Abstract:
Structure is provided for controlling the electrical stress on one or more optical fibers in a high voltage environment.

Description:
BACKGROUND  
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to stress control structure for optical fibers in regions of high electrical stress down to a lower electrical stress, or zero electrical stress or ground. In particular, it involves selections and arrangements of dielectric layers and other layers associated with optical fibers originating elsewhere or originating in or traversing regions of high electrical field strength and the control of the stress to which the optical fibers are exposed. 
         [0003]    2. Description of the Art 
         [0004]    The advantages of optical fibers over other media for the conveyance of digital and electrical signals are well known. However, optical fibers placed in regions of high electrical field strength are susceptible to damage and failure from localized dielectric breakdown of the electrical insulation system in the vicinity of the optical fibers. This condition is intensified when a discontinuity exists in the shielding of a high voltage cable, sensing device or other electrical equipment. 
         [0005]    The following and all other referenced patents and applications are incorporated herein by reference in their entirety. Definitions herein prevail over definitions in such references. 
         [0006]    Some of the prior art is very useful in pointing out solutions to various problem areas. U.S. Pat. No. 6,215,940, for High Voltage Insulator for Optical Fibers, issued Apr. 10, 2001, which reference is incorporated herein, describes the various embodiments of flexible insulator sleeves, characteristics of the insulative support rod, (corresponding to insulative central structural element herein), preserving integrity of the optical fibers, cushioning between the optical fibers and the support rod, problem of voids, (air gaps), moisture, and other aspects. This reference also points out that the length of the rod, (insulative central structural element herein), depends primarily upon the level of the (voltage) system on which it is used. 
         [0007]    Also, U.S. Pat. No. 6,015,625, for Stress Control for Termination of a High Voltage Cable, issued Jan. 18, 2000, is incorporated herein by reference, providing test and design information as to stress control layers, including permittivity requirements, with improved corona discharge extinction, withstanding of power frequency voltage and impulse voltages, and in meeting the requirements of IEEE Standard Test procedures, STD 48-1990. That patent also provides helpful information as to exterior elastomeric coverings, (sleeves), and other types of exterior coverings, elimination of air pockets, as well as superiority of stress control material comprised of epihalohydrin rather than EPDM. 
         [0008]    Elements:
       1. Insulative Central Structural Element: The insulative central structural element is the innermost portion of the particular structure. Although other shapes may be used, it may be constructed of a longitudinal, central, circular, or somewhat circular, column, forming the body of the elongated portion of the structure. Whereas circular or near circular is the preferred shape, elliptical, triangular, square, hexagonal, octagonal and other multi-sided symmetric and asymmetric shapes might conceivably be used.
           The insulative central structural element is formed from a material with good insulative properties at high voltages. In various embodiments of the invention, the structure may be comprised of ceramic, glass, porcelain, plastic, polycarbonate, phenol, or other materials which are suitably insulative and provide the strength required to support the physical structure. Although it is not necessary, the insulative central structural element may be made of high dielectric material.   In one embodiment, such structure may be a high voltage cable with all it accompanying layers.   The thickness of the insulative central structural element is determined by the dielectric strength of the insulator material, the applied high-voltage and the needed safety factor as determined by the application. Typical thickness for the insulative central structural element is between 0.5 and 1.25 inches for an applied high-voltage of 35 kV.   Voltages in high voltage distribution systems range from 4 kV to 75 kV with common voltages of, say, only as examples, 15 kV, 25 kV, 35 kV, 46 kV and 69 kV. The dielectric capability and size of the insulative central structural element would, of course, take into account differences in intended use.   
           2. Stress Control Layer: The stress control layer consists of a high permittivity material that is tightly-fitted around the insulative central structural element in such a way so as to create a void-free or substantially void-free interface. This high permittivity material manages the high-electrical stresses by creating a more evenly distributed e-field over the surface of the structure, significantly reducing the potential for insulation breakdowns that could result in partial discharge, withstand or Basic Insulation Level (BIL) failures. (Partial discharge occurs when a localized dielectric breakdown of the electrical insulation system under high voltage stress but not of sufficient magnitude to bridge the space between two conductors.)
           The thickness of the stress control layer is typically 50 to 200 mils and in various embodiments of the invention, the material may be comprised of Ethylene-Propylene (EP), Ethylene-Propylene-Diene-Monomer (EPDM), rubber or silicon or other suitable materials having properties, with various additives, fillers and conductive particles so as to provide the needed permittivity.   
           3. Dielectric Layer: The dielectric layer consists of an elastomeric material with high dielectric strength that, in conjunction with a second layer of elastomeric material, form two layers that encapsulate the optical fibers, minimizing voids in the vicinity of the fibers. In one preferred embodiment, two dielectric layers are composed of the same material with self-fusing properties to minimize voids and with sufficient pliability to conform to the fiber radius; however the self-fusing property is not strictly required, but it is an advantageous embodiment.
           In various embodiments of the invention, the dielectric layer may be comprised of rubber, silicon, or other elastic materials with various additives and fillers to provide the needed dielectric material properties.   
           4. Optical Fiber: The optical fiber is a cylindrical dielectric waveguide, typically made of low-loss glass or plastic comprised of a central core which guides the optical signal surrounding by a cladding material of slightly lower refractive index. Light impinging on the core-cladding interface at an angle greater than the critical angle undergoes total internal reflection and is guided through the core. Fiber optical cable is familiar to those skilled in the art. The optical fiber, in this application, is used for the transmission of analog or digital data through a region of high electrical stress. “Light” as used herein includes, of course, much more of the electromagnetic spectrum than visible light. Both infrared and ultraviolet frequencies might be used. Infrared and near infrared are most common. Such wider meaning is commonly known in the art.   5. Dielectric Layer: The dielectric layer consists of an elastomeric material with high dielectric strength that, in conjunction with a first layer of elastomeric material, form two layers that encapsulate the optical fibers, minimizing voids in the vicinity of the fibers.   6. External Insulator: An external layer of insulative material is placed around the structure. The external insulator is very tightly-fitting. The external insulator may include “skirts” or “sheds” or “wings” to decrease the susceptibility to external arcing or tracking; known to those skilled in the art, or, as described, for example in U.S. Pat. No. 6,215,940. The external insulator must prevent electrical tracking and must be strong, tight-fitting, hydrophobic and be an effective “weathershed”, providing protection against weather and harsh environmental conditions. It may be composed of highly insulative rubbers, silicone, various polymers and other suitable materials.
           Depending of the application, the external insulator may not be required since this is primarily necessitated by exposure to weathering and harsh environmental conditions. In various embodiments of the invention, this layer can be eliminated or replaced with an alternate external layer suitable for the physical conditions to which the structure will be exposed.   In the preferred embodiment, the construction elements 1-6, as referenced above, describe the materials and methods used to construct the stress control insulating device of the invention. It is conceivable that either one or both of the dielectric layers (elements 3 and 5 above) could be omitted. In alternate embodiments of the invention, the various construction elements could be combined, performing the same physical and electrical functions, albeit with a reduced number of physical elements, but accomplishing the same functions.   
               
 
         [0023]    A description of a current and voltage sensor, such as might be used in combination with the inventions herein, is found in patent application Ser. No. 13/5,534,886, filed Jul. 19, 2012, entitled, “OPTICAL SENSOR ASSEMBLY FOR INSTALLATION ON A CURRENT CARRYING CABLE. (Applicant will furnish updated information when received.) Such application is incorporated herein for reference purposes. 
         [0024]    The various items used in the electrical constructions herein are commercially available. Most can be ordered by size, length, width, dielectric constant, conductivity and other characteristics. The needs for cable elements can be met over a wide range of voltages. Many suitable products are available from several suppliers. Of course, a wide range of optical fibers are also readily available. 
       SUMMARY OF THE INVENTION 
       [0025]    “The present invention relates to providing control of the electrical stress on one or more optical fibers in a high voltage environment. The optical fibers may originate in a high voltage environment or elsewhere and then pass into such high voltage environment. Various layers of materials having particular electrical characteristics are described for reducing the high stress electrical field down to a lower stress electrical field or zero stress electrical field or to ground level, as the case may be, in order to protect the optical fibers and, of course, the information they are transmitting.” 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]      FIG. 1  is a view of an embodiment of the invention, illustrating broadly, a stress control structure reducing a high electrical stress down to a lower electrical stress or zero electrical stress, providing for optical current fibers to pass from the high electrical stress area down to the lower electrical stress or to a zero electrical stress area. In this embodiment, the optical fibers originate in the high electrical stress area. 
           [0027]      FIG. 1A  is a view of another embodiment of the stress control structure, illustrating optical current fibers originating elsewhere than within the stress control structure. 
           [0028]      FIG. 1B  is an illustration of a high voltage structure extending into a channel in the stress control structure. 
           [0029]      FIG. 2  is an embodiment in which is shown a cross-section of the layers outside the stress control layer which is shown in relief, and illustrating the disposition of a plurality of optical fibers in helical fashion between two layers of dielectric material. 
           [0030]      FIG. 2A  is a close-up view of the optical cables in relief emerging from between dielectric layers., shown in cross-section. 
           [0031]      FIG. 2B  is a cross-section view, showing the disposition of the optical cables in relief between dielectric layers shown in cross-section. 
           [0032]      FIG. 2A  is a close-up cross-section of an optical fiber disposed between two conformable dielectric layers. 
           [0033]      FIG. 3  is a breakaway view of the outer dielectric layer and the outer protective layer, down to the optical fibers layer, showing them disposed in meandering fashion. 
           [0034]      FIG. 4  is another embodiment, a breakaway of the outer protective layer, showing the optical fibers layer against the stress control layer, and showing a single optical fiber disposed in curvilinear fashion. Multiple optical fibers could be similarly disposed. Also represented in this embodiment are a sprayed on dielectric layer or a painted on dielectric layer or a molded in place dielectric layer or extruded or molded and applied dielectric layer over the optical fiber. 
           [0035]      FIG. 5  is an exterior view of a high electrical stress termination of a high voltage cable, showing Sections taken on lines  6 - 6  for  FIG. 6  and at  8 - 8  for  FIG. 8 . 
           [0036]      FIG. 6  is a cross-section view taken on line  6 - 6  of the embodiment of  FIG. 5 , illustrating the high voltage cable terminating within the stress control structure and having optical fibers therein. 
           [0037]      FIG. 6A  is a cross-section view similar to that of  FIG. 6 , but with the optical fibers placed between the stress control layer and an outer dielectric layer. 
           [0038]      FIG. 7  is an embodiment of  FIG. 5 , showing the outer protective cover and an outer dielectric layer broken away to show the optical fibers helically wound around an inner dielectric layer. 
           [0039]      FIG. 8  is a cross-section view taken on line  8 - 8 , of the embodiment of  FIG. 5 . 
           [0040]      FIG. 9  is an exterior view of a termination of a high voltage cable. 
           [0041]      FIG. 10  is a cross-section view taken on line  10 - 10  in  FIG. 9 . of a termination of a high voltage cable, having optical fibers helically wound therein, which continue through the termination. 
           [0042]      FIG. 10A  is a cross-section view similar to that taken for  FIG. 10 , but having the electric cable extend through the device of the invention. 
           [0043]      FIG. 10B  is a cross-section of the device of the invention comprising a joint between the ends of two cables. 
           [0044]      FIG. 11  is an exterior view of termination with two layers broken away, the outer protective layer and the first dielectric layer, showing the optical fiber cables helically wound on the inner dielectric layer. 
           [0045]      FIG. 12  is a cross-section view of the termination taken at  12 - 12  in  FIG. 9 . 
           [0046]      FIG. 13  is an exterior view of an electrical sensor attached to a high voltage cable. 
           [0047]      FIG. 14  is cross-section view of the body of the device of the invention showing the various layers and the optical fibers for the current sensor as well as optical fibers for the voltage sensor. 
       
    
    
     DETAILED DESCRIPTION 
       [0048]      FIG. 1  is a stress control structure  1 , having optical fibers originating therein, in the high electrical stress area  2  of the stress control structure and passing out of the stress control structure through the low electrical stress or zero electrical stress area  3  as shown by exterior cables  4 . 
         [0049]      FIG. 1A  illustrates a similar stress control structure  1 . However, the optical fibers  5  extend into the high electrical stress area  2  from elsewhere. Then the optical fibers  5  extend to the low electrical stress  3  and on out of the stress control structure, as shown by cables  5 . 
         [0050]      FIG. 1B  is an illustration of a high voltage structure  6  extending into a channel in the stress control structure  1 . Also shown is the embodiment in which the optical fibers  5 , (combined optical fibers), originate “elsewhere” and extend through the stress control structure  1  and provide output to optical fibers  4 , (combined optical fibers). 
         [0051]      FIG. 2  is a view of the stress control structure that is partially broken away down to the stress control layer  12 , which is the high permittivity, (high-dielectric), layer shown in relief, showing the two meandering dielectric layers  7  and  8  surrounding the individual fiber optical wires  9  and it also shows the outer protective layer  11 . 
         [0052]      FIG. 2  is a magnified cross-section of a small portion of the stress control layer  12 , showing an optical fiber wire  9  surrounded by two dielectric layers  7  and  8  and a protective outer layer  11 . 
         [0053]      FIG. 2A  is a close-up view of the optical cables  9 , shown in relief, emerging from between dielectric layers  7  and  8 , shown in cross-section. Outer protective layer  11  is also shown in cross-section. 
         [0054]      FIG. 2B  is a cross-section view, showing the disposition of the optical cables  9  between dielectric layers  7  and  8 . Outer protective layer  11  is also shown in cross-section. 
         [0055]      FIG. 3  is an exterior view of the stress control structure  1 , having a breakaway of the outer layer  11  down to outer dielectric layer  7 , down to the optical fibers  9 , showing them disposed in meandering fashion between dielectric layer  7  and stress control layer  12  of high permittivity. 
         [0056]      FIG. 4  is another embodiment, a breakaway of the outer protective layer  11 , showing the optical fiber layer  9  disposed in curvilinear fashion against the stress control layer  12 . Multiple optical fibers could be similarly disposed. Also represented in this embodiment are a sprayed on dielectric layer  13  which, instead, may be painted on dielectric layer or a molded in place dielectric layer or extruded or extruded or molded and applied dielectric layer over the optical fiber or a combination thereof. Dielectric layer  13  may be a tape-like strip or a wide band of dielectric. Also, the optical fiber  9  may be disposed linearly rather than curvilinearly. Such technique could be used at whatever level the optical fibers are located, between dielectric layers or otherwise. 
         [0057]      FIG. 5  is an exterior view of the stress control structure  1  illustrating the termination of a high voltage structure  6  and showing the Sections taken at  6 - 6 ,  FIG. 6  and at  8 - 8 ,  FIG. 8 . 
         [0058]      FIG. 6  is a cross-section view taken on line  6 - 6  of the embodiment of  FIG. 5 , illustrating the high voltage structure  6  terminating within the stress control structure  1  and having optical fibers  9  therein. Also shown are outer protective layer  11 , outer dielectric layer  7 , inner dielectric layer  8 , stress control layer  12 , which is of high permittivity, (high dielectric), and the central longitudinal structure  14 , which may, for example, be polycarbonate. A grounded metallic tube or cup  10  is shown which helps to establish the lower stress field or zero stress field at the bottom end of the embodiment. 
         [0059]      FIG. 6  also illustrates a channel which extends into, but not through, the high voltage insulator  14  of the stress control structure  1 , in which high voltage structure  6  is located. Such channel permits a wide use of the embodiment in connection with various other equipment. It is also noted in  FIG. 10A , below, that the channel extends completely through the stress control structure  1 , and is occupied by high voltage cable  20 . This channel allows for wide use in connection with cables and other equipment. 
         [0060]      FIG. 6A  is a cross-section similar to the view of  FIG. 6 , but with the optical fibers  9  placed between the stress control layer  12  and the outer dielectric layer  7 . In this embodiment, no inner dielectric layer  8  is used. 
         [0061]      FIG. 7  is an embodiment of  FIG. 5 , showing the outer protective layer  11  and the outer dielectric layer  7  broken away to show the optical fibers  9  helically wound around an inner dielectric layer  8 . 
         [0062]      FIG. 8  is a cross-section view taken on line  8 - 8 , of the embodiment of  FIG. 5 . Shown are the high voltage structure  6 , the insulator  14 , metal cup or tube  10 , at lower electrical stress or zero electrical stress, (ground), stress control layer  12 , inner dielectric layer  8 , optical fibers  9 , outer dielectric layer  7 , and outer protective layer  11 . 
         [0063]      FIG. 9  is an exterior view of the stress control structure  1  used in the termination of a high voltage cable, by lower electrical stress or zero electrical stress metal lug  21  at the top of the embodiment.  FIG. 10  is taken on section line  10 - 10  and  FIG. 12  is taken on section  12 - 12 . 
         [0064]      FIG. 10  is a cross-section view taken on line  10 - 10  in  FIG. 9  of a termination of a high voltage cable  20  having optical fibers  9  helically wound therein, which optical fibers continue throughout the termination. There is an outer protective layer  11 , and an inner dielectric layer  8  with optical fibers  9  placed against it and covered by outer dielectric layer  7 . Inside of inner dielectric layer  8 , is stress control layer  12 , high voltage insulator  14  and in its interior is the high voltage cable  20 . Also shown are a semiconductive layer  22  with metal ground wires thereon (not shown) and entering optical fibers, (combined),  5  and leaving optical fibers, (combined)  4 . In this embodiment, the high voltage cable  20 , its high voltage insulator  14 , (and any other elements coming with the particular high voltage cable), form the longitudinal structure of the embodiment. 
         [0065]      FIG. 10A  is a cross-section view similar to that taken on line  10 - 10  of  FIG. 9 , for  FIG. 10 , but having the high voltage cable  20  extending through the stress control structure  1 , with no terminating lug. 
         [0066]      FIG. 10B  is a cross-section similar to the lower half of the stress control structure  1  of  FIG. 10  having the lower half inverted and placed above the lower half, to show a joint connection  25  of two high voltage cables  23  and  24 . The stress control layer  12  widens to fill in the gap at the joint connection  25 . The joint connection  25  may be a butt weld, soldered joint, crimping together, metal band or a tube or by any other suitable electrical joint. 
         [0067]      FIG. 11  is an exterior view of a termination embodiment with two layers broken away, the outer protective layer  11  and the outer dielectric layer  7 , showing the optical fibers  9  helically wound on the inner dielectric layer  8 . 
         [0068]      FIG. 12  is a cross-section view of the termination taken at  12 - 12  in  FIG. 9 . The layers shown are the high voltage cable  20 , the cable insulator  14 , the cable semiconductive layer  22 , having metal ground wires (not shown), stress control layer  12  which is of high permittivity, inner dielectric layer  8 , optical fibers  9 , outer dielectric layer  7 , and outer protective layer  11 . 
         [0069]      FIG. 13  is an exterior view of an electrical sensor assembly  30  attached to a high voltage cable  31 . The stress control structure  1  provides the support for the electrical sensor assembly  30 , providing for passage therethrough of the optical fibers  4  associated with the electrical sensor assembly  30 . The outer protective layer  11  of the stress control structure  1  can be seen as well as the combined optical fibers  4 . 
         [0070]      FIG. 14  is an exterior view of the electrical sensor assembly  30  disposed on top of a cross-section view of stress control structure  1 . The various layers are those shown and described with reference to  FIG. 6 . In addition, the optical fibers  9  at the top of the drawing, associated with the current sensor portion of the electrical sensor assembly  30  are shown passing through the body of the stress control structure  1  and out into combined optical fibers  4 . There is shown a voltage sensor  32  in the stress control structure  1 , (sensing voltage of the high voltage cable  31 ). Optical fibers  9 , associated with voltage sensor  32 , pass through the lower end of the stress control structure and out into combined optical fibers  4 . 
         [0071]    The various items used in the electrical constructions herein are commercially available. All can be ordered by size, length, width, dielectric constant, conductivity or other relevant characteristics. The needs can be met over a wide range of voltages. Many suitable products are available from several suppliers. Of course, a wide range of optical fibers are also readily available.