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11 KV cables.

broadgage
Until fairly recently, underground 11 KV cables were invariably 3 core, paper insulated, lead covered, with an earthed steel wire armouring. Then came the more modern plastic insulated cables, with earthed armour and a red plastic over sheath to distinguish them from LV cables.


However the modern trend seems to be a bundle a 3 single core cables, each with an aluminium core, plastic insulation, copper wire armouring and a red plastic sheath.

Why is this used ? It seems to me that 3 single core cables would be more costly to manufacture, and more labour intensive to joint or terminate, than one 3 core cable.

I fail to see the advantage, but presume that there must be some advantage, or why make the change ?
  • 0
    im not sure, but on our transformers, the bundled cable enters through the same gland, so I assume its a new type of gland to accommodate the three separate cables


    33kV was always 3 separate cables, round here at least
  • 0
    It is actually easier to make the single-core cables that have quite thick insulation usually of xlpe, the extrusion processes and test can easily follow one another in a straight line, whereas the paper insulated 3 cores were slow and complex to make. I am sure that the modern red ones cost less, and are probably much easier to joint or terminate. The red identification is a great advantage when faced with several cables in a trench! The insulation is also much better, the oil is no longer present to leak out and be replaced with water, so reliability is improved. They are also easier to handle on site, easier to bend around obstacles as the cables can move relative to one another.
  • 0
    Johno12345:

    the bundled cable enters through the same gland, so I assume its a new type of gland to accommodate the three separate cables

    The gland plates will be non ferrous to avoid the heating effect of circulating eddy currents or perhaps slots between the holes, preferably if indoor transformers. If the plates are painted, use a magnet to test if unsure.


    Jaymack
  • 0
    davezawadi (David Stone):

    They are also easier to handle on site, easier to bend around obstacles as the cables can move relative to one another.


    The cleating is superior too since the cable bursting strength is increased.


    jaymack 


  • 0
    Just out of curiosity, do DNOs care about phase order? I.e. as long as they are in the correct phase rotation sequence, do they care whether the correct of the three red cables coming into the substation is identified as 'red' phase?


    Or do they do a large-scale equivalent of a ring final circuit, where order would then be important?
  • 0
    yeah, i know about the eddy currents but on our install, the three cables are bunched together, heatshrinked together with the earth tape for the last 6 inches, which all then go through a metallic 'stuffing gland' into the transformer box. 


    thats a fairly modern outdoors transformer installation
  • 0
    DNOs tend where possible to run  11Kv cables in a ring formation, in order that any substation on that ring may receive power from either direction.

    Contrary to domestic ring final circuits, DNO practice is often to run with the ring "open" at one point, this point being selected so as to minimise losses. If a fault occurs, then the faulty section of the ring can be isolated by operation of the "ring main switches" in the substations, and supply restored before repairs are completed.
  • 0
    I think some of the 11kV singles cables use EPR insulation.  They first came a just singles (not pretwisted) and then changed to pretwisted,  I think they are single core for ease of manufacture.  It also helps bending, handling etc. They are much lighter than the old PICAS or lead sheathed variety.  The red fades badly after a few years in some soils so cant be relied on.  Phase is important on rings - there are standards for radials but no way to confirm in most cases.  The non twisted version needs decent trefoil cleats or bands.  There is no armour but there is a [copper] screen.  Care needs to be taken at higher fault levels as the rating of the screen can be exceeded and in some cases with high fault levels this type of cable simply isn't suitable.  IIRC these cables were first used in the mid 1980s in the UK.


  • 0
    broadgage:

    DNOs tend where possible to run  11Kv cables in a ring formation, in order that any substation on that ring may receive power from either direction.

    Contrary to domestic ring final circuits, DNO practice is often to run with the ring "open" at one point, this point being selected so as to minimise losses. If a fault occurs, then the faulty section of the ring can be isolated by operation of the "ring main switches" in the substations, and supply restored before repairs are completed.


    Hmm I had assumed that in normal operation the ring was closed and only opened at two isolators if one (or more than one adjacent) point at the substation required isolation.. 


  • Former Community Member
    0 Former Community Member
    Hmm I had assumed that in normal operation the ring was closed and only opened at two isolators if one (or more than one adjacent) point at the substation required isolation.. 


    Invariably they are operated as mid point open ring to limit fault current ebee.


    Regards


    BOD

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