Transformer circuits

Hi everyone hope you fine.
1.I'm studying the auto transformer and i have reached where it says apparent power by conduction and induction ..This the first time i see this question .I'm familiar with the power P=VI..If any one could explain this please?
see the two attached files one for the question and the other for the answer

2.the other question is for a three phase transformer .If each single transformer is of 20KVA ..if a three are tied together to form three phase transformer, what is the power that reaches the load if the three of the are configured in delta or wye ??

waiting for help:shock:

Imagine if the tap was right at the top of the transformer, all the power would be transferred by conduction. So how much power is transferred directly from the input to the output? Its 2000 X 500 = 1MVA. How much is transferred via the magnetic field? 2000 X 50 = 100KVA.
If it was a large step down transformer say, 220 to 22V, the the results would be the other way around with bulk of the power transferred by the magnetic field.
Frank

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Suppose each transformer was rated at 400 V @ 50 A. So connecting them Phase to neutral would give your 20 KVA. In star configuration, because the non phase end is connected via another winding to a voltage that has 400 V@ angle 120 degrees connected to it, You need more voltage to get the same output. Its √3 times as much, but with a suitable tap you will get the same power out. Without the extra tap its been derated by 1/√3, becaus it can't get the full voltage on its primary.
In Y formation, the winding has the same voltage, but there is a parallel current path, feeding current into the other windings so the transformer must be derated because of the excess √3 current.
frank