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Transformer Tails Sizing

fiftyhertz
Hi All,


I am trying to nail down sizing the tails from the TX into a switchpanel.


I have reviewed the the information in GN1 and understand the methods of installing to get the best current sharing capabilities. 


The issue is getting reasonbly sized AWA singles.


Example: 1500KVA TX - FLC 2165A.


Using Table 4D3 (to run to 70C) I cannot even get Trefoil 3 sets of 4 x 630mm2 (809A) cables to work, due to a grouping factor of 0.54?


How is the dealt with practically?
  • 0
    fiftyhertz:

    . . . Using Table 4D3 (to run to 70C) I cannot even get Trefoil 3 sets of 4 x 630mm2 (809A) cables to work . . . 


    Your cables are XLPE? Why not use the 90 degree figures -I would expect the connections at each end to be suitably rated? 


  • 0
    Hmm the trench may need to be a bit wider than you thought, or indeed the example width of  450mm -  you did not say the magic word 'trench' to begin with....


    OK, look here   either derate the single core rating of your AWA or whatever by the amount in 4C6 relative to the one wire case, not the already de-rated trefoil case.


    Or look at the wide spaced trefoil on tray case,  (more on that here) and realise that with enough room for air flow  it can becomes more or less the same.

    another old thread on non-trefoil groups.


    In the limit you could have multiple separated trenches, but do not, as it will make length matching a sod.

    There are tricks involving backfill with specially graded sands and things to improve the dissipation, but see how close you are without that.


    M.
  • 0
    After further looking I’m not sure that can be applied to cables in trenches (sorry I wasn’t clear). It seems the grouping factors make it near impossible to get realistic figures..
  • 0
    Mike,


    Thanks for the reply thats great.


    I wasnt aware that you could use the 2 dia cable rule when in a trench (Table 4C6 makes no mention). Is table 4C6 assuming that all cable are grouped together with 50mm in between? 


    This makes life a lot easier if you can disregard the grouping factor!


    Thanks

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  • 0
    Normally the trefoil (or quadrafoil with neutral - is that a word ? you know what I mean I hope ) groups are kept a bundle diameter or two apart on the tray/ rack or whatever , so there is air flow between and no big extra group of groups  derating factor - have you over done the derating  by mistake ?

    (an old discussion thread may help)


    How many bundles did you have in parallel, and are you assuming aluminium core or copper core singles. Perhaps start at 3 or  4 bundles at 500- 700a per bundle and see how that looks, then try one size up or one down from there.  The size and number of the transformer terminals can be also be used as a clue to what the makers are expecting.  Each bundle needs all 3 phases and  a neutral nearby, NOT a bundle of phase 1 and another bundle of phase 2 etc.

    Come back if it makes no sense or does not seem to fit..

    M.

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