Correct option is B
When operating transformers in parallel, several conditions must be met to ensure proper operation and avoid issues such as circulating currents, imbalanced load sharing, or potential damage to the transformers. These conditions include:
Same Polarity: Ensuring that the transformers have the same polarity is crucial. If the polarity is not the same, the voltages of the transformers could oppose each other, leading to circulating currents and possible damage.
Same Voltage Ratio: The primary and secondary voltage ratios must be the same. This ensures that the transformers share the load proportionately without circulating currents caused by different secondary voltages.
Same Phase Sequence and Phase Angle: The phase sequence and phase angle must match to ensure that the transformers work together correctly and do not produce phase shifts that could cause issues in the system.
Similar Impedance: The impedance of the transformers should be similar (preferably within ±10%). This ensures that the load is shared proportionately according to the kVA ratings of the transformers.
The same efficiency, kVA rating, or the same number of turns on the secondary side are not strict requirements for parallel operation, though having similar characteristics can improve load sharing and efficiency. However, the most critical factor among the given options for safe and effective parallel operation is:
(b) same polarity
