Star-Star (Y-Y) Connection of Three Phase Transformers
Advantages of Star-Star Connection
- Phase voltage = (Line Voltage)/√3 . The voltage gets reduced in-phase and this, in turn, reduces the number of turns per phase and the amount of insulation.
- There is no phase displacement between the primary and secondary voltages
- Star points on both sides make it possible to provide a neutral connection
Disadvantages of Star-Star Connection
If the load on the secondary side of the transformer is unbalanced, the phase voltages of load side change unless the load-star point is earthed. The difficulty of shifting neutral can be overcome by connecting the primary star point to the star point of the generator.
The primary of the transformer draws a magnetizing current which has third and fifth harmonics. If neutral of the primary winding is not connected to neutral of the generator, the third and fifth harmonics current cannot flow hence the flux in the core cannot be of a sinusoidal wave and, therefore, the voltages will be distorted. By connecting primary neutral to the generator neutral, the path for the return of these third and fifth harmonics current is provided and, therefore, the trouble of distortion of voltages is overcome.
Even if neutral point of primary is connected to neutral of generator or earthed, still if third harmonic is present in the alternator voltage form, it will appear on secondary side. Though the secondary line voltages do not contain third harmonic voltage; but the 3rd harmonic voltages are additive in the neutral and causes the current in the neutral of tipple frequency (3rd harmonic) which will cause interference to the nearby communication system.
Thus harmonic phase voltages may be high in-shell type 3-phase transformers.
The star-star connection is rarely used owing to the difficulties associated with the exciting current.
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