the prior art.
It is a specific object of the present invention to increase the dielectric strength and reduce the dielectric losses of a high voltage cable by disposing therein an insulation of optimized thickness and dielectric strength.
It is a more specific object of the present invention to increase the dielectric strength both radially and longitudinally of a coaxial high voltage cable by disposing therein in an insulation which more uniformly distributes the voltage gradient between the conductor and shield at terminations and splices.
These objects are accomplished in one form by constructing the insulation of the cable in at least three layers. An insulating material of low specific inductive capacity (hereinafter SIC) is utilized in two or more layers and an insulating material of high SIC is utilized as a field refracting barrier between the layers of low SIC material. Each layer of high SIC material is thin relative to each layer of low SIC material to optimize the dielectric strength of the insulation for minimized dielectric losses.
BRIEF DESCRIPTION OF THE DRAWING
The manner in which these and other objects of the invention are achieved will be best understood by reference to the following description, the appended claims, and the figures of the attached drawing wherein:
FIG. 1 is a cross-sectional view of a high voltage cable having the multiple layer insulation construction of this invention;
FIG. 2 is a longitudinal cross-sectional view thereof with a simplified termination made thereto;
FIG. 3 is a side elevational view thereof with comparison voltage gradient lines emitting therefrom to illustrate the improved electrical conditions resulting at terminations and splices due to the multiple layer insulation construction of this invention;
FIG. 4 is a cross-sectional view of another high voltage cable having the multiple layer insulation construction of this invention.
FIG. 5 is a longitudinal cross-sectional view of a joint between two high voltage cables having the multiple layer insulation construction of this invention; and
FIG. 6 is a cross-sectional view of a further construction of high voltage cable having the multiple layer insulation of this invention and having semi-conductive layers adjacent the core and shield.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Turning now to the drawings, FIG. 1 illustrates a high voltage cable 10 having the multiple layer insulation construction of this invention. As is the case for most high voltage applications, the cable 10 includes a load conductor 12 and a shield conductor 14 in coaxial arrangement. However, the concept of this invention could be applied to high voltage cables having only a load conductor. An insulation 16 of multiple layer construction is disposed between the load conductor 12 and the shield conductor 14. The bulk of the insulation 16 is a low SIC insulating material and is disposed in two layers 18. However, high SIC insulating material is disposed as layer 20 between the layers 18.
It is to be understood that insulating materials have particular electrical characteristics by which they are clearly distinguishable from semi-conductive materials used so frequently in high voltage cables. An insulating material is characterized by a very high resistivity at room temperature (above 1010 ohms-centimeters), a good dielectric strength (above 100 volts per mil), and measurable values of SIC; whereas a semi-conductive material is characterized by a room temperature resistivity below 1010 ohms-centimeters, virtually no dielectric strength and immeasurably high values of SIC. Typical of the insulating materials utilized in high voltage cables are natural or synthetic rubber compositions and thermoplastic polyolefins, such as polyethylene and its compositions, or blends thereof.
Increasing the SIC of insulating materials by adding thereto an appropriate quantity of titanium dioxide, carbon, or other materials in particulate form is known. In the practice of this invention, care must be exercised in selecting the modifying agent and quantity thereof employed, to assure that the room temperature resistivity is not lowered to the point where a conversion to semi-conductive material occurs. It has been found that titanium dioxide is the preferred modifying agent because significant quantities thereof can be utilized in raising SIC values of insulating materials, with only minimal adverse effects to the room temperature resistivity thereof.
Considerable dielectric losses are encountered in electrical transmission lines, due to their tremendous length, especially when a shield is coaxially arranged about the load conductor within the high voltage cable utilized therein. Of course, the dielectric losses are directly proportional to the electrical capacitance existing between the coaxial conductors. Therefore, such losses can be decreased by either decreasing the dielectric constant of the insulation between the coaxial conductors or by increasing the radial distance therebetween. Since the dielectric strength across the insulation must be sufficient to preclude electrical breakdown between the conductors, the dielectric losses will be minimized when the insulation is of optimized dielectric constant and radial thickness.
Although minimized dielectric losses may appear to be merely a matter of arithmetic, the radial thickness of insulation attainable from conventional extrusion processes is limited for many reasons. Since the insulating material is semi-fluid when applied by such processes, it only solidifies after curing for a period of time. Therefore, as the radial thickness of insulation applied by a single extrusion pass is increased, the greater the tendency is for it to run or sag into irregular configurations. Furthermore, bubbles or air pockets are inherently present in the semi-fluid insulation applied by an extrusion process and therefore, voids are created in the insulation by those bubbles which are entrapped upon solidification. Of course, as the radial thickness increases for a single extrusion pass, the greater the number of voids entrapped therein. Since ionization occurs at each void, the dielectric strength across the insulation will be decreased thereby to a value less than arithmetically anticipated When the insulation is applied in multiple layers by several extrusion passes, the voids therein will be fewer; however, impurities are inherent on the outer surface of each layer and electrical stress concentrates at these impurities to decrease the dielectric strength of the insulation.
For vulcanized insulations, there are also by-products from the curing process that are entrapped and must gradually dissipate outward. The thicker the wall, the more difficult it is for these by-products to dissipate.
Generally, the cable 10 is utilized for electrical power transmission lines and is connected to conduct current through the load conductor 12 at high voltages with the shield conductor 14 grounded. The multiple layers 18 and 20 of the insulation 16 are configured and arranged to optimize both the dielectric strength thereacross and the dielectric losses of the cable 10. This is so because fewer voids exist in the low SIC layers 18 than would otherwise have existed therein if the optimum thickness thereof had been applied as a single layer. Furthermore, the high SIC layer 20 establishes a refraction barrier to relax the electrical stress concentrations which result at the surface impurities on the innermost layer 18. As is well known from electrical field theory, the electric field intensity and the electric flux density are coincident vectors which refract at the boundary of different dielectric materials in such a manner that the tangent function ratio of the angles to the normal is equal to the dielectric constant ratio of the materials. In addition to the refractive properties at discontinuities, it is commonly known that the electric flux component, normal to a boundary between two dielectrics, will be equal in either side of the boundary. Relationship between electric flux and electric stress is the SIC; i.e. D =εE, where D is the flux, ε the actual permittivity, and E the electric stress. With the high SIC (high ε) layer interposed between the low SIC layers, the electric stress in this layer is reduced by the ratio of these SIC's. Therefore, the electrical stress concentration normally encountered at the surface impurities of the innermost layer 18 are decreased in proportion to the SIC ratio of the insulating materials and for best results, this ratio should be maximized.
Terminations and splices to the cable 10 are also simplified, due to the refraction barrier. As illustrated in FIG. 5, a splice is made by joining the discontinuous ends of the core 12 with a splicing lug 21 to establish electrical continuity therebetween. The shield 14 includes means 15 for establishing electrical continuity around the splicing lug 21 at a distance therefrom. Of course, many shield continuity means 15 are known in the art, such as the very simple soldered connection. Furthermore, the air space thereby created between the splicing lug 21 and shield 14 serves as an electrical insulation therebetween. However, any insulation material could otherwise be disposed to occupy the air space, if desired. As illustrated, in FIG. 2, a termination is made by peeling the shield conductor 14 back from the end of the cable 10 and by removing the insulation 16 from a portion of the load conductor 12. A lug 22 is affixed to the load conductor 12 and moisture seals 24 and 26 are applied between the insulation 16 and the shield conductor 14 and lug 22 respectively, as shown in phantom lines.
The shielding seal 24 is simplified relative to those in common use because no stress relief cone exists therein. Such cones would normally be required to disperse the voltage gradient between the load conductor 12 and the shield conductor 14 at the end of the cable 10 and would be precisely shaped and positioned within the shielding seal 24 which would, therefore, be made more complicated and costly. The voltage gradient is dispersed by the cable 10 without the use of a stress relief cone due to the effect of the refraction barrier on the electric field intensity and electric flux density vectors, as previously discussed. Of course, the ability of a cable to withstand breakdown at terminations and splices increases as the voltage gradient is dispersed in broader patterns. A typical voltage gradient pattern at a termination with the high voltage cable of this invention is shown in FIG. 3 where the voltage gradient pattern of conventional high voltage cable is illustrated in phantom lines for purposes of comparison. Although only two equipotential lines are shown at the 50 percent and 75 percent values of line voltage, the voltage gradient is dispersed to a greater extent with the high voltage cable of this invention at any percentage value. Relating this comparison to the use of stress relief cones at terminations and splices, conventional high voltage cable requires such cones above 5KV whereas the high voltage cable of this invention only requires such cones above 35KV. Although the cable of this invention will withstand breakdown at terminations and splices for higher line voltages, stress relief cones can be utilized therewith to disperse the voltage gradient still further and thereby enhance the breakdown capability for special applications.
The refraction barrier can be attained within the insulation 16 without regard to thickness of layer 20 or the ratio of thicknesses between layers 18 and 20. However, the principles of electrical field theory teach that in an insulation having equal thicknesses of high and low SIC materials, the voltage gradient distributed across each material is inversely proportional to the ratio of the SIC values. Therefore, the greater the difference in SIC values, the less the voltage gradient across the high SIC material and the greater the voltage gradient across the low SIC material. As explained previously, dielectric losses are minimized in the cable 10 when the insulation 16 is of optimum thickness and dielectric strength, so that layers 18 should be of much greater thickness than layer 20 to distribute the overwhelming portion of the voltage gradient across the low SIC material.
The practicalities of cable fabrication processes and material limitations, however, must always be considered. As to SIC values for known insulating materials, it has been found that 2-4.5 should be considered as low and 10-50 should be considered as high. With these insulating materials and the known extrusion processes, a thickness of up to approximately 350 mils can be applied to have very few voids or configuration irregularities, while a thickness of approximately 40 mils is as thin as can be applied. Considering these practicalities, the insulation 16 of one typical high voltage cable 10 made according to this invention has an overall thickness of approximately 560 mils with the low SIC material layers 18 being a relatively greater thickness than the high SIC material layer 20 by a factor of 7. As to the relative nature of the SIC values, the high to low ratio thereof can be from approximately 2.5 to approximately 25.
Another benefit of this invention is the electron trapping or energy absorption that occurs at the interface between the thin high SIC layer and the thick low SIC materials. This further improves the dielectric strength of this construction as compared to conventional designs having the same overall dimensions.
Depending on the application, more than two layers of low SIC material may be utilized in a high voltage cable under the concept of this invention. Additional layers of high SIC material can also be disposed in a cable having the refraction barrier of this invention for conventional purposes. A cable 10' exemplifying these possibilities is shown in FIG. 4 where because of the similarities which exist with cable 10 of FIG. 1, like items are identified by the same reference numerals but with a prime (') added. The low SIC material is disposed in three layers 18' with layers 20' of high SIC materials disposed between each adjoining layer 18'. Additional layers 28 of high SIC materials are disposed between the insulation 16' and each of the coaxial conductors 12' and 14' for conventional reasons. Furthermore, semi-conductive materials are utilized in high voltage cables for conventional purposes and a cable having the refraction barrier of this invention could also include such semiconductive materials. A cable 10" exemplifying such possibilities is shown in FIG. 6 where because of the similarities which exist with cable 10 of FIG. 1, like terms areidentified by the same reference numerals, but with a double prime (") added. An insulation 16" of multiple layer construction is disposed between a load conductor 12" and a shield conductor 14". The bulk of the insulation 16" is a low SIC insulating material and is disposed in two layers 18". However, high SIC insulating material is disposed as layer 20" between the layers 18". Additional layers 30 of semi-conductive material are disposed to separate the insulation 16" from the core 12" and the shield 14".
Those skilled in the art should readily appreciate that the dielectric losses encountered with the high voltage cable of this invention are minimized due to the use of the field refraction barrier within the insulation to optimize thickness of the cable. Furthermore, the field refraction barrier is also effective in dispersing the voltage gradient at terminations and splices of cables having coaxial conductors and therefore, stress relief cones can be omitted therefrom in many applications.
It should be understood that the present disclosure has been made only by way of example and that numerous changes in details of construction and the combination or arrangement of parts may be resorted to without departing from the true spirit and scope of the invention. Therefore, the present disclosure should be construed as illustrative only rather than limiting.