# Phasor Connection of Three Phase Transformers | Electrical Machine | GATE Preparation | EE

Real Life Application The application of Phasor connection of Three Phase transformers is very important in the transmission and distribution of Power. It is the three phase system which has been adopted world over to generate, transmit and distribute electrical power. Therefore to change the level of voltages in the system three phase transformers should be used. Whenever a transformer is installed for transmission or distribution, the phase difference is one of the deciding factors for the type of transformer to be chosen.

Explanation To understand the concept with clarity it’s important to have basic idea of working of Transformers and Three Phase Circuits. Three number of identical single phase transformers can be suitably connected for use in a three phase system and such a three phase transformer is called a bank of three phase transformer. Alternatively, a three phase transformer can be constructed as a single unit. In a single phase transformer, we have only two coils namely primary and secondary. Primary is energized with single phase supply and load is connected across the secondary. However, in a 3- phase transformer there will be 3 numbers of primary coils and 3 numbers of secondary coils. So these 3 primary coils and the three secondary coils are to be properly connected so that the voltage level of a balanced 3-phase supply may be changed to another 3-phase balanced system of different voltage level. The possible connections of three phase transformers are Star Star, Star Delta, Delta Star, Delta Delta. For example Star Delta means primary side has Star connection and secondary side has Delta. The secondary voltages of a 3-phase transformer may undergo a phase shift of either +30° leading or -30° lagging or 0° i.e, no phase shift or 180° reversal with respective line or phase to neutral voltages. On the name plate of a three phase transformer, the vector group is mentioned. Typical representation of the Vector Group could be Yd1 or Dy11 etc. The first capital latter Y indicates that the primary is connected in star and the second lower case latter d indicates delta connection of the secondary side. The third numerical figure conveys the angle of phase shift based on clock convention. The minute hand is used to represent the primary phase to neutral voltage and always shown to occupy the position 12. The hour hand represents the secondary phase to neutral voltage and may, depending upon phase shift, occupy position other than 12 such as 6 or 1 or 11. For any given three phase transformer the phase difference can be found out from the connection of the primary and secondary terminals, which is explained in this video lecture.