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/

Copyright Oxford University Press, Inc. 2001 under license agreement with Books24x7

Products & Services
Power Transformers
Power transformers convert power-level voltages from one level or phase configuration to another.  They can include features for electrical isolation, power distribution, and control and instrumentation applications.
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.
Isolation Transformers
Isolation transformers provide electrical decoupling or isolation to two connected circuits. They have a turn ratio of 1:1.
Autotransformers
Autotransformers are a special type of power transformer with only one winding.

Topics of Interest

4.11 The Constant-Current Transformer Thus far we have devoted our attention to the study of constant-potential transformers, for which the load voltage is essentially constant and the current varies...

15.1 Winding Polarity A transformer consists of two windings viz., primary and secondary coupled to a common magnetic core. International standards define the polarity of the primary and secondary...

A5.1 THE 9 kVA THREE-PHASE TRANSFORMER BANK A Y Y connected three-phase transformer bank with 416/416 V of rated line-to-line voltages consists of three single-phase transformers. The type numbers of...

4.2 Construction of a Transformer In order to keep the core loss to a minimum, the core of a transformer is built up of thin laminations of highly permeable ferromagnetic material such as...

A neutral is a connection to an electrical system. In a three-phase WYE configuration, one end of each of the three generator or transformer windings is connected to a common point. This common point...