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Understanding Three-Phase Transformers

Ralph Fehr, Ph.D., P.E.

Understanding Three-Phase Transformers briefly reviews the fundamentals of single-phase transformers, beginning with the ideal transformer model and the circuit model which accounts for losses. Next, a presentation of the per-unit system of calculation is made, stressing why it is useful for transformer problems. Then, a comprehensive view of transformer impedance is presented, explaining why impedance is stated in percent and not ohms, and detailing how impedance is measured. The significance of transformer impedance on system operation, particularly voltage drop and short circuit current, is also explained.

Next, the course explores the most common three-phase connections the delta and the wye. Three-phase transformations (the most widely used transformer connections in industry) are explored, including a universal method of analyzing any transformer connection using a per-unit current approach. The delta-wye transformation will be studied in detail, particularly the reasons for the magnitude change and phase shift. Then, application of the per-unit current analysis method will be shown for transformer connections more complicated than the delta-wye.

This course includes a multiple-choice quiz at the end, which is designed to enhance the understanding of the course materials.

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