Answer / dinesh

it is the percentage of voltage variation... suppose u take
-15 to +5 means,

From 100% of the transformer voltage u can increase 5%(105%) or decrease 15% (85%).... + defines increase and - defines decrease...

This is done by varying the number of turns by the OLTC...

and the next question, suppose u have 400 turns at 100% then at +5% it is 420 turns and -15% means 340 turns.....

Answer / k.prakashchandra

That is correct.As the tapping is done on primary side, with
+5% tap you are decreasing secondary voltage by 5% or you
use that tap when the primary voltage increases by 5% to
maintain the secondary voltage steady.Then N1/N2 = 105/100

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