# Choice of Transformer connections

1.Star-star(Yy0 or Yy6) connections:This type  of Transformer is most economical for small current and high voltage transformers.

• The number of turns per phase and the amount of insulation is minimum because phase voltage =1/√3 of line voltage.
• There is no phase displacement between the primary  and secondary voltages.
• It is possible to provide a neutral connection with star points available on both side.

• If the load on the secondary side of transformer is unbalanced,the phase voltages of load side change unless the load star point is earthed.
• The primary of the transformer draws a magnetizing current which has third and fifth harmonics.The third and fifth harmonics cannot flow if neutral of primary winding is not connected to neutral of generator.Therefore the flux in core cannot be of sinusoidal wave and the voltages will be distorted.The trouble of distortion can be eliminated by connecting primary neutral to the generator neutral which provides the return path for third and fifth harmonics.
• If neutral point of primary is connected to neutral of generator and still the third harmonics is present in the alternator voltage form,it will appear on secondary side.Though the secondary line voltage do not contain third harmonics voltage ,but the 3 rd harmonic voltages are additive in the neutral and causes the current in the neutral of tipple frequency and causes the interference to the nearby communication system.

Note:

• Third harmonics phase voltages may be high in shell type 3-phase transformers.
• Star-star connection is rarely used as it has difficulties associated with the exciting current.

2.Delta-Delta(Dd0 or Dd6) connections:This type of connections is mainly used in a system which carry large currents on low voltages and especially when continuity of service must be maintained even though one of the phase develops fault.

• There is no phase displacement between the primary and secondary voltages.
• There is no distortion of flux as third harmonic component of magnetizing current can flow in the delta connected primary winding without flowing in the line wires.
• The cross section of conductor is reduced because the phase current is 1/√3 times of line current.
• Unbalancing of loads on secondary side causes no difficulty.

• Requires more insulation as compared to Y-Y connections.
• Absence of star point is disadvantageous because if one of the line gets earthed due to fault,maximum voltage between windings and core will be full line voltage.

3.star-Delta(Yd1 or Yd11) connections:This type of transformer is used where the voltage is to be stepped down.It is used at the end of transmission line.The neutral of primary winding is earthed in this type of transformer.The line voltage ratio is 1/√3 times of transformer turn-ratio and secondary line voltages have a phase shift of ±30° with respect to primary line voltages.In the hv side of the transformer insulation is stressed only to the extent of 57.7% of the line voltages.so,there is some saving in the cost of insulation.

4.Delta-star(Dy1 or Dy11) connections:This type of connection is used where the voltage is to step up .It is used at the beginning of a ht transmission system.The line voltage ratio is √3 times of transformer turn-ratio and the secondary line voltages have a phase shift of ±30° with respect to primary line voltages.

• on the high voltage side of a transformer,insulation is stressed only to the extent of 57.7% of line to line voltage.
• Neutral point is stable and will not float when load is unbalanced.
• As there is no distortion of flux because existence  Δ connection allows a path for the 3 rd harmonic contents.

• Delta star transformer cannot be operated in parallel with star-star or delta-delta connected transformer even if the voltage ratio are adjusted.

Applications:

The neutral of the secondary is grounded to provide 3-phase,4-wire system and can be used in distribution systems as it can be used to serve three-phase power equipment’s and single phase lighting circuits.

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