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Parallel TX Tails - Neutral Arrangement

MW (aka Inrush - for those who remember me!)
So this is the new look forum... Impressive!


I have a question regarding AWA transformer tails. I'm looking at a situation where I need three parallel cables per phase.


My question is related to the neutral conductor arrangement. The neutrals need to be installed between the inter-group trefoils in order to maintain equal current sharing. Therefore, I can either have two neutral conductors of slightly larger CSA to give approx. the same CSA as the overall parallel phase conductors. Or, alternatively, three neutrals of the same CSA as the phase conductors split (oddly) between the two groups.


What convention is normally adopted in this situation?


Many thanks in advance.
  • 0
    Forgive me, I'm not completely following:  Why two groups not three, if you have three parallel conductors for each phase?
  • Former Community Member
    0 Former Community Member
    I would use 3no Quadrafoil Groups - you don't say what ampacity you want but for up to say 2.0MVA the quadrafoil would be easiest for you


    Google that and look for Ellis Patents cable cleats


    For each of the 3 groups try and contra rotate the phase conductors - each group of 4 contains L1, L2, L3 and N


    Regards


    OMS

  • 0
    this depends on the mechanics, but if the run is long and the desire is to avoid stray magnetic fields, and so keep the inductive volt drop down, it is common to have L1 L2 L3 in each bundle, and then however many such bundles you need can be separated, and neutrals between. This assumes neutral current is less than phase currents, but if for some reason it is not expected to be you can have 4 cables per bundle (then strictly not trefoil , more of a 4 leaf clover..).

    The key thing is the magnetic situation wants you to keep flow and return currents close, so the cancellation is good and the fields roll of with distance as fast as possible, but cable size and thermal considerations want spacing for cooling.
  • 0

    I would use 3no Quadrafoil Groups - you don't say what ampacity you want but for up to say 2.0MVA the quadrafoil would be easiest for you


    Google that and look for Ellis Patents cable cleats


    For each of the 3 groups try and contra rotate the phase conductors - each group of 4 contains L1, L2, L3 and N


    Regards


    OMS




    So there is no problem with having a neutral on the outside of the last group? The reason for the question is IET GN 6 implies the neutrals should only be installed between groups. Therefore, this would only allow two neutrals for three groups, if you follow?


  • Former Community Member
    0 Former Community Member
    If you use quadrafoil, then each neutral is with the group of 3 phases - so a block of 4 cables (2 x 2) - spaced by whatever you need to suit your grouping assessment (ideally bang tight together, and accept the current derating factors))


    You can put two neutrals between the groups if you insist on using trefoil - or you can go for two tiers of 6 cables in flat formation.


    Basically, whatever you choose for the physical lay up will have less or more implication on the current sharing and thus the cable sizing.


    I would try to avoid the outer neural scenario in favour of 2 together between group 1 and 2 and one neutral between group 2 and 3


    You could also examine why you want 3 parallel conductors rather than 2 or 4 for example - assuming 3 is the best fit, then again I would suggest quadrafoil arrangements


    Regards


    OMS
  • 0
    My first post was started before  OMS put his in, but if you want N the same size area as the phases then you may find
    https://www.powerandcables.com/product/product-category/quad-cleats/


    is helpful - basically 4 cables in each  bundle ( 3Ls and one N) behaves thermally very much as 3 for nearly all 3 phase loads but those with the most appalling power factor - just because it is not one of the standard arrangements, does not mean you cannot do it, nor that it needs any special consideration or derating.

    Rotating the bundles so N is on top in one, bottom right in another and bottom left in another is a bit of a refinement - it should give further far field cancellation, but it is a very marginal installation  ( or one with a small signal magnetic instrument test lab alongside perhaps !!)  where that matters, and then there are probably better measures.

    https://www.ellispatents.co.uk/wp-content/uploads/2016/08/Ellis-Black-Book-1st-Edition-1.pdf

    meas also makes the same assertions about rating on P16
  • 0
    Thanks all (especially OMS and Mike). As the quad layout (for three parallel phase conductors) is not mentioned in GN6 any chance you can point me in the direction of any reference/technical information covering this arrangement?


    Thanks,
  • Former Community Member
    0 Former Community Member
    I've used these quad cleats https://www.etscablecomponents.com/cable-cleats-cable-ties/quad-cable-cleats-4way/vulcan-stainless-steel-quadrofoil-cable-cleats-vrq.html 


    On a recent 2MVA transformer project, I used 4 sets of parallel AWAs for the LV tails.

    Reassuring to read that OMS is an advocate of this method.


    Next question, to single point or solid bond the armouring?


    Regards


    Parsley
  • 0
    Thanks, planning to solid bond the armour.
  • 0
    That black book reference, page 16, is as close to maker's instructions as you will get. It seems ERA labs did some tests for Ellis to generate some rating advice.

    A report produced by ERA on behalf of Ellis delivered the following guidelines:Current ratings, given in BS7671, for cables in touching trefoil formation are (also) appropriate for cables in quad bundles. Derating factors, given in BS7671,for cables in touching trefoil formation are (also) appropriate for cables in quad bundles. Voltage drops for circuits in quad-formation should be calculated using the values tabulated in BS7671 for cables in flat touching formation When considering multiple cables per phase, the advice given in Guidance Note No.6 for trefoil groups is applicable to quad bundles. The induced voltage in the neutral conductor of a quad group is minimal and can be ignored.





    So basically by selecting the relevant bits from the tables that are there for trefoil and flat touching , you can create a new column, for quads, and the makers of the cable clamps back you up on it.


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