Three-Phase Transformers | Electrical Machines for Electrical Engg. - Electrical Engineering (EE) PDF Download

The three-phase transformers are mainly of two types:

1. Single unit three-phase transformer.
2. Three unit three-phase transformer.

Figure: Three phase shell type transformer 

Advantages of Bank of three units:
In case of damage of one winding the power can be transmitted through the two units by using an open delta connection, so 50% power can be transferred.

Advantages of single unit transformer:

1. These transformers use less space.
2. These transformers are lighter, smaller, and cheaper.
3. These transformers are slightly more efficient.

Disadvantages of single unit transformer:
If the single winding of the transformer gets damaged, then we have to change the complete unit.

Connections of three-phase transformer:
A three-phase transformer has three transformers connected in it, either separately or combined on one core. We can connect the primary and secondary winding of a 3-phase transformer in either a star (Y) or delta (∆). There are four ways to connect the 3-phase transformer bank:

1. ∆ - ∆ (Delta primary - Delta secondary)
2. Y - Y (Star primary - Star secondary)
3. ∆ - Y (Delta primary - Star secondary)
4. Y - ∆ (Star primary - Delta secondary)

For connecting the transformers into star or delta, we have to assume that the transformers we are connecting, are all of the same KVA ratings.

What are the factors that affect the choice of connections?
The factors that affect the choice are as follows:

1. We have to check the availability of a neutral connection for grounding, protection or load current.
2. Insulation to ground and voltage stress.
3. We have to check that the path for the flow of third harmonic and zero sequence current is available or not.
4. When one circuit is out of service, we need the partial capacity.
5. Parallel operation with other transformers.
6. We have to check the economic considerations.

1. Delta - Delta (∆ - ∆) Connection
In delta-delta connection the line voltage of the transformer is equal to the supply voltage of the transformer.


The above diagram shows the delta-delta connection of three windings of single phase transformer. The secondary winding a₁ a₂ corresponds to the primary winding A₁A₂, b₁b₂ corresponds to B₁B₂, and c₁c₂ corresponds to C₁C₂, similarly 'a' corresponds to A, 'b' corresponds to B and 'c' corresponds to C. The terminals 'a₁' and A₁ have the same polarity. The Phasor diagram drawn above is for lagging power factor cos Φ. For balanced conditions, the line current is three times the Phase current.
The turn ratio for 3 phase transformer is

And the current ratio when the magnetizing current is neglected is

We can see from the above-drawn diagram that both the primary and secondary line voltages are in phase. This connection is called 0° - connection.

If we reverse the connections of the phase winding, we obtain the phase difference of 180° between the primary and secondary systems. This connection is called 180° - connection.

The advantage of ∆ - ∆ Transformation:

1. The delta-delta connection is good for balanced and unbalanced loading.
2. If a third harmonic is present, it circulates in the closed path and therefore does not appear in the output voltage wave.
3. The main advantage of ∆ - ∆ transformer is that if one transformer stops working, then the other two transformers will keep on working. This is called an open delta connection.

The disadvantage of ∆ - ∆ Transformation:
The disadvantage of ∆ - ∆ transformer is that it does not contain a neutral point and this can only be used when neither primary nor secondary requires neutral, and the required voltage is low and moderate.

2. Star-Star (Y - Y) Connection
Fig: Star-Star connection of transformer (0◦ Phase Shift) 

The voltage ratios for ideal transformer are:

And current ratios are:

There are two serious problems in star-star connection:

1. In the star-star connection when the load is unbalanced and neutral is not provided, then the phase voltage tends to become severely unbalanced. Therefore, the star-star connection is not suitable for unbalanced loading.
2. The magnetizing current of any transformer is very non-sinusoidal and contains a very large third harmonic, which is necessary to overcome saturation to produce a sinusoidal flux.

3. Delta-Star (∆ - Y) Connection
In ∆ - Y connection of 3-phase transformers, the primary line voltage is equal to the primary phase voltage (V_LP = V_pP). The relationship between secondary voltages is  therefore, the line-line voltage ratio of this connection is


Figure: Delta - star connection of transformer (Phase shift 30◦ lead), (b) Phasor diagram 

The phasor diagram drawn above shows the delta-star connection supplying a balanced load at power factor cos Φ lagging. It is seen from the phasor diagram that the secondary phase voltage V_an leads primary phase voltage V_AN by 30◦. Similarly, V_bn leads V_BN by 30◦ and V_cn leads V_CN by 30◦. This is also the phase relationship between the respective line-to-line voltages. This connection is called +30◦ connection.

4. Star-Delta (Y - ∆) Connection
The Y - ∆ connection of three-phase transformers is shown below. In this connection, the primary line voltage is equal totimes the primary phase voltageThe secondary line voltage is equal to the secondary phase voltage (V_LS = V_pS). The voltage ration of each phase is

Therefore line-to-line voltage ratio of a Y - ∆ connection is

Figure: Y - ∆ connection of transformer (Phase shift of 30◦ lead) 

When there is a phase shift of 30◦ lead between respective line-to-line voltages then this type of connection is called as +30◦ connection.
And when there is a phase shift of 30◦ lag between the line-to-line voltages then the connection is known as -30◦ connection.
The ∆ - Y connection or Y - ∆ connection has no problem with unbalanced loads and third harmonics.