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Question

A 100 MVA, 220 kV/110 kV ΔΔ,3ϕ 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 __________%.
  1. 3.7

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Solution

The correct option is A 3.7
Base current in secondary side,
I2(base)=100×1063×110×103=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=80×1063×110×103=420A

(Is)p.u.=420525cos1(0.8)

=0.836.87 p.u.

Per unit no load voltage of the transformer is

ENL=Vs+Iz
=10+(0.836.87)(0.02+j0.05)

=1.0371.24 p.u.

According to definition of V.R =No load voltage - Full load voltageFull load voltage×100%

V.R.=1.03711×100%=3.7%

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