Abstract:
The current invention relates to self-supporting cables that often are aerial mounted between cable fixing points ( 800 ) and where the conductors in the cables act as the bearing elements. In this type of cables, slip-page between the surfaces of different layers in the cable is undesirable. On the other hand, it must be possible to easily bend the cable, even for larger dimensions. Both these requirements are difficult to meet with the solutions from prior art. The present invention overcomes this by introducing an intermediate layer ( 130 ) in the cable ( 100 ) located between and adhered to the surfaces ( 112, 121 ) of the layers and having a frictional inner structure allowing the two surfaces ( 112, 121 ) to slip relatively each other in longitudinal direction enough so that the cable ( 100 ) can be bent but prevents the two surfaces { 112, 121 ) from slipping in response to an inwardly directed radial pressure force (F) at the cable fixing points ( 800 ).

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a self-supporting cable. 
       BACKGROUND 
       [0002]    It is known from prior art to make aerial cables self-supporting by using separate supporting elements. These could for example be a separate messenger wire of steel. This wire could be mounted along the cable as illustrated in the European patent EP0461794. The cable could also be twisted around the messenger wire in a spiral. 
         [0003]    It is also known to provide cables of improved tensile strength by embedding supporting elements in the cable insulation as described in U.S. Pat. No. 4,956,523. 
         [0004]    A disadvantage of using these supporting elements is that the cables become expensive to produce. A cable with a supporting element also becomes heavier and for steel messengers there is often a demand that the messenger wire should be grounded for safety reasons which complicates the mounting in cable fixing points. 
         [0005]    An electrical cable comprises one or several conductors that are made out of aluminum or copper. One solution is therefore to let the conductor itself act as the supporting element. 
         [0006]    The conductors are normally surrounded by a plurality of different layers or shields, conductor shields, insulation shields, screen etc. If the different layers and/or conductors within the cable are not adhered to each other it becomes easy to bend the cable as the layers/conductors can stretch and slip relatively each other. This slippage is however undesirable for self-supporting cables. To overcome the slippage an inwardly directed radial pressure force to the cable in the cable fixing points can be applied so that the slippage is avoided. This force needs however to be very strong and has the disadvantage of damaging the outermost layers of the cable. 
         [0007]    A solution to avoid the slippage is to simply make the different layers/conductors adhere to each other (for example by gluing or melting). This has however the disadvantage that the cable will become difficult to bend and it will also be very difficult to separate the different layers/conductors from each other without damaging the cable when jointing or terminating. 
         [0008]    In U.S. Pat. No. 6,288,339 layers with undulations are disclosed. This solution has the effect that the layers can slip relative each other to some extent when the cable is bent, but in response to a relatively low inwardly directed radial pressure force the undulated layers cam into each other whereby the slippage is avoided. However, the flexibility becomes somewhat limited for large dimension cables. 
       SUMMARY 
       [0009]    It is the object of the invention to obviate at least some of the above disadvantages and to provide an improved self-supporting cable. 
         [0010]    The problems and disadvantages are in the invention solved by an intermediate portion in the cable positioned between and adhered to the outer surface of an inner portion (e.g. a core with conductors) and the inner surface of an outer portion (e.g. a shield and/or a sheath). The intermediate portion has a frictional inner structure allowing the two surfaces to slip relatively each other in longitudinal direction enough so that the cable can be bent but prevents the two surfaces from slipping in response to an inwardly directed radial pressure force at cable fixing points. 
         [0011]    The tension forces and the gravitational force acting on the cable between said cable fixing points can now be transmitted into the conductors and the cable will become self-supporting. 
         [0012]    As an option, the intermediate portion is further arranged to split in response to an outwardly directed radial force applied to the outer portion so that the outer portion can easily be separated from the inner portion. 
         [0013]    An advantage with the invention is that the cable is both easy to bend and can be mounted in cable fixing points such as dead end spirals without slippage between the layers. This applies also to large diameter cables. 
         [0014]    Another advantage is that the orientation of the structure of the intermediate portion is not critical which makes the cable easier and less expensive to produce. 
         [0015]    Yet another advantage is that the intermediate portion also reduces vibrations and oscillations when the cable is subject to strong winds. 
         [0016]    The invention will now be described in more detail and with preferred embodiments and referring to accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIGS. 1   a  and  1   b  are block diagrams illustrating a radial and a longitudinal cross section of one embodiment of a cable according to the invention. 
           [0018]      FIGS. 2 and 3  are block diagrams illustrating a longitudinal cross section of two additional embodiments of a cable according to the invention. 
           [0019]      FIGS. 4   a  and  4   b  are block diagrams illustrating a bent cable and a cable subject to an inwardly directed radial pressure force. 
           [0020]      FIGS. 5   a ,  5   b  and  5   c  are block diagrams illustrating the behavior of the fibrous structure in the intermediate portion. 
           [0021]      FIG. 6  is a block diagram illustrating a longitudinal cross section of a cable according to the invention with a separated outer portion. 
           [0022]      FIG. 7   a  is a block diagram illustrating a 3-core high voltage power cable comprising the present invention. 
           [0023]      FIG. 7   b  is a block diagram illustrating a 1 kV power cable comprising the present invention. 
           [0024]      FIG. 8  is a block diagram illustrating a cable fixing point. 
       
    
    
     DETAILED DESCRIPTION 
       [0025]      FIGS. 1   a  and  1   b  illustrates a radial and a longitudinal cross-section of a cable  100  according to the present invention. The cable  100  in  FIGS. 1   a  and  1   b  comprises an inner portion  110  with an outer surface  112 , an outer portion  120  with an inner surface  121  and an intermediate portion  130 . The inner portion  110  comprises one or several conductors  111 . Each conductor  111  often consists of a plurality of metal wires  115  (normally aluminum or copper). The inner portion  110  and the outer portion  120  can consist of one or several layers of different types, plastic isolating layer, metal shield, semi conductive shield, sheath etc. An example on a cable  200  with an outer portion  210  comprising a metal shield  211  and a plastic layer  212  is illustrated in  FIG. 2 . The plastic layer  212  has penetrated between the wires of the metal shield  211  by melting in the extrusion process. 
         [0026]    The embodiments of the invention illustrated by  FIGS. 1   a ,  1   b  and  2  comprise only one intermediate portion  130 . The inventive concept is however not limited to one intermediate portion  130  only but several intermediate portions can be used. This is illustrated in  FIG. 3 . What in  FIG. 2  comprises a cable  200  with an outer portion  210  can in principle be regarded as the inner portion  310  of a cable  300  with yet another intermediate portion  330  and yet another outer portion  320 . 
         [0027]    The main principle of the invention is illustrated in  FIGS. 4   a  and  4   b . The intermediate portion  130  is adhered to the two surfaces  112 , 121  and having a frictional inner structure allowing the two surfaces  112 , 121  to slip relatively each other in longitudinal direction so that the cable  100  can be bent as illustrated in  FIG. 4   a.    
         [0028]    The friction within the inner structure of the intermediate portion  130  is further adapted to increase in response to an inwardly directed radial pressure force F at cable fixing points as to prevent the two surfaces from slipping. This is illustrated in  FIG. 4   b.    
         [0029]    The tension forces and the gravitational force acting on the cable  100  between the cable fixing points can now be transmitted into the conductors  111  wherewith the cable  100  becomes self-supporting by virtue of the intrinsic mechanical strength of the conductors  111 . 
         [0030]    A preferred embodiment of an intermediate portion  130  comprises at least one sheet of a non-woven material adhered to the two surfaces  112 ,  121 . It has been observed that a non-woven material with a fibrous structure is particular suitable. One example of such a non-woven material is crepe paper, or crêpe paper. Crepe paper is tissue paper typically having a thickness between 0.20 and 0.60 mm that has been coated with sizing and then “creped” to create gathers. Sizing is a material such as glue, gum, or starch, added to paper pulp to add sheen and stiffness, among other things. This gives crepe paper a distinct texture quite different from untreated tissue paper. Crepe paper has also the characteristics of being easy to stretch. Using adhered crepe paper as the intermediate portion  130 , the friction within the crepe paper allows the cable  100  to easily be bent to some extent but when subject to the radial pressure force F the friction between the fibers in the crepe paper quickly increases and prevents the two surfaces  112 ,  121  from slipping. Crepe paper is relatively inexpensive, easy to wrap around the inner portion  110  of the cable  100  and has the same characteristics independent of orientation. It is also possible to use two or more sheets of crepe paper that are wrapped around each other. 
         [0031]    The behavior of the fibrous structure is illustrated in  FIGS. 5   a  to  5   c . When the cable  100  is not subject to any inwardly directed radial force, the fibers  511  in the intermediate portion  130  allow the two surfaces  112 ,  121  to slip to some extent relative each other as illustrated in  FIGS. 5   a  and  5   b . When subject to an inwardly directed radial force F, as in  FIG. 5   c , the friction between the fibers  511  quickly increases already when the thickness of the fibrous structure has decreased a few percent. 
         [0032]    If the surfaces  112 ,  121  belong to plastic layers (which often is the case), it is possible to adhere the crepe paper to the two surfaces  112 ,  121  by heating. After the crepe paper has been wrapped around the inner plastic layer of the inner portion  110 , the extrusion process melts the outer plastic layer on the crepe paper. The temperature in the extrusion process is set to be sufficient to also melt the outer surface  112  of the inner plastic layer at the same time. In the melting process, the two surfaces  112 , 121  of the plastic layers penetrate into the fibrous structure of the crepe paper whereby it becomes adhered to the two surfaces  112 , 121 . 
         [0033]    This adhering process also works if the outer portion  120  comprises a metal shield  211  as illustrated in  FIG. 2 . In this case the outer plastic layer both penetrates between the wires of the shield  211  and reaches and penetrates into the fibrous structure of the crepe paper. 
         [0034]    Making the intermediate portion  130  to adhere to both the inner and outer portion  110 , 120  in one manufacturing step is a great advantage. Although not being a preferred embodiment, the intermediate portion  130  can also be adhered to the surfaces  112 ,  121  by gluing. 
         [0035]    The fibrous structure of the crepe paper further allows it easily to be split. This is illustrated in  FIG. 6 . This feature makes it easy to separate the outer portion  120  from the inner portion  110  of the cable  100  without damage by applying an outwardly directed radial force S to the outer portion  120 . This feature is a great advantage when jointing or terminating the cable  100 . 
         [0036]    Yet another feature of the invention is that the intermediate portion  130  also reduces vibrations and oscillations of the cable  100 . Vibrations and oscillations can occur when the cable  100  is subject to strong winds and can cause the cable  100  to come loose from its fixing points. The frictional structure of the intermediate portion  130  reduces the vibrations and oscillations as it transforms the kinetic energy from the relative movement between the two surfaces  112 , 121  to thermal energy (heat) due to the friction. 
         [0037]    Although the  FIGS. 1 to 6  only illustrate cables with one conductor  111 , the inner portion  110  of the cable  100  can comprise a plurality of conductors. Two examples of this are illustrated in  FIGS. 7   a  and  7   b.    
         [0038]    The cable  700  in  FIG. 7   a  is a high voltage AXCES type of cable for 12 kV where the inner portion comprises three conductors  701 ,  708 ,  709  made of aluminum. Around each conductor  701  an inner conductive layer  702  of polyethylene, PE is extruded. Around the inner conductive layer  702  an insulation layer  703  of cross-linked polyethylene, PEX or XLPE is triple extruded. Around the insulation layer  703  a second conductive polyethylene layer  704  is extruded. 
         [0039]    Around this inner portion, comprising the three conductors  701 ,  708 ,  709  each with its conductive and insulating layers  702 , 703 , 704 , the intermediate portion  705  is mounted. The outer portion comprises screen wires or foil normally of copper or aluminum (not shown) wrapped around the intermediate portion  705 . Finally, a black LLD PE (linear low density polyethylene) sheath  706  is extruded over the screen. The intermediate portion  705  comprises here a sheet of crepe paper. The LLD PE sheath  706  has penetrated through the copper shield and into the texture of the crepe paper  705  during the extrusion process. During the same process, the heat has also made the crepe paper  705  to adhere to the second conductive PE layer  704 . 
         [0040]    The cable  710  in  FIG. 7   b  is a N1XE type of cable for 1 kV with four conductors  711 ,  717 ,  718 ,  719 . As this cable  710  is made for lower voltage the dimensions of the conductors  711 ,  717 ,  718 ,  719  are smaller. The four conductors  711 ,  717 ,  718 ,  719  can for example be of solid round copper (as in  FIG. 7   b ), stranded round copper or of stranded sector shaped aluminum depending on cross section area. In this cable  710 , the inner portion comprises the four conductors  711 ,  717 ,  718 ,  719  each having an insulation layer  712  of cross-linked polyethylene. Around the four conductors  711 ,  717 ,  718 ,  719  an inner covering  713  is extruded. Around this inner covering  713  the intermediate portion  714  of crepe paper is mounted and the outer portion of the cable comprises a black polyethylene sheath  715  extruded over the crepe paper  714 . In a similar way as for the AXCES type of cable  700  above, the crepe paper  714  is adhered to the outer surface of the inner covering  713  and the inner surface of the polyethylene sheath  715  during the extrusion. 
         [0041]    An example of a cable fixing point used for self-supporting cables is a so called dead end spiral. An example of a dead end spiral is illustrated in  FIG. 8 . In the fixing point  800 , a metal wire  810  is twisted around the cable  100  in a spiral  811 . The other end of the wire  810  is fixed to a pole  820 . In order to not damage the outer layers of the cable  100  in the fixing point  800 , the radial pressure forces F applied to the cable  100  must be relatively low. Therefore the spiral  811  extends up to two meters along the cable in order to distribute the radial pressure forces F to the cable. By applying relatively weak forces F to a cable  100  according to the present invention, tension forces T and the gravitational force G acting on the cable  100  are transmitted into the conductors  111  without slippage between the layers in the cable  100 . 
         [0042]    Although the embodiments described above mainly address electrical cables, the inventive concept can also be used for optical cables having an inner portion with a sufficient mechanical strength that allows the cable to be self-supporting.