Three Phase Transformer Interconnections

The phase windings of three-phase transformers are interconnected to give a three-phase, three-or four-wire supply through three different modes of connection. They are

1. mesh or delta connection
2. star connection
3. zigzag connection

and each of these can be achieved in two ways.

The primary and secondary can be treated in two ways, so at least twelve methods of connection are possible. The twelve methods are shown in the figure.

These are arranged in four main groups, as given below, according to the phase displacement which exists between the line voltages on the two sides of the transformer.

• Group  1 : Zero phase displacement (Yy 0, Dd 0, Dz 0) 
• Group 2: 180° phase displacement (Yy 6, Dd 6, Dz 6) 
• Group 3 : 30° lag phase displacement (Dy 1, Yd 1, Yz 1) 
• Group 4 : 30° lead phase displacement (D y11, Y d11, Y z11)

The most popular method of interconnection off transformer is the delta-star connection (4,dy11).

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It has been observed in group 4, when HV line voltage is maximum the lv line voltage has been advanced 30°. With group no. 1 there is no phase displacement, but the group no 2 gives phase displacement of 180°. Group no 3 causes the lv line voltage to lag by 30°.

For parallel operation of transformers, the essential requirement is that the transformer connections belong to the same main group.

The Zig-Zag connection is one example of a sectionalized winding and its effect is to reduce third harmonics in line-to-neutral voltages, as well as in line-to-line voltages. 

15% more turns are required for a given total voltage per phase compared with a normal phase connection, which may necessitate an increase in the frame size over that normally used for the given rating. Nevertheless, the advantages of zig-zag arrangement may offset the cost; unbalanced loads on the secondary side are distributed better on the primary side. 

The zig-zag/star connection has been employed where delta connections were mechanically weak (on account of a large number of turns and small copper sections) in HV transformers; also for rectifiers.