From Electric Machinery and Transformers, Third Edition

4.10 Three-Phase Transformers

Since most of the power generated and transmitted over long distances is of the three-phase type, we can use three exactly alike single-phase transformers to form a single three-phase transformer. For economic reasons, however, a three-phase transformer is designed to have all six windings on a common magnetic core. A common magnetic, core, three-phase transformer can also be either a core type (Figure 4.35) or a shell type (Figure 4.36).

Figure 4.35: A core type three-phase transformer.

Figure 4.36: A shell-type three-phase transformer.

Since the third harmonic flux created by each winding is in phase, a shell-type transformer is preferred because it provides an external path for this flux. In other words, the voltage waveforms are less distorted for a shell-type transformer than for a core-type transformer of similar ratings.

The three windings on either side of a three-phase transformer can be connected either in wye ( Y) or in delta ( ?). Therefore, a three-phase transformer can be connected in four possible ways: Y/ Y, Y/ ?, ?/ Y, and ?/ ?. Some of the advantages and drawbacks of each connection are highlighted below.

Y/Y Connection

A Y/ Y connection for the primary and secondary windings of a three-phase transformer is depicted in Figure 4.37. The line-to-line voltage on each side of the three-phase transformer is ?3 times the nominal voltage of the single-phase transformer. The main advantage of a Y/

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