Star - Delta Transformer

 

• It is used at the substation end of the transmission line. 
• The third harmonic current flows in the secondary delta which produces sinusoidal flux.

Primary winding is star connected

• Phase voltage = Line voltage / √ 3
• Phase current = Line current
• Voltage transformation ratio K = Secondary phase voltage / Primary phase voltage

Secondary winding is delta connected

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

• As the phase current is less in the secondary 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.

Relation between secondary line voltage and primary line voltage

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

Therefore

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

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

Advantages

• The neutral of the primary winding is earthed in order to avoid distortion in the voltage waveform.
• The third harmonic current flows in the delta connected phase winding but don’t flow in the line voltage.
• As the primary winding is star connected, it requires less number of turns due to phase voltage is equal to 57.7% of the line voltage.

Disadvantages

• There is 30° phase shift between primary and secondary line voltages. 
• It means that the star – delta transformer cannot be parallel with star – star or delta – delta transformer.

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