Correct Answer - Option 2 : 400 kVA
Concept:
When unequal kVA ratings of transformers are operating in parallel, the transformer with the lowest kVA rating is loaded fully then the total allowable kVA loading is calculated.
Consider transformer 1 rating S1 kVA with leakage impedance z1, transformer 2 rating S2 kVA with leakage impedance z2 (S1 > S2)
Then total loading applied without overloading any transformer is
\(S = {S_2}\left( {\frac{{{z_1} + {z_2}}}{{{z_1}}}} \right)\)
Calculation:
Given S1 = 600 kVA, S2 = 300 kVA, z1 = 0.15 pu, z2 = 0.05 pu
Maximum permissible kVA loading is
\(S = 300\left( {\frac{{0.15 + 0.05}}{{0.15}}} \right)\)
S = 400 kVA
