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/

Products & Services
Single Phase Transformers
Single-phase transformers output single-phase AC power, typically while stepping-up or stepping-down the voltage between two circuits.
Distribution Transformers
Distribution transformers are pole-type transformers that supply relatively small amounts of power to residences. They are used at the end of an electrical utility’s delivery system.
Three Phase Transformers
Three phase transformers are designed to supply electric power to three-phase systems.
Autotransformers are a special type of power transformer with only one winding.
Transformer Test Equipment
Transformer Test Equipment consists of specialized test modules or systems used to test and/or monitor electrical and mechanical parameters of transformers and other related devices.

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