A star circuit is one in which similar ends of three resistances are connected to a common point 'N' called a star point or neutral point. It is also called wye ( connection or Tee (T) connection, because of its shape.

A delta circuit is one in which dissimilar ends of three elements are connected together.It is also called mesh connection or pi (π) connected because of its shape. There is no neutral point in the case of delta connection.

For long-distance transmission and distribution, star connection is preferred because it requires little insulation and has a neutral that helps to balance the circuit. Delta connections require more insulation and have issues with unbalanced currents, hence they are preferable for transmission and distribution over shorter distances.

In general, star connections are employed in situations that need a neutral and two distinct voltages, such as our distribution system. When a neutral conductor is not required, such as during the transmission of high-voltage electricity, a delta connection is often preferred. Additionally, delta connections are preferred when controlling third harmonics.

Higher reliability of the Delta connection is the major advantage. The secondary will continue to produce full voltage on all three phases even if one of the three primary windings fails. The only criteria is that the load must be supported by the final two phases.

The main advantage of star connection over delta is, the neutral is available and it can be grounded to prevent voltage distortion. Third harmonics are not a concern for the star-delta connection because they circulate in the delta loop on the secondary side.