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For the parallel operation of single phase transformers it is necessary that they should have?

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For the parallel operation of single phase transformers, it is necessary that they should have same polarity.

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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.

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