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Current imbalance in splitting parallel circuits

Accel
Hello Gents,


I am currently tasked in helping to install cables connecting the low side of a 2.5MVA transformer to an LV switchboard. Cable size and amount is 4 sets of trefoil + neutral 630mm single core SWA(total 16 630mm singles). They were split into 2 sets per group W shape from the LV connection before meeting at the high level containment. One group is 3m longer. I know it might lead to current imbalance because parallel circuits are far from each other but how bad is it? Distance between the TX and LV switchboard is rougly 15 meters.
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    If you mean 18m and 21m length of trefoil are in parallel, then currents can be expected to  split in the ratio of 21/18 assuming the termination impedance is small compared to the bulk resistances of the wires i.e. the lugging at the ends is done correctly.. Any one trefoil should be in balance with itself, that is to say the LLL(N) currents in any one close bundle will sum to near zero, but the currents that are summing to zero  may not be the same as another trefoil/quadfoil that is not the same length.

     

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    How about per phase currents coming out from the switchboard assuming all my loads will be 3 phase? I assume that they are also balance as well since the incomer is a trefoil/quadfoil config
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    hoping I have understood what you are asking yes,  if you have no neutral. Each trefoil bundle L1/L2/L3 will have zero sum that is the 3 phases will be in balance.

    However, the trefoils will not share equally, as they are not the same length - that is to say the  L1 current in a short trefoil will be higher than the L1 current in  a longer one - after all the voltage across the ends is fixed, as they are connected to each other at both ends, but the two L1 conductors are not the same length.
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    My next question would be a little off topic. It is about the heat dissipition from the trefoils. I^2r says the longer cables would dissipate more heat. Am I just worrying too much for the 3meter difference?
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    Well for starters V2 * r is the one you want - the current is not equal for the varying lengths in parallal, but the voltages between the ends are, so the shorter ones will run hotter.

    But given you know how the current is splitting you can calculate this,

    what are the two lengths, and how are they trefoils routed, in terms of airflow, tray/basket  etc?


    As a bit of rough and ready, you have 2.5MVA, and if we assume that is 833 kVA per phase for equal loading, so 3.3ka per phase. If it was balanced,  then 3300/4 =  833A per core.


    But it is not right. If the lengths are 15 and 18m, we expect a share in the ratio of 15/18 or 0.83:1 or by the miracle of square roots, 0.91 to  1.09 . Between friends the current in the long one will be ~10% lower than equal share, and the current in the short one will be ~10% over.


    then the long  ones one will carry 833-83= 750A and the short ones more like 833+83 = 916A

    So how does this stack with the rating for 630mmsq cable in your layout ?  On tray or basket as trefoil/quadfoil it would be about 1000A per core, so perfectly OK so long as the trefoils themselves are kept spaced to allow air to pass between. less good if restricted.


    PS I'm hoping SWA was a typo, and you mean AWA, or you may be onto a loser when you come to buy it. Are you glanding the armour at both ends ? There will be armour current to consider too.


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