Three Phase Transformers

Topics: Transformer, Alternating current, Magnetic core Pages: 10 (2832 words) Published: January 30, 2011
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 step-down applications. The secondary winding current is about 58% of the load current. On the primary side the voltages are from line to neutral, whereas the voltages are from line to line on the secondary side. Therefore, the voltage and the current in the primary are out of ohase with the voltage and the current in the secondary. In a Y/Δ connection the distortion in the waveform of the induced voltages is not as drastic as it is in a Y/Y-connected transformer when the neutral is not connected to the ground the reason is that the distorted currents in the primary give rise to a circulating current in the Δ-connected secondary.(4:256) [pic]

Figure (2) Y/ Δ connected three-phase transformer

c) Δ/Y Connection

This connection as shown in figure below is proper for a step-up application. However, this connection is now being exploited to satisfy the requirements of both three-phase and the single-phase loads. In this case, we use a four-wire secondary. The single-phase loads are taken care of by the three line-to-neutral circuits. An attempt is invariably made to distribute the single-phase loads almost equally among three-phases.(4:256) [pic]

Figure (3) Δ/Y connected three-phase transformer

d) Δ/Δ Connection

as shown below the three transformers with the primary and secondary windings connected as Δ/Δ. The line-to-line voltage on either side is equal to the corresponding phase voltage. Therefore, this arrangement is useful when the voltages are not very high. The advantage of this connection is that even under unbalanced loads the three-phase load voltages remain substantially equal. This disadvantage of Δ/Δ connection is the absence of a neutral terminal on either side. Another drawback is that the electrical insulation is stressed to the line voltage. Therefore, a Δ-connection winding requires more expensive insulation than a Y-connected winding for the same power rating.(4:255) [pic]

Figure (4) Δ/Δ connected three-phase transformer

C. Analysis of...
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