TRANSFORMERS IN GENERAL USE

1. Star - Star YyO or Yy6
2. Delta - Delta DdO or Dd6
3. Star - Delta Yd
Delta - Star Dy
4. Star - Zigzag Yz
5. Delta - Zigzag Dz

Star-Star (Yy0 or Yy6)

Most economical connection in HV power system to interconnect two delta system and to provide neutral for grounding both of them.
Tertiary winding stabilises the neutral conditions.

In star connected transformers, load can be connected between line and neutral, only if

(a) the source side transformers is delta connected or

(b) the source side is star connected with neutral connected back to the source neutral.

Delta - Delta (Dd 0 or Dd 6)

This is an economical connection for large low voltage transformers.

Large unbalance of load can be met without difficulty.

Third harmonics are damped out as the windings are closed mesh.

But the absence of a star point is dis-advantageous in some cases.

Delta - Star (Dy or Yd)

Common for distribution transformers.

Star point facilitates mixed loading of three phase and single phase consumer connections.

The delta winding carry third harmonics and stabilises star point potential.

Delta-Star connections is used for step-up generating stations.

If HV winding is star connected there will be saving in cost of insulation.

But delta connected HV winding is common in distribution network, for feeding motors and lighting loads from LV side.

Star-Zig-zag or Delta-Zig-zag (Yz or Dz)

These connections are employed where delta connections are weak. Interconnection of phases in zigzag winding effects a reduction of third harmonic voltages and at the same time permits unbalanced loading.

This connection may be used with either delta connected or star connected winding either for step-up or step-down transformers. In either case, the zigzag winding produces the same angular displacement as a delta winding, and at the same time provides a neutral for earthing purposes.

The amount of copper required from a zigzag winding in 15% more than a corresponding star or delta winding. This is extensively used for earthing transformer.