Why the stator of the turbo-generators normally has a star winding, instead of a delta winding?

One of the main reasons for the larger generators producing voltages not more than around 22kV is the insulation restrictions. Higher voltages between the windings require more insulation in the stator windings which turn out not to be feasible for larger output voltages.

In case of a star-connected winding, the line voltage is √3 times the phase voltage. And the winding insulation depends on the phase voltage levels. For a delta-connected winding, line voltage is equal to phase voltage. Thus, for the same line voltage generated, amount of insulation required in a star winding will be lesser(due to lesser phase voltage) than that required in a delta winding.

VL = √3Vp               For star connection

   VL = Vp                   For Delta connection

• Insulation between the winding depends on the voltage levels
• Heating of the winding depends on the current levels.

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