Abstract:
There is provided an arrangement of components for use in a power line communication system. The arrangement includes (a) an inductive coupler having a core with an aperture through which a coaxial power cable is routed, where the coaxial power cable has a center conductor and an outer conductor; and (b) a lead being routed through the aperture, where the lead connects the outer conductor to a termination.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   The present application is claiming priority to U.S. Provisional Application No. 60/429,172, filed Nov. 26, 2002. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to power line communications (PLC), and more particularly, to placement of an inductive coupler around a cable having a shield or sheath of neutral conductors. The present invention is particularly advantageous in a case where the cable is an underground power line. 
   2. Description of the Related Art 
   In a power line communication system, a data signal may be coupled onto and off of a power line via an inductive coupler, such as described in U.S. Pat. No. 6,452,482. Inductive couplers may be placed around an uninsulated phase conductor or an insulated phase conductor. However, in underground power distribution systems, there is often no physical access to, or space around, a section of a phase conductor or a center conductor in the vicinity of a cable termination. Even when such access exists, work rules may require de-energizing the cable prior to attachment of the coupler. This process is inconvenient and requires personnel at both ends of a cable segment, and may sometimes affect service to power customers. 
   Many underground cables are built with a solid coaxial shield, grounded at each end, the main purpose of which is to provide shielding. Other underground cables are built with a plurality of wires wound spirally around an insulated core, where the plurality of wires serve as a neutral conductor. This shield or neutral conductor sheath is terminated just short of the end of a center conductor. The length of the unsheathed center conductor can be very short, often too short to allow installation of an inductive coupler. 
   In contrast, a shielded or sheathed cable segment located slightly away from the cable termination is generally available for coupler attachment while the cable is energized. However, efficiency of inductive coupling is reduced by the cable&#39;s shield or sheath. This is due to signal current induced in the shield or sheath, of magnitude similar to that in the center conductor, but of opposite phase. Since an output of the inductive coupler is proportional to a phasor sum of current passing through an aperture of the coupler, signal currents in the center conductor and shield or sheath conductors will tend to cancel, greatly reducing amplitude of a coupled signal. 
   SUMMARY OF THE INVENTION 
   An embodiment of the present invention is a placement of an inductive coupler around a sheathed or shielded coaxial power cable. The placement includes routing a wire that terminates the sheath or shield as a third conductor through an aperture of the coupler. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a diagram of an arrangement of an inductive coupler around a coaxial power cable. 
       FIG. 2  is a schematic diagram of a circuit for the arrangement of FIG.  1 . 
       FIG. 3  is a diagram of an alternative arrangement of an inductive coupler around a coaxial power cable. 
   

   DESCRIPTION OF THE INVENTION 
   In a coaxial cable having a center conductor that carries a signal current, a concentric shield (or sheath) serves as a return circuit, carrying a current of similar magnitude but opposite phase to the signal current. When such a cable is passed through a core of a an inductive coupler, magnetic fluxes generated by the two currents tend to cancel, greatly reducing a ratio of current in a secondary winding of the coupler to current in the cable&#39;s center conductor. Thus, placing an inductive coupler around a shielded coaxial cable provides poor coupling to signal currents carried in the central conductor. 
   Consider a wire that connects the shield to its termination as a “drain” wire, completing a circuit for noise signals or power current to electrical ground. Routing the drain wire back through the coupler passes the current in the shield through the coupler core twice, once in the shield in one direct and once through the drain wire in the opposite direction, and essentially cancels the effect of the shield current. 
     FIG. 1  is a diagram of an arrangement of an inductive coupler around a coaxial power cable. It shows an inductive coupler  100  having a magnetic core  105  having an aperture  110  and a secondary winding  115 , connected to a communications device  120 . A coaxial cable  125 , which may be a power cable, has a center conductor  130 , a core insulation  135 , and a shield (or neutral conductor)  140 . A lead, i.e., a drain wire  145 , connects shield  140  to ground  146 . In the embodiment shown in  FIG. 1 , cable  125  passes through aperture  110  from left to right, and drain wire  145  passes through inductive coupler  100  from right to left. 
   Ideally, a magnitude of signal current I 1  in center conductor  130  is equal to a magnitude of signal current I 2  in shield  140 , which is, in turn, equal to a magnitude of signal current I 3  in drain wire  145 . The net magnetomotive force in inductive coupler  100  due to coaxial cable  125  and drain wire  145  is I 1  minus I 2  plus I 3 , which equals I 1 . This has the effect of electrically “peeling back” shield  140  and providing coupling between the signal current in the center conductor, i.e., I 1 , and signal current in secondary winding  115 , as well as to communications device  120 . 
   Should center conductor  130  be energized at kilovolt potentials, then a high voltage termination device with a stress cone should be placed at each terminus of coaxial cable  125 .  FIG. 1  does not show such a termination, as the termination does not affect the operation or placement of inductive coupler  100 . 
     FIG. 2  is a schematic diagram of the arrangement of FIG.  1 . Again, the sum of currents I 1 , I 2  and I 3  through coaxial cable  125  and drain wire  145  is equal to current I 1  in central conductor  130 . 
   Should drain wire  145  be passed multiple times through aperture  110 , with current I 3  flowing in the direction indicated in  FIG. 1 , then the corresponding sum of currents, including multiples of I 3 , would yield a signal current in the secondary winding  115  proportional to I 1 . 
     FIG. 3  is a diagram identical to  FIG. 1  except that I 3  in a lead, i.e., a drain wire  300 , passes through aperture  110  in the same direction as I 2 . With the magnitudes of I 1 , I 2 , and I 3  identical to each other, a net magnetomotive force in inductive coupler  100  for the arrangement of coaxial cable  125  and drain wire  300  shown in  FIG. 3  is I 1  minus I 2  minus I 3 , which equals minus I 1 . Since inverted phase does not affect a data signal, this arrangement again “peels back” shield  140  to providing coupling between the signal current in the center conductor, i.e., I 1 , and signal current in secondary winding  115 , as well as to communications device  120 . By extension, winding drain wire  300  around core  105  more than one time also recovers signal current proportional to I 1  or multiples thereof. 
   The present invention also contemplates coupling a signal to a coaxial cable that is not a power cable. Also, central conductor  130  may be replaced by a plurality of conductors surrounded by any outer shield. 
   It should be understood that various alternatives and modifications of the present invention could be devised by those skilled in the art. Nevertheless, the present invention is intended to embrace all such alternatives, modifications and variances that fall within the scope of the appended claims.