Three Phase Transformers

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Topics: Transformer
A. Introduction
Three phase transformers are used throughout industry to change values of three-phase voltage and current. Since three-phase power is the most common way in which power is produced, transmitted, and used, an understanding of how three phase transformer connections are made is essential. In this section it will discuss different types of three phase transformers connections.
A three-phase transformer is constructed by winding three single-phase transformers on a single core. These transformers are put into an enclosure, which is then filled with dielectric oil. The dielectric oil performs several functions. Since it is a dielectric, a nonconductor of electricity, it provides electrical insulation between the windings and the case. It is also used to help provide cooling and to prevent the formation of moisture, which can deteriorate the winding insulation.

B. Three-Phase Transformer Connections
There are only 4 possible transformer combinations:
● Delta-to-Delta - use: industrial applications
● Delta to Wye – use: most common; commercial and industrial
● Wye to Delta – use: high voltage transmissions
● Wye-to-Wye – use: rare, don’t use causes harmonics and balancing problems.

a) Y/Y Connection

A Y/Y connection for the primary and secondary windings of a three-phase transformer is shown in the figure below. 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 Y/Y connection is that we have access to the neutral terminal on each side and it can be grounded if desired. Without grounding the neutral terminals, the Y/Y operation satisfactory only when the three-phase load is balanced. The electrical insulation is stressed only to about 58% of the line voltage in a Y-connected transformer.(4:225)
[pic]
Figure (1) Y/Y connected three-phase transformer

b) Y/Δ Connection

This connection as shown in figure below is very suitable for



References: 1- Criss, John and Larry Lawhead. "Using Transformer Differential Relay Iop Characteristics to Measure Near Trip Conditions." IEEE (1996) : 103- 108. 2- Yamayee, Ziz. A. and Juan L. Bala, Electromechanical Energy Devices and Power Systems, Gonzaga University, 1994, 64-109. 3- Gebert, Kenneth L and Edwards, Kenneth R . Transformers principles and applications. Chicago: American Technical Society, 1974. 4- Guru, Bhag S and Huseyin R. Electric Machinery and Transformers. New York: Oxford University Press, 2001. 6- Seitlinger, Walter. "Phase Shifting Transformers." VA TECH Transmission and Distribution, 2001 : 1- 7..

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