- 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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