Three Phase Transformers
For Generation of electrical power three-phase alternators are employed at the power generating station. Usually large power is generated at high voltage. Transmission is generally accomplished at higher voltage of 132KV, 220KV, 400KV, and 765KV. For which purpose 3-phase transformers are necessary to step up the generated voltage to that of the transmission line. At load centre, these voltages are stepped down to 66KV, 33KV, or 11KV, for distribution purpose. For distribution purpose 3-phase transformer also required. So that three phase transformers plays very important role in transmission and distribution of electric power.
Advantages of Three Phase Transformers
- It is light in weight.
- It occupies less floor space.
- Its cost is less than 3 single phase transformers of equal rating.
Disadvantages of Three Phase Transformer
The main disadvantage of this transformer is that one of the phases becomes faulty, and then whole of transformer is to be replaced.
Three Phase Transformers Connections
The windings of three phase transformers may be connected in Y(star) or Δ(delta) in the same manner as for three single phase transformers. There are various type of 3-phase transformers available according to arrangement of connections. The most common connections are given below.
- Star – Star (Y – Y) connection.
- Delta – Delta (Δ – Δ) connection.
- Star – Delta (Y – Δ) connection.
- Delta – Star (Δ – Y) connection.
- Open – Delta (V – V) connection.
- Scott connection or T-T connection.
Star – Star (Y –Y) connection
This connection is most economical for small, high-voltage transformers. The insulation required is minimum for these connections. It given line voltage times phase voltage. There is no phase shift between primary and secondary voltage.
Delta – Delta (Δ – Δ) connection
This connection is used for large current and low voltage. The conductor is required of smaller x-section as the phase current is 58% of the line current.
Star – Delta (Y – Δ) connection
This connection is used at the substation end of the transmission line where the voltage is to be stepped down. There is a 300 shift between primary and secondary line voltages.
Delta – Star (Δ – Y) connection
This connection is used at generating station because voltage is generated at low level and it need to be stepped up for transmission purpose. So that, it is used at the beginning of the high tension transmission system. This connection is also used in distribution because it give neutral point at secondary side. It is useful for 3-phase 4-wire service. It can be used to serve both the 3-phase machine and single phase lighting circuits.
Open – Delta or V – V connection
If one of the phase is removed and 3 – phase supply is connected to the primary of the transformer. In this case 3 – phase voltage will appear at secondary. The arrangement is called open-delta or V-V connection.
The conversion of three phase transformer into single phase or two phase such available transformer is called scott connection transformer.