# Three-Phase Transformer Winding Connections

In a single-phase as well as three-phase transformer the windings are placed on the core structure which is made up of a laminated steel core. This core structure which supports the two windings of the transformer is placed inside the transformer tank (immersed in the oil for cooling). The terminals of the primary and secondary windings are brought out through the transformer tank.

But before bringing out, the three windings (each winding for each phase) of primary and secondary are connected inside the transformer. Let us see the possible way of connecting the primary and secondary of a three-phase transformer.

## Star-Star (Y - Y) Connection :

In a star-star type of connection, the terminals of a three-phase winding on both primary and secondary sides are connected in the shape of the letter 'Y'. Since, both primary and secondary windings are connected in the same manner (i.e., star). It is known as the Star-to-Star connection. This type of connection is most economical for transformers that work for small currents and high voltages.

## Delta-Delta (Î” - Î”) Connection :

In this type of connection, the end of one coil of a three-phase winding is connected to the end of the other coil. In this way, the three coils are connected such that it forms a shape delta. The delta-connected windings can generate large currents with low values of voltages. The below figure shows the Î” - Î” connections and voltage triangles.

## Star-Delta (Y - Î”) Connection :

This type of winding connection is used where we need to decrease the voltages levels, for example, at the end of a transmission line. Here, the neutral of the transformer's primary winding is earthed.

In this system transformation ratio, K is equal to the ratio of secondary phase voltage to the primary phase voltage. Also third harmonic current flow in the Î” - connected winding can control the system even when it is unbalanced with minimized distortion in the waveform of the flux.

On the primary side of the transformer, the winding is connected in a star. Here in the star connection line voltage is √3 times of phase voltage which decreases the effective use of per phase insulation required. But there will be a phase displacement between the primary and secondary voltages.

## Delta-Star (Î” - Y) Connection :

It is a combination of delta-connected winding on the primary side and star connection on the secondary side as shown below. The delta-star connection can be used where there is a need in increasing the voltage levels or stepping up.

This type of connection is well suited for distribution purposes due to the availability of a 3-phase, 4-wire system at the secondary side. But limited to its applications due to the existence of a phase shift between primary and secondary winding.

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