Abstract:
The present invention relates to a system for data transmission over an electricity distribution network. The system comprises an electrical apparatus having a shielded electrical cable connected thereto. The cable has a core surrounded by a metallic sheath and armour surrounding the metallic sheath. The armour is disengaged from around the metallic sheath at a position where the cable enters the apparatus and is supported relative to the apparatus at a position spaced from that position such that a portion of the metallic sheath is exposed.

Description:
[0001]    This application is entitled to the benefit of, and incorporates by reference essential subject matter disclosed in PCT Application No. PCT/GB2006/001549 filed on Apr. 28, 2006 and Great Britain Application No. 0508673.1 filed on Apr. 28, 2005. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Technical Field 
         [0003]    The present invention relates to data transmission, and in particular to data transmission over an electricity distribution network. 
         [0004]    2. Background Information 
         [0005]    Electricity distribution networks are extensively used to transmit electricity from electricity generating stations, such as power stations, to domestic and industrial consumers. It has been realised that the electricity distribution network provides an infrastructure connecting thousands of consumers which can be used to transmit more than just electricity. For example, the distribution network can be used for the transmission of data. 
         [0006]    Existing systems in which the electricity distribution network is used to transmit data are called ‘Power Line Communications’ (PLC) and ‘Broadband over Power Lines’ (BPL). In these systems a high frequency data signal is added, or inserted, onto an electrical power cable using an inductive coil or capacitor arranged around or near the cable. A corresponding inductive coil or capacitor is then used to remove, or extract, the data signal at the other end. These systems allow a data signal to be transmitted between two parties over an existing power cable without the need for a separate data communication infrastructure. Furthermore, the transmission of data does not interfere with the primary use of the cable, that is to transmit electricity. 
         [0007]    There are however a number of problems associated with implementing PLC on existing electricity distribution networks, particularly where the infrastructure is old or where the parts are inaccessible. 
         [0008]    In many parts of a distribution network (such as high voltage overhead power cables) the electrical cables are completely exposed or are coated with a thin protective layer or sheath. In these situations, access to the cables is largely unhindered and as a result data can be injected onto the cable easily using a small inductive coil. However, there are many parts of a network in which the cables are shielded for insulation and/or protection purposes such as shielded cables used in underground applications. Shielded cables typically comprise a core which carries the power and which is surrounded by a metallic sheath and a series of mild steel windings or tape (referred to as armour) running around and along the length of the cable. In situations where cables are shielded, large inductive coils or capacitors are needed to inject the data signal through the cable shield and to the conductive cores of the cable. It is of course possible to use large inductive coils to inject a data signal through the shield but installation and mounting of these coils on existing cables has made their use commercially unviable. Furthermore, the quality of the data signal which can be injected onto or extracted from a shielded cable can be diminished due to the cable shield and armour configuration. 
         [0009]    The problems of using armoured cables in PLC applications is a limiting factor wherever access to the cable core is required to inject or extract a data signal. For example, PLC networks often require repeaters to ‘boost’ the signal strength where the signal is required to travel over extended distances. Again, armoured cables mean that access to cable cores to inject the signal requires a substantial increase in the cost of PLC repeaters. 
         [0010]    The problems discussed above also arise when an electrical cable is connected to apparatus such as transformers, termination boxes and the like. In these situations the cables are shielded (using mild steel armour) for safety purposes. In a conventional connection of a shielded electrical cable to a termination box, the shielding of the cable is mechanically coupled to the termination box by a mechanical clamp. The clamp grips the steel armour to mechanically support the cable against the termination box housing and prevents the cable pulling away from the termination, box, also electrically bonding the armours to the potential of the metallic shield. The cores of the cable, which are surrounded by the armour and insulation, feed directly into the termination box thereby providing the required electrical path. 
         [0011]    Because termination boxes are often sealed and inaccessible the only way to implement PLC in these situations is to install an inductive coil around the shielded cable before the cable enters the termination box. 
         [0012]    In general, the use of shielded cables with inaccessible cable boxes within an electricity distribution network restricts access to the cable cores and makes the implementation of PLC to these parts of the network difficult and expensive. One option to overcome the problems of implementing PLC with shielded cables is to replace each, of the transformers or termination boxes with an equivalent which provides access to the cable core. However, it will be appreciated that the cost of replacing transformers and termination boxes in an electricity distribution network is both disruptive to the electricity supply and extremely expensive. 
         [0013]    There is therefore a need for apparatus which allows PLC to be implemented on an existing distribution network in which shielded cables are used without the need for substantial modifications to the equipment. Specifically, there is a need for apparatus which removes the requirement for large and expensive network modifications and which does not compromise the electrical safety or the mechanical integrity of the connection. 
       DISCLOSURE OF THE INVENTION 
       [0014]    From a first aspect, therefore, the invention provides an electrical system comprising an electrical apparatus and a shielded electrical cable connected to said apparatus, said cable having a core, a metallic sheath around the core and a surrounding armour; wherein said armour is disengaged from around said metallic sheath at a location where the sheath enters the apparatus, and is supported relative to said apparatus at a position spaced from said location, such, that a portion, of the sheath is exposed between said location and the armour. 
         [0015]    With such an arrangement, a portion of the sheath is exposed where the cable enters the apparatus (for example a transformer, switchgear, or a termination box) thereby making the sheath accessible for PLC equipment and components. 
         [0016]    Preferably the armour is connected to the apparatus by a coupling which maintains the desired positional relationship between the apparatus and the armour and which may also, depending on the orientation of the apparatus and cable, support the armour on the apparatus. 
         [0017]    Preferably the coupling comprises a first end arranged to receive the armour and a second end spaced from, but connected to, said first end, which second end is arranged to be mounted to a part of the apparatus. 
         [0018]    The invention also extends to such a coupling and from a second aspect provides a coupling for connecting a shielded electrical cable having a core and surrounding armour to an electrical apparatus, said coupling comprising a first end arranged to receive the armour of the cable and a second end spaced from, but connected to, said first end, which second end is arranged to be mounted to a part of the apparatus. 
         [0019]    The second end is connectable to the apparatus in any suitable manner, but preferably it is configured to engage circumferentially around a part of the apparatus, for example the gland of a termination box. 
         [0020]    The connection may be fixed, i.e. permanent, or may be releasable such that the coupling can be connected and disconnected. 
         [0021]    Preferably, the second end is arranged to clamp circumferentially around a part of the apparatus. It may, therefore, be formed of one or more parts which together extend around the circumference of the part. For example, it may comprise a collar arranged to surround the part, with means to allow the collar to be tightened around the part. 
         [0022]    Preferably the collar is formed of two or more components arranged to connect together and to tighten around the part. Each part may comprise flanges to receive tightening means such as tightening bolts. 
         [0023]    The first end of the coupling is preferably arranged to clamp the cable armour between two surfaces. The surfaces may be any shape between which the armour can be interposed. Preferably, the clamping surfaces are substantially parallel surfaces and are preferably arranged at an angle to the axis of the cable, most preferably perpendicular to the axis of the cable. 
         [0024]    The first end of the coupling is preferably formed of two plates or discs having a central bore sufficiently large to allow the core of the cable to pass therethrough (i.e. a bore diameter at least as large as the diameter of the core of the cable). The plates may engage against one another using any suitable mechanical connection or joint which may be permanent or releasable. 
         [0025]    The plates may each be formed of two or more components arranged to connect together to encircle the cable core and to cooperate so as to mechanically secure the cable armour. Preferably the joints between the respective plate components are offset circumferentially from each other. 
         [0026]    The coupling preferably comprises one or more connection members extending between the first and second ends of the coupling. The connection member (s) may be a rod or rods, for example. Preferably, three circumferentially spaced connection rods are provided. 
         [0027]    The connection member(s) may be provided with means such that the first and second ends can be moved relative to one another along the axis of the cable. In a preferred embodiment a turnbuckle arrangement may be used. This allows a cable to be pulled towards or pushed away from a termination box, in use. 
         [0028]    Preferably, electrical insulation is provided between the first and second ends of the coupling. For example, in the turnbuckle arrangement described above, the turnbuckle body may be formed of an insulating material, for example nylon. 
         [0029]    The first end of the coupling preferably provides a surface surrounding an exposed length of cable sheath for receiving an inductive component. Preferably, it provides a surface perpendicular to the axis of the cable to receive an inductive component. The inductive component may be any suitable inductor but is preferably an inductive coil. 
         [0030]    The invention also extends to apparatus including an inductor, and from a yet further aspect, therefore, provides an electrical system comprising an electrical apparatus and a shielded electrical cable mounted to said apparatus, said cable having a core surrounded by a metallic sheath and by armour; wherein said armour is disengaged from around said sheath at a location where the cable enters the apparatus, such that a portion of the sheath is exposed between said location and the armour, and inductance means arranged around said exposed portion of said sheath. 
         [0031]    As discussed above, PLC applications also require the use of repeaters to ‘boost’ the data signal carried on the shielded cable such that the signal can be transmitted over extended distances. In PLC repeaters a signal is extracted from the cable, amplified and then injected back onto the cable. 
         [0032]    Viewed from a further aspect, an invention disclosed herein provides a signal repeater on an electrical cable comprising a current carrying core, a metallic sheath arranged around the core and armour around the metallic sheath, wherein the armour is removed over a length of cable to expose a section of said metallic sheath; a central part of said exposed section of metallic sheath is removed to expose a length of the core and to leave two lengths of sheath projecting from said armour; respective inductors are arranged on said projecting lengths of sheath; and respective earth paths run from respective end regions of said projecting metallic sheath back under said inductors to an external earth path extending between both ends of the repeater, said earth paths being insulated from the respective metallic sheath lengths between the end region of the sheath lengths and their connection to the external earth path. 
         [0033]    The break in the metallic sheath breaks the electrical continuity of the sheath along the length of the cable, as is required, because it is thought that the signal may in fact be assisted by being carried on the sheath and the cable core. 
         [0034]    Preferably the inductors are coaxial with the cable. 
         [0035]    The earth path can be provided by any suitable insulated conductor, but it is preferably provided by an insulated conductive braid connecting the end region of the sheath and passing under the respective inductors, i.e. between the sheath and the inner surface of the inductor. 
         [0036]    The external earth path is preferably provided by one or more external conductive members. Preferably, however, it is provided by a conductive cage which surrounds the cable. This provides not only the necessary earth path but also protects the underlying components from accidental damage by cutting, particularly when the repeater is buried underground, as is likely to be the case. 
         [0037]    Preferably the inductors are received closely within the cage for support. 
         [0038]    The first and second inductors are preferably connected to a data processor arranged to receive an input signal from the first inductor, to amplify the signal and to output the amplified signal to the second inductor. 
         [0039]    Thus, with this arrangement the core of the cable is accessible in the repeater thereby allowing a data signal to be easily extracted and injected onto the cable. 
         [0040]    Furthermore, the earth path required for the operation of the inductors is maintained. 
         [0041]    Viewed from a still further broad aspect, there is provided an electrical conductor having a current carrying core, a metallic sheath arranged around the core and armour around the metallic sheath, wherein the armour is removed over a length of conductor to expose a section of said metallic sheath and an inductor is arranged around the exposed section of metallic sheath. 
         [0042]    The electrical cables/conductors disclosed herein may be formed with a single core or, alternatively, may be formed of a plurality of cores. Each core may be formed of any suitable conductive material. 
         [0043]    The core or cores may be surrounded by an insulating layer which may be of any suitable material. The core (or cores) may for example be surrounded by a paper insulating layer. In this arrangement when part of the metallic sheath is removed, a length of insulating material surrounding the core is exposed. The cores may be individually surrounded by an insulating layer. 
         [0044]    The metallic sheath may be formed of any suitable material. For example, the metallic sheath may be formed of lead. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0045]    Embodiments of the invention will now be described, by way of example only, and with reference to the accompanying drawings in which: 
           [0046]      FIG. 1  shows a coupling arrangement according to the present invention; 
           [0047]      FIG. 2  shows the gland clamp of the coupling of  FIG. 1 ; 
           [0048]      FIG. 3  shows the construction of a conventional  3  phase 1 IkV electricity cable; 
           [0049]      FIGS. 4A and 4B  illustrate the armour clamp of the coupling of  FIG. 1 ; 
           [0050]      FIG. 5  shows the coupling arrangement of  FIG. 1  with an inductive coil mounted therein; 
           [0051]      FIG. 6  shows, schematically, a repeater for use with a power line communication system. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0052]      FIG. 1  shows a coupling according to a preferred embodiment of the present invention. The coupling  1  is arranged to connect the gland  2  of an electrical termination box (not shown) to a three phase 1 IkV shielded electrical cable  3 . 
         [0053]    The coupling  1  comprises a gland clamp  4 , which clamps circumferentially around the gland  2 , and an armour clamp  5  which clamps the armour of the cable  3 . The gland clamp and the armour clamp are connected by a plurality of connecting rods  6 ,  7 ,  8 . 
         [0054]    With reference to  FIG. 2 , the gland clamp  2  is formed of 2 semicircular elements  21 ,  22 . Electrical termination box glands have standard diameters and the gland clamp diameter is selected according to the termination box to which the gland will be selected. The gland clamp elements  21 ,  22  are provided with tightening flanges  23 ,  24  which allow the gland clamp elements  21 ,  22  to be arranged around the gland  2  and then bolted together (using bolts not shown) to circumferentially clamp against the gland  2 . 
         [0055]    The gland clamp elements  21 ,  22  are further provided with flanges  25 ,  26 ,  27  spaced equally around the circumference of the gland clamp  2 . These receive the upper ends of the connecting rods  6 ,  7 ,  8 . The connecting rods are, in this embodiment, formed of M10 threaded bar and connect the gland clamp connection flanges  25 ,  26 ,  27  with corresponding connection flanges  49 ,  50 ,  51  of the armour clamp  5 , as show in  FIG. 4B . 
         [0056]    The connecting rods  6 ,  7 ,  8  are variable in length, each having a turnbuckle  9 . The turnbuckle  9  is formed of a cylindrical length of nylon having two threaded bores in either end and which bores are arranged to receive the M10 threads of the connection members  6 ,  1 ,  8 . The connection rods are each formed of a first portion  7   a  which extends between the turnbuckle  9  and the gland clamp  4  and a second portion  7   b  which extends between the turnbuckle  9  and the armour clamp  5 . As a turnbuckle  9  is provided on each of the connecting members  6 ,  7 ,  8  (only connection element  7  being visible in  FIG. 1 ) the gland clamp  4  is electrically insulated from the armour clamp  5 . 
         [0057]    The armour clamp  5  is mechanically connected to the armour of the 3 phase shielded cable  3 .  FIG. 3  illustrates the components of a shielded cable. 
         [0058]    A shielded cable  31  as shown in  FIG. 3  generally comprises three separate aluminium cores  32 ,  33 ,  34  each surrounded by a paper insulating layer (not shown). The 3 cores  32 ,  33 ,  34  are surrounded by a lead sheath  35  which acts to protect and support the individual cores. The cable is surrounded by a series of mild steel wires helically wound around and along the length of the cable to form the cable armour  36 . The armour  36  acts to shield and protect the cable. The lead sheath  35  and armour  36  are in electrical contact so as to be at the same electrical potential. 
         [0059]      FIG. 1  shows the arrangement of the armour  36  and its connection to the armour clamp  5 . In assembling the coupling, the armour  36  of the cable  3  is clipped such that the armour  36  can be pulled away from the lead sheath  35 . The armour  36  is then pulled away from the lead sheath  35  and bent back so as to lie perpendicular to the axis of the cable  3  as shown in  FIG. 1 . The armour  36  is then positioned between the armour clamp elements as shown in  FIGS. 4A and 4B . 
         [0060]      FIG. 4A  show the armour clamp  3  which, in the preferred embodiment, is assembled of 4 elements  41 ,  42 ,  43 ,  44 . Elements  41 ,  42 ,  43 ,  44  are assembled as shown in the figure in  4 A and bolted together using bolt holes  45 ,  46 ,  47 ,  48  as shown in  FIG. 4B  (in plan view). The armour clamp is also provided with connection flanges  49 ,  50 ,  51 , as described above. 
         [0061]    Once the bent back armour  36  is in position between the 4 armour clamp elements, the armour clamp  5  is tightened up using bolts (not shown) arranged in bolt holes  45 ,  46 ,  47 ,  48  to clamp the cable armour in the armour clamp. The armour  36 , and therefore the cable  3 , is thereby rigidly connected to the armour clamp  5  as shown in  FIG. 1 . 
         [0062]    Once the armour  36  has been connected to the armour clamp the connection rods  6 ,  7 ,  8  can be bolted to the connection flanges of the gland clamp  4  and armour clamp  5  as discussed above. The armour  36  and cable  3  are thereby mechanically supported by the gland  2  thorough the coupling  1 . 
         [0063]    The lead sheath  35  passes through the centre of the coupling and into the electrical termination box (of which the gland  2  is a part). As shown in  FIG. 1  the gland  2  is wiped, or soldered, onto the lead sheath  35  by a wipe  10  which ensures that the gland  2  and lead sheath  35  are at the same electrical potential. This arrangement ensures that the earth path connecting the gland  2  to the cable  3  runs through the centre of coupling via the lead sheath  35  and not via the connecting rods  6 ,  7 ,  8 . The current carrying core of the cable is connected inside the termination box. The wipe  10  will also provide some mechanical support for the sheath  35  and thus the cable  3 . 
         [0064]    The lead sheath  35  of the cable  3  is exposed between the gland clamp  4  and the armour clamp  5 , and as shown in  FIG. 5 , an inductive coil  51  is placed around the exposed length of the sheath. 
         [0065]    The arrangement shown in  FIG. 5  is generally the same as that shown in  FIG. 1  except that in this embodiment the connecting rods are formed of an electrically insulating material and the insulating elements  9  shown in  FIG. 1  are therefore not required. 
         [0066]    As shown in  FIG. 5 , an inductive coil  51  (only half of which is illustrated for clarity) is positioned on the upper horizontal surface  52  provided by the armour clamp  5 . The inductive coil is thereby positioned adjacent to and coaxial with the lead sheath  35 . The inductive coil is connected to known signal generating and/or decoding equipment (not shown) for generating the data signals which are to be injected and/or extracted to or from the cable  3 . In particular, signals may be injected or extracted using either Time Division Multiplexing (TDM) or Frequency Division Multiplexing (FDM). Both of these techniques are well known in the data transmission art and do not need to be discussed further here. Preferably, however, TDM is used. 
         [0067]    As mentioned above, the gland  2  and the lead sheath  35  of cable  3  are at the same electrical potential. The connecting rods  6 ,  7 ,  8  are electrically insulating, thereby ensuring that an earth path passes along the sheath  35  from the armour  36  through the centre of the inductive coil  51 . To ensure proper operation of the inductor  51 , a further earth path should be provided, e.g. by a copper bar  11  ( FIG. 1 ) which also passes along the surface of the sheath  35  back from the gland  2  through the inductor  51  to the armour. 
         [0068]      FIG. 6  shows, schematically, a PLC repeater suitable for use with shielded electrical cables of the type already described above with reference to  FIG. 3 . The cable would typically connect one electricity sub-station to another on a medium voltage (1 IkV) distribution network. A repeater as shown in  FIG. 6  would typically be placed at intervals along the length of the cable. 
         [0069]    The central core  62  of the cable  61  remains uninterrupted such that electricity continues to be transmitted along the cable when the repeater is installed. In 
         [0070]      FIG. 6 , the PLC signal propagates from left to right; i.e. the left of the cable can be considered to be the input and the right to be the output. 
         [0071]    In the repeater, the armour  63  of the cable is terminated at a certain point to expose the underlying lead sheath  64 ,  65 . The lead sheath  64 ,  65  is also cut away to expose a section of current carrying cores&#39; insulation  62 . The exposed lengths  64 ,  65  of lead sheath are provided with terminations  68  made from stress relieving tape arranged around the sheath termination point. Typically the tape would be a stress mastic tapes. 
         [0072]    As shown, first and second inductive coils  69 ,  70  are arranged over the exposed lead sheath lengths  64 ,  65  on the respective ends of the repeater. The first inductive coil  69  is arranged to extract a data signal and the second inductive coil  70  is arranged to inject a data signal. The first and second inductive coils are electrically connected to a known PLC repeater circuit (not shown) which receives and amplifies the extracted signal from inductive coil  69  and feeds the amplified output to the second inductive coil  70  for injection back onto the cable  61 . The data signal can thereby travel over extended distances using a series of repeaters disposed along the length of the cable. The data transmission, as in the earlier embodiment, preferably uses TDM or FDM techniques. 
         [0073]    Now, respective collars  71  are mounted at one end of the respective exposed sheath lengths  64 ,  65 . These collars  71  are connected electrically to the armour  63 , but are insulated from the underlying lead sheath sections  64 ,  65  by insulators  72 . The collars  71  mount, on their outer surfaces  73 , an earth screen cage  74 . This cage  74  is formed of a plurality of aluminium strips arranged around the cable or as a perforated sheet member. The cage  74  is at the same potential as the armour  63  as it is connected to the armour  63  through the collars  71 . It is, however, insulated from the underlying sheath sections  64 ,  65 . 
         [0074]    However, an earth path is provided from the sheath sections  64 ,  65  to the cage  74  through respective insulated braids or cables  75  which are connected at one end; e.g. by solder  76  to the part of the sheath lengths  64 ,  65  beyond the coils  69 ,  70 . The braids  75  pass under the coils  69 ,  70  and are affixed at their other ends to the collars  71 , through which they are electrically connected to the cage  74 . This arrangement thus provides a PLC repeater having an earth path between the input and output of the repeater which passes through each of the inductive coils, which is required for satisfactory extraction and injection of the signal. 
         [0075]    The cage  74  may be encapsulated within a cover (not shown) which will prevent ingress of solids and moisture into the repeater.