A 400 kVA transformer has a primary winding resistance of 0.5 ohm and a secondary winding resistance of 0.001 ohm. The iron loss is 2.5 Kw and the primary and secondary voltages are 5 kV and 320 V respectively. If the power factor of the load is 0.85, determine the efficiency of the transformer (i) on full load and (ii) on half load.
Rated output = 400 kVA = 400x103 kVA
Full load secondary current, I2 = Rated output/V2 = 1250 A
Total resistance referred to secondary, re2 = r2+r1(V2/V1)2 = 0.033 ohm Full load copper loss, Pc = I22 re2 = 51.5625 Kw
Iron loss, Pi = 2.5 x103 watts
(i) Transformer efficiency at full load and 0.85 pf
(ii) Transformer efficiency at half load and 0.85 pf
2. Find all day efficiency of a transformer having maximum efficiency of 98% at 15 kVA at unity power factor. Compare its all day efficiencies for the following load cycles:
a. Full load of 20 kVA, 12 hours per day and no load rest of the day. b. Full load, 4 hours per day and 0.4 full load rest of the day.
Assume the load to operate on upf all day