Three Phase Transformers
The three-phase transformers are mainly of two types:
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:
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:
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. 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:
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:
3. Delta-Star (∆ - Y) Connection
In ∆ - Y connection of 3-phase transformers, the primary line voltage is equal to the primary phase voltage (VLP = VpP). The relationship between secondary voltages is VLS= VpS therefore, the line-line voltage ratio of this connection is
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 Van leads primary phase voltage VAN by 30◦. Similarly, Vbn leads VBN by 30◦ and Vcn leads VCN 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 to times the primary phase voltage (VLP= VpP). The secondary line voltage is equal to the secondary phase voltage (VLS=VpS). 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.