Which of the following is a requirement for the operation of transformers in parallel?

For transformers to operate in parallel effectively, it is essential that they have the same phase sequence. This ensures that the voltages produced by each transformer are in phase with one another, allowing for proper load sharing and preventing potential issues such as circulating currents or unbalanced loading. If transformers operate with different phase sequences, they could end up causing serious operational problems, including damage and inefficiency.

Having the same turns ratio is also crucial, but it ensures that the voltage outputs are compatible rather than directly correlating to phase alignment, making phase sequence the more critical requirement for effective parallel operation. While load current ratings and insulation type can affect overall performance and safety, they do not directly impact the basic requirement for synchronizing the operation of transformers in parallel concerning their electrical characteristics.