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25/02/2021

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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