Which equation is used for finding the unbalanced current in a 3-phase WYE transformer?

The equation for finding the unbalanced current in a 3-phase WYE transformer utilizes the square root of the sum of the squares of the phase currents, adjusted by subtracting the product of the phase currents pairs. This is essential in systems where unbalanced conditions occur, as it accurately accounts for the vector nature of alternating currents which can lead to cancellation effects or amplification of certain components.

The formulation in the correct answer captures the essence of how to evaluate the unbalanced current by considering not just the magnitudes of the phase currents but also their phase relationships. In a balanced system, the phase currents would equal each other, leading to zero unbalanced current, while in an unbalanced system, the equation allows for clarity in determining the degree of imbalance present.

The incorrect options either fail to consider the vector nature of alternating current or do not provide a valid way to account for the mutual effects between phase currents. This makes those options less effective for determining unbalanced conditions in practical applications like a WYE transformer setup. Thus, the chosen equation stands as the most reliable for engineers and technicians working with three-phase systems.