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.
Three Phase Transformers
Three phase transformers are designed to supply electric power to three-phase systems.
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.
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.

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...