Question

A three phase star/delta transformer is used step down the voltage. If primary voltage and secondary current are 11 kV and 423 A respectively. Turns ratio of transformer is 5, then output kVA is

• A. 930.6
• B. 1100.2
• C. 840.1
• D. 537.3

Answer 1

The correct option is A 930.6

Primary is star connected and secondary is delta connected.

$(V_L{)}_{primary}=11000V$

$(V_{ph}{)}_{primary}=\frac{11000}{\sqrt{3}}V$

$\frac{(V_{ph}{)}_{sec}}{(V_{ph}{)}_{pri}}=\frac{1}{5}$

$\therefore \text{Turns ratio}={\left(\frac{\text{High voltage}}{\text{Low voltage}}\right)}_{phase}$

$\therefore$ $(V_{ph}{)}_{sec}=\frac{11000}{5\sqrt{3}}V$

$(V_{ph}{)}_{\Delta }=(V_L{)}_{\Delta }$

$\text{Output kVA =}\sqrt{3}{V}_L{I}_L$

$=\sqrt{3}\times \frac{11000}{5\sqrt{3}}\times 423=930.6kVA$