When two transformers are connected in parallel, their phase shifts must be identical if not, a short circuit will occur when the transformers are energized.” Basic Idea of Winding: This condition is called “no phase shift.” But when the primary and secondary windings are connected differently, the secondary voltage waveforms will differ from the corresponding primary voltage waveforms by 30 electrical degrees. It’s important to remember that the secondary voltage waveforms are in phase with the primary waveforms when the primary and secondary windings are connected the same way. This means that a 3-phase transformer can have its primary and secondary windings connected the same (delta-delta or star-star), or differently (delta-star or star-delta). The secondary windings are connected similarly. The two most common configurations are the delta, in which the polarity end of one winding is connected to the non-polarity end of the next, and the star, in which all three non-polarities (or polarity) ends are connected together. The primary windings are connected in one of several ways.
Separate single-phase transformers can be used and externally interconnected to yield the same results as a 3-phase unit. Three phase transformer consists of three sets of primary windings, one for each phase, and three sets of secondary windings wound on the same iron core.
