A three phase A-Y connected transformer with 50 MVA, 33:11 kV is protected with a differential relay. The CT ratios given as 600: 3A (primary side) and 2500: 3 A (secondary side), determine the relay current setting for faults drawing up to 300% of rated transformer current. a) 4.09 A b) 3.24 A c) 3.94 A d) 5.68 A

The nameplate on a 25-MVA, 60-Hz single-phase transformer indicates that it has a voltage rating of 8.0-kV:78-kV. A short-circuit test from the high-voltage side (low-voltage winding shortcircuited) gives readings of 4.53 kV, 321 A, and 77.5 kW. An open-circuit test is conducted fromthe low-voltage side and the corresponding instrument readings are 8.0 kV, 39.6 A, and 86.2 kW. (a) Calculate the equivalent series impedance of the transformer as referred to the high-voltageterminals.(b) Calculate the equivalent series impedance of the transformer as referred to the low-voltageterminals.(c) Making appropriate approximations, draw a T equivalent circuit for the transformer.(d) Determine the efficiency of the transformer if it is supplying rated load (unity power factor)at rated voltage at its low-voltage terminal