# Three Phase Transformer Interconnections

- Link: https://studyelectrical.com/2014/08/three-phase-transformer-interconnections.html
- Published: 2014-08-13T11:48:00+00:00
- Author: 

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).

![Three Phase Transformer Connections](https://studyelectrical.com/wp-content/uploads/
2014/08/Three-Phase-Transformer-Connections.png)

Three Phase Transformer Connections

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](https://studyelectrical.com/2014/10/parallel-operation-of-single-phase-transformers.html),
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.

There are several methods available for transforming three-phase voltage to higher
or lower voltage. Next, we will discuss the most common connection in the following
sections.

 * [Star –  Star Connection](http://www.studyelectrical.com/2014/11/star-star-connection-of-three-phase-transformer.html)
 * Delta – Delta Connection
 * Delta – Star Connection
 * Star – Delta Connection
 * Open Delta Connection (V-V)
 * Scott Connection (T-T)

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