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Depends on the design criteria. A grounded star transformer winding will provide zero sequence (ground) current, so it can be used for polarizing of protective relays. The extra ground current may be a requirement of the system to guarantee protective device operation.

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The three phase windings in a generator / transformer / motor can be interconnected in two ways:

STAR

If the similar ends of the coils are connected together and the other ends are connected to the 3 incoming supply service lines, that is called "star connection". The internally-connected point is brought out as the "neutral" connection. The service "line-to-line" voltage (between any 2 lines) is 1.732 times the voltage between any line and neutral. Thus we can get two voltages for distribution within a building or site to supply lighting and power. However the service line currents are the same as the phase winding currents.

DELTA

If the dissimilar ends of each winding are labelled A and B and the three windings are then connected together B->A, B->A, B->A, and the 3 incomiing supply service lines are then connected to the junctions, it is called DELTA. Here the values of line voltage and phase voltage are the same while the service line currents are 1.732 times the phase winding currents.

Additional Answer

In Britain, the secondary windings of a three-phase distribution transformer are always star- (wye-) connected, with the star (neutral) point earthed. This provides a three-phase distribution system, with 400 V between lines, and 230 V between line and neutral. Residences are connected line-to-neutral, providing 230-V single-phase service; premises that require three-phase supplies are provided with three lines and a neutral.

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