Single-phase transformers are typically connected with a subtractive polarity with the exception of small transformers per IEEE C57 series, which are connected with an additive polarity. Three phase transformers are connected as wye or delta. Therefore, we use vector group to identify which winding is connected wye or delta as well as which voltage phase leads or lags. Clock notation is used to express the phase angle displacement between the high voltage and low voltage phase. It is important to understand the vector group and clock notation, which can help in understanding phase rotation and phase sequence.
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