A$500-$kVA,$3-$phase,$50-$Hz $$transformer has a voltage ratio $($line $$voltages$)$of

$3311-$kV $$and is $$deltastar connected. The resistances per phase are : high voltage $35,$low voltage

$0\mathrm{.}876$and the iron loss is$3050$W$.$Calculate the value of $$efficiency at $$full$-$load and one$-$half of $$full$-$

load respectively $($a$)$at $$unity p$.$f$.$and $($b$)0\mathrm{.}8$p$.$f$.$

$($Electrical $$Machinery$,$ Madras Univ. $1985$