Delta - Star Transformer

• The primary winding of transformer is connected in delta whereas the secondary winding is connected in star. 
• It is used as the beginning of the transmission line to step up the transmission line voltage.

Primary winding is delta connected

• Line voltage = Phase voltage
• Phase current = Line current  / √ 3

As the phase current is less in the primary side, cross section of the conductor is small as compared to star side. As the line voltage is equal to phase voltage, more insulation requires as compared to star connection.

Secondary winding is star connected

• Phase voltage = Line voltage / √ 3
• Phase current = Line current

Voltage transformation ratio K = Secondary phase voltage / Primary phase voltage

Relation between secondary line voltage and primary line voltage

• Secondary phase voltage = K × Primary phase voltage
• Secondary line voltage / √ 3 = K × Primary line voltage

Therefore

• Secondary line voltage =  √ 3× K × Primary line voltage

Relation between secondary line current and primary line current

Voltage transformation ratio K = Primary phase current / Secondary phase current

• Secondary phase current = Primary phase current / K
• Secondary line current = Primary line current / √ 3× K

Advantages

• As the neutral is available at the secondary side, it can be used for three phase, four wire supply system. 
• As the secondary neutral is earthed, it can handle unbalanced current. As the delta connected primary winding allows path for third harmonic current, no distortion of flux.

Disadvantages

• As the secondary voltage is not in phase with the primary, it is not possible to operate in parallel with star – star or delta – delta connected transformer.

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