A. a) Same voltage ratio of transformer: If two transformers of different voltage ratio, are connected in parallel with same primary supply voltage, there will be a difference in secondary voltages. If they are connected to same bus, there will be a circulating current between secondaries and therefore between primaries too. As the internal impedance of transformer is small, a small voltage difference may cause sufficiently high circulating current causing unnecessary extra I2R loss. b) Same percentage impedance: impedance of transformers running in parallel are inversely proportional to their MVA rating c) Same polarity: Polarity of all transformers run in parallel should be same otherwise huge circulating current flows in the transformer but no load will be fed from these transformers. d) Same phase sequence: If they are not in same phase sequence, during the cycle, each pair of phases will be short circuited.