# Three Phase Transformer Connections

Last Updated on November 4, 2023 by Electricalvolt

In three phase transformer connections, a transformer can be connected to either the star or delta. As we know, in a three-phase electrical system, there are two types of connection used to connect the three-phase. They are the star connection and the delta connection.

In the star connection, each end of the three phases is connected to a common point, and the other ends of each phase are taken out as the line terminal. On the other hand, in the delta connection, one end of a phase is connected to another end of the second phase such that the three phases form a closed loop. In the case of star and delta connections, there is a phase difference of 120° electrical between any two phases.

The following figure shows the star and delta connected three-phase electrical system.

The star and delta connections are widely used to form connections of three-phase transformer windings. Using these two connections, we can also form some other types of connections of transformer windings that we will cover in this article.

## Three-Phase Transformer Connections

A three-phase transformer consists of three primary windings and three secondary windings. Each pair of primary and secondary windings is provided for each of the three phases in a three-phase electrical system.

Some common types of winding connections used in three-phase transformers are listed here:

• Star-Star Connection
• Delta-Delta Connection
• Star-Delta Connection
• Delta-Star Connection
• Open-Delta Connection
• Scott Connection

These six types of three-phase transformer connections are commonly used in practical transformers in three-phase systems.

Let us now explore each of these three-phase transformer connections in detail.

### (1). Star-Star Connection / Y-Y Connection

In the star-star connection, primary and secondary windings are connected in a star configuration, as shown in the following figure.

Let V1 and V2 be the phase voltage on the primary and secondary sides of the transformer, respectively. If the line voltages on the primary and secondary sides are VL1 and VL2, respectively. Then, the transformation ratio is given by,

We know, in the star connection,

Then, the transformation ratio will be,

### Characteristics of Star-Star Connection:

The following are major characteristics of star-star connection:

• If the load is unbalanced, the phase voltages will also be unbalanced.
• The star-star connection suffers from a third-harmonic component due to the absence of a closed path for their circulation. This can cause the breakdown of the insulation.
• To eliminate the problem of third harmonics due to unbalanced load conditions, we can provide a solid neutral ground.

### (2). Delta-Delta Connection / Δ-Δ Connection

In the delta-delta three phase transformer connection, primary and secondary windings are connected in the delta configuration, as shown in the following figure.

In the case of a delta connection, the line voltage and phase voltage are the same. Thus, the transformation ratio of a delta-delta connection is given by,

### Characteristics of Delta-Delta Connection:

The following are the main characteristics of the delta-delta connection:

• The delta-delta transformer is used in the transmission system as there is no need for neutral.
• The delta-delta-connected transformer can be used for both balanced and unbalanced loads.
• Delta-delta connection provides a closed path for the circulation of the third harmonics, so they do not appear in the output voltage.

### (3). Star-Delta Connection / Wye-Delta (Y-Δ) Connection

In the star-delta connection, the primary windings are connected in a star configuration, and the secondary windings are connected in a delta configuration.

This type of transformer connection is mainly used in generating stations where the primary winding neutral is grounded.

In the case of the star-delta connection of the transformer, the transformation ratio is given by,

### Characteristics of Star-Delta Connection:

The star-delta connection has the following major characteristics:

• It provides a close circuit for the circulation of the third harmonic component in the voltage.
• The star-delta connection provides a neutral on the primary side for grounding.
• In the star-delta connection, the secondary voltage is shifted by 30° with respect to the primary voltage.

### (4). Delta-Star Connection / Delta-Wye (Δ-Y) Connection

In the delta-star transformer connection, the primary winding is connected in the delta configuration, while the secondary winding is connected in the star configuration.

This type of transformer connection is mainly used in distribution transformers to create a 3-phase 4-wire electrical system.

The transformation ratio of transformers having a delta-star connection is given by,

### Characteristics of Delta-Star Connection:

The major characteristics of delta-star connection are given below:

• Due to a delta loop, the delta-star connection provides a closed path for the third harmonics in voltage.
• The secondary voltage lags the primary voltage by an angle of 30°.
• The delta-star connection provides a neutral on the secondary side to obtain a single-phase supply as well.

### (5). Open Delta (V-V) Connection

In the open-delta connection, two single-phase transformers are used to supply three-phase power to a load. This connection is possible in the case of a three-phase transformer bank.

In this type of transformer connection, if one of the three single-phase transformers connected in a delta-delta connection is taken out of service for maintenance, the resulting connection becomes an open delta or V-V connection.

In the open delta connection, the remaining two transformers supply the three-phase load with a capacity reduced to 57.7%.

The open delta connection of the transformer is used in emergencies when one of the three transformers is removed from the circuit for repair.

### (6). Scott (T-T) Connection

The Scott connection is another type of transformer connection in which two single-phase transformers are used for 3-phase to 2-phase conversion.

In this type of connection, the two single-phase transformers are connected electrically and not magnetically. Here, one transformer is called the main transformer, and another transformer is called a teaser transformer.

The Scott connection is used in several industrial applications like electric trains, electric furnaces, etc.

## Conclusion

In conclusion, this is all about three-phase transformer connections. A three-phase transformer has three primary and three secondary windings that can be connected in either star or delta configuration. The type of connection depends on the applications of the transformer.