For the parallel operation of single phase transformers, it is necessary that they should have same polarity.
Necessary conditions required to be satisfied in order to ensure successful parallel operation of single phase transformers:
1. Only transformers having the same phase displacement between primary and secondary voltage can operate in parallel. The clock-hour which indicate the phase displacement is stamped on the transformer rating plate, e.g. Dyn11, Yd11, 2. Terminals with the same polarity on HV- and LV side shall be connected in parallel, 3. Transformers should have approximately the same voltage ratio, within +-0.05% 4. The short-circuit impedance voltage should be the same, within +-10%, 5. The power rating of the transformers should not deviate more than 1:3, 6. Tap changers should have tap position giving voltage ratios as close as possible. 7. The phase sequence must be followed strictly on HV and LV side connection.