Generally either star and delta conections for generator is possible. If the windings of a generator are joined through a star connection, fewer turns will be required than with a delta connection, but the conductor will need a larger cross section. Hence, while star connections are more advantageous at high voltages (the windings can be designed for voltages that are times lower than line voltages), delta connections are preferred for heavy generator load currents.
An advantage of the delta connected AC generator is that if one phase becomes damaged or open, the remaining two phases can still deliver three-phase power. The capacity of the generator is reduced to 57.7% of what it was with all three phases in operation.
As IEEE 142 stated, generators produce slightly nonsinusoidal voltages because of saturation and imperfect winding and flux distribution. Industrial generators therefore produce odd harmonic voltages, with the third harmonic voltage being as much as 10%. These harmonic voltages can cause heating from circulating currents in a closed loop. This is one reason why most industrial generators have their internal windings connected in wye rather than delta. The third harmonic voltages produced in the generator’s windings are in phase and additive. This would cause third harmonic current to circulate within the deltaconnected windings, as shown in Figure below. The circulating current would createadditional heating within the generator thereby reducing some of its thermal capacity.
Generators that operate in a delta connection allow for this in their design. If the generator windings are designed with a two-thirds pitch, the third harmonic voltage can be suppressed, but the zero-sequence impedance will be lowered increasing the ground-fault current.