Which of the following is NOT a necessary requirement for transformers to operate in parallel?

For transformers to operate in parallel effectively, they must share certain characteristics to ensure proper load sharing and avoid circulating currents that can lead to operational issues. Among the listed requirements, having the same voltage ratio is indeed a critical aspect, but what is being asked is what is NOT necessary.

Having the same frequency is essential because transformers designed for different frequencies may operate inefficiently or produce heating and losses. Similar phase angle shift is also important, as differences can lead to phase misalignment and imbalanced loading. Same phase sequence is crucial as it determines how the phases align between different transformers; mismatched phase sequences can result in severe disturbances.

However, while it is ideal for transformers to have the same voltage ratio for optimal operation, it is not an absolute requirement for parallel operation. Transformers can operate in parallel even if they have different voltage ratios; they will simply share loads based on their respective voltage ratios. As a result, a transformer with a lower voltage ratio will take a higher share of the load. Thus, having the same voltage ratio is not a necessary requirement, making it the correct choice in this context.