Question

Three identical single phase transformer are connected in $\Delta /\Delta$ to supply a balanced three phase load at unity power factor. Each transformer carries rated current at rated voltage. Each transformer has a core loss of 0.01 p.u and ohmic loss of 0.02 p.u. at rated voltage and rated current respectively. One transformer develops fault and got removed from the circuit. For the same load kVA, find the percentage increase in losses.

• A. 33.33
• B. 35.71
• C. 50.00
• D. 55.56

Answer 1

The correct option is D 55.56

$\text{Total = 3(0.01 + 0.02)}$
$\text{= 0.09 p.u}$

The current carried by each transformer in open delta is $\sqrt{3}$ p.u and core losses in each transformer remain unchanged.

$\text{Now, copper loss}=2(\sqrt{3{)}^2}\times 0.02=0.12$
$\text{Core losses = 2 (0.01) = 0.02}$
$\text{Total losses = 0.02 + 0.12 = 0.14}$

$\text{\% increase in losses}=\frac{0.14-0.09}{0.09}\times 100=55.56\%$