# Three-Phase Transformer Connections: Advantages and Disadvantages

The three-phase transformer comprises three individual transformers that may be independent or interconnected through a single core. The transformer’s primary and secondary can be connected in either a star or delta configuration, with the ability to do so independently. The 3-phase transformer bank can have four potential connections.

## 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 sides.

• 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 the neutral point of the primary is connected to the neutral of the generator and still the third harmonics is present in the alternator voltage form, it will appear on the secondary side. Though the secondary line voltage does not contain third harmonics voltage, the 3rd harmonic voltages are additive in the neutral and cause the current in the neutral of tipple frequency and cause 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.

## Delta-Delta (Dd0 or Dd6) connection

This type of connection is mainly used in a system that carries large currents on low voltages and especially when continuity of service must be maintained even though one of the phases develops a 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.

## 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 the transmission line. The neutral of the primary winding is earthed in this type of transformer. The line voltage ratio is 1/√3 times the transformer turn ratio and secondary line voltages have a phase shift of ±30° concerning primary line voltages. On the hv side of the transformer, insulation is stressed only to the extent of 57.7% of the line voltages. so, there are some savings in the cost of insulation.

## Delta-star(Dy1 or Dy11) connection

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 the existence  Δ connection allows a path for the 3rd harmonic contents.