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Question

A 100 MVA, 220 kV/110 kV Δ−Δ,3−ϕ power transformer has a resistance of 0.02 per unit and a reactance of 0.05 per unit.The transformer supplies a load of 80 MVA at 0.8 pf lagging. The percentage voltage regulation of the transformer is __________%.3.7

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Solution

The correct option is A 3.7Base current in secondary side, I2(base)=100×106√3×110×103=524.86 A Since the transformer supplies a load of 80MVA at 0.8 pf lagging, so secondary line current of the transformer is Is=80×106√3×110×103=420A (Is)p.u.=420525∠cos−1(0.8) =0.8∠−36.87∘ p.u. Per unit no load voltage of the transformer is ENL=Vs+Iz =1∠0∘+(0.8∠−36.87∘)(0.02+j0.05) =1.037∠1.24∘ p.u. According to definition of V.R =No load voltage - Full load voltageFull load voltage×100% V.R.=1.037−11×100%=3.7%

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