Abstract:
A cable has a conductor and an insulator over the conductor. The conductor includes a first layer of at least one conductor element, a second layer of conductor elements, disposed over the first layer, and an intermediate water blocking layer, between the first and second conductor layers. The intermediate water block layering has at least one longitudinally applied water swellable yarn and at least one helically applied water swellable yarn.

Description:
FIELD OF THE INVENTION 
     The present invention relates to power cables. More particularly, the present invention relates to water blocked power cables. 
     BACKGROUND 
     Medium voltage power cables (5 kV to 46 kV) must meet various safety standards set by the cable standard organizations. Such standards include AEIC CS 8 (Association of Edison Illuminating Companies), ICEA S-94-649 (Insulated Cable Engineering Association, CSA C68.3 (Canadian Standards Association) and UL 1072 (Underwriters Laboratories) 
     Additional standards may be applied when these power cables are made water blocked. Such cables must meet additional standards, such as ICEA T-31-610 which is a test for resistance for longitudinal water penetration. 
     In the prior art “strandblock” has been used to meet these standards. Strandblock is essentially a process whereby a strand filling compound, such as a gel or other water blocking agent is applied between the gaps and spaces between individual elements of a conductor core, blocking the penetration of water. Although many improvements have been made over the years, Strandblock remains a specialized slow and unclean process which is costly and difficult to maintain. 
     Water swellable powders are a form of polymers that, when brought into contact with water, expand, trapping the water within, forming a water locking gel-like substance. One example of a water swellable powder is formed with polymers that are in the form of polymeric chains coiled, and lined with carboxyl groups. When these water swellable powders are wetted, the carboxyl groups change to be negatively charged, forcing the chains to “uncoil,” resulting in a rapidly forming water absorbing gel. 
     Water swellable powders of this type are commercially available for use in the cable industry. One form for such powders is to be applied to yarns or tapes impregnated with the powders. These yarns and tapes may be applied at various stages during the cable construction. 
     For example, a typical prior art cable as shown in  FIG. 1  includes a cable core of aluminum alloy or copper conductor elements (strands, wires), which are assembled to form a conductor. Around this conductor, a layer of insulating material, such as polyethylene (PE) is applied. In the case of medium and high voltage cables, this layer of insulating material is applied between two layers of semi-conductor material, such as polyolefin with carbon black. The cable may contain other cable components such as metallic screens, armour, jacket etc. 
     In one prior art arrangement as shown in CA 2 394 846, a water swellable tape is applied around the external copper screen, under the jacket so as to form an improved water blocking structure. 
     In another prior art arrangement, water blocking yarns may be fashioned for insertion within the conductor itself for use as a water protection measure. However, the simple application of water swellable yarns to a conductor core does not necessarily provide adequate water protection to meet the necessary safety standards such as those met by the strandblock method outlined above. 
     SUMMARY 
     The present invention in accordance with various exemplary embodiments described herein, improves upon the prior art solutions by providing a novel water blocking arrangement for power cables, employing a combination of helically wound and longitudinally arranged water blocking yarns in between the conductor elements forming the conductor. 
     To this end, in accordance with one embodiment, a conductor is provided having a first layer of at least one conductor element and an additional layer or layer of conductor elements. A water blocking layer is arranged between each layer of conductor elements, where the intermediate water blocking layer has at least one longitudinally applied water swellable yarn and at least one helically applied water swellable yarn. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a prior art cross section image of a power cable. 
         FIG. 2  is a cross section of a power cables constructed according to one embodiment of the present invention; 
         FIG. 3  is close up exploded view of the conductor core of the power cable as shown in  FIG. 2 ; and 
         FIG. 4  is an illustration of a device for helically winding the water swellable yarn as shown in  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION 
     In one embodiment of the present invention as shown in  FIG. 2 , a power cable  10  is shown having a conductor core  11  (or conductor), a first semi-conductor layer  20 , an insulator layer  22 , a second semi-conductor layer  24 , a metal shielding  26 , and an outer jacket  28 . Optionally, a water swellable tape  30  may be applied around metal shielding  26 , between shielding  26  and jacket  28 . According to one arrangement, a water swellable yarn  40  is applied within conductor core  10  as described in more detail below. 
     As shown in  FIGS. 2 and 3 , Conductor  11  is typically formed of as multi-layer core of conductor elements  12 , each element  12  being a metal wire, such as copper wire or aluminum alloy wire. A first layer  14  is formed of a single conductor element  12 , with second layer  16  being formed of six conductor elements  12 , disposed around the outer circumference of first layer  14 . A third layer  18  is formed of additional conductor elements  12  disposed around the outer circumference of second layer  16 . 
     It is understood that various different forms of multi-layer conductors  11  are within the contemplation of the present invention. For the purposes of illustration the three layer conductor  11  shown in  FIG. 3  is used to exemplify the salient features of the present invention. 
     Likewise, it is noted that the arrangement shown is typical for medium voltage power cables. However, it is understood that the features of the present invention may be applied to any cable arrangement. 
     First semi conductor layer  20 , insulator layer  22 , and second semi conductor layer  24  form the primary insulation around conductor  11 . Typically, semiconductor layers  20  and  24  are made from a semi-conductive polymer, such a PE (Poly ethylene) with embedded carbon black particles. Insulator layer  22  may be formed of PE or other non-conducting polymers suitable for the voltages/current to be carried through cable  10 . It is understood that the material/polymer selections for first semi conductor layer  20 , insulator layer  22 , and second semi conductor layer  24  is by way of example and is in no way intended to limit the scope of the invention. 
     Metal shielding  26  is preferably a copper screen/mesh used to provide structural integrity to cable  10  as well as to protect against cable damage caused by short circuit currents in cable  10 . Metal shield  26  in this capacity acts to equalize electrical stresses around insulation layers  20 ,  22  and  24 . 
     An optional outer jacket  28  is typically an extruded polymer such as PE used to provide an outer water proof non conductive coating to cable  10 . The optional water swellable tape  30  wound around metal shielding  26  under jacket  28  is used to provide added protection against water ingress into the cable and to prevent such water from progressing longitudinally down the length of the cable under jacket  28 . 
     Turning now to the construction of conductor  11  of cable  10 , in one embodiment of the present invention, as illustrated in  FIG. 3 , one or more water swellable yarns  40  are applied helically around each layer of conductor  11 . Additionally, one or more water swellable yarns  40  are also applied longitudinally over each layer of conductor elements  12 . The outermost layer of conductor elements  12  of conductor  11  may not have a water swellable yarn  40  as it is directly convered by the previously described semi-conductor layer  20 . 
     Thus, yarns  40  are applied both longitudinally and helically around each conductor layer of conductor  11 . 
     In one embodiment of the present invention, water swellable yarn  40  has an average diameter (measured in a hypothetical circular disposition of the yarn) in the range of 0.08″ to 0.102″. As noted above, water swellable yarn  40  is applied both helically and longitudinally over each conductor element layer. This arrangement provides a distinct advantage when attempting to replace the “standblock” tar used in the prior art, as the combination of helically wound water swellable yarns  40  as well as longitudinally arranged yarns  40  form a robust waterswellable layer that provide not only circumferential water ingress protection but also is effective at filling the interstices between one conductor element layer and the next conductor element layers, particularly in the open areas between the cylindrically shaped conductor elements. 
     By using the water swellable yarn  40  arrangement as described above, the overall cable  10  diameter may be reduced, relative to a cable employing the standblock technique, making the cable less expensive to produce. Additionally, the equipment necessary for producing the present design is also less costly than the equipment necessary to apply the strandblock compound. Moreover, in addition to cost savings, the water swellable yarns  40  are more robust than the strandblock compound and will return to a rounded (better filling) shape, whereas the strandblcok filler may flatten out under pressure making less effective. 
     Yet another advantage of the present arrangement, is that water swellbale yarns  40  are more environmentally friendly because they do not require the cleaning solvents needed to maintain a strandblock arrangement. Likewise, the recovery process (i.e. re-use of scrap) of conductors is made significantly easier. 
     In one arrangement of the present invention, regarding the application of yarns  40 , the helically applied portion of water swellable yarns  40  are applied at a rate of substantially 1″ to 5″ per revolution around a given layer. Such an arrangement gives good water ingress protection while not adversely affecting the conductor element conductivity, such as between the conductor elements in different layers. 
     In one embodiment of the present invention, as shown in  FIG. 4 , a yarn winding device  52  is utilized in a line production unit  50  to generate core  11  as described above. As shown in  FIG. 4 , yarn winding device  52  is installed in line production assembly  50  in such a manner and at a location for applying water swellable yarn  40  over a first conductor layer  14 , before the application/winding of second layer  16 . It is understood that additional winding devices may be employed for additional layers, such as an additional winding device  52  at a point on production assembly line  50  after the application/stranding of second conductor layer  16  and third conductor layer  18 . 
     In one arrangement, the longitudinally applied water swellable yarns  40  may be pulled through line production assembly  10  with their respective conductor elements. For the helically wound yarns  40 , winding device  52  operates a rotation speed of approximately 500 revolutions per minute, which operating on line speed of substantially 12 meters per minute to 64 meters per minute, results in the helically winding rate of yarns  40 , to fall in the range of 1″ to 5″ per revolution on their respective conductor layer. 
     While only certain features of the invention have been illustrated and described herein, many modifications, substitutions, changes or equivalents will now occur to those skilled in the art. It is therefore, to be understood that this application is intended to cover all such modifications and changes that fall within the true spirit of the invention.