Question

A 100 MVA, 220 kV/110 kV $\Delta -\Delta ,3-\varphi$ 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 __________%.

• A. 3.7

Answer 1

The correct option is A 3.7
$\text{Base current in secondary side,}$
$I_{2_{\left(base\right)}}=\frac{100\times {10}^6}{\sqrt{3}\times 110\times {10}^3}=524.86A$

Since the transformer supplies a load of 80MVA at 0.8 pf lagging, so secondary line current of the transformer is

$I_s=\frac{80\times {10}^6}{\sqrt{3}\times 110\times {10}^3}=420A$

$(I_s{)}_{p.u.}=\frac{420}{525}\angle co{s}^{-1}(0.8)$

$=0.8\angle -{36.87}^{\circ }p.u.$

$\text{Per unit no load voltage of the transformer is}$

$E_{NL}=V_s+I_z$
$=1\angle 0^{\circ }+(0.8\angle -{36.87}^{\circ })(0.02+j0.05)$

$=1.037\angle {1.24}^{\circ }p.u.$

$\text{According to definition of V.R}=\frac{\text{No load voltage - Full load voltage}}{\text{Full load voltage}}\times 100\%$

$V.R.=\frac{1.037-1}{1}\times 100\%=3.7\%$