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Three-phase transformer connections and vector groups for beginners
Three-phase transformer connections and vector groups for beginners (photo credit: postmapostma.co.za)

Transformer connections

In theory a three-phase transformer works like three separate single-phase transformers with shared limbs in which the magnetic circuit for the outer limbs is longer than for the centre limb.

The voltage transformation is determined by the ratio between the number of turns on the primary and secondary sides assuming what are known as even connections, Yy, Dd and Zz.

Schematic diagram of a three-phase transformer
Schematic diagram of a three-phase transformer

In the three-phase transformer we can change the transformation by going from star to delta connection. This gives us mixed connections. In the case of mixed connections the ratio between the main voltages on the primary and secondary sides is not equal to the ratio between the number of turns

The transformation ratio is calculated as equal to the ratio between the main primary and secondary voltages with no load independently of the ratio between the numbers of turns.

Star, delta and zigzag connections

Three-phase transformers can have their windings connected in various ways: star, delta or zigzag connection.The connection type must be specified on the transformer’s rating plate.

Star, delta or zigzag connection is indicated by the letters Y, D and Z for the side with the highest voltage and y, d and z for the side with the lowest voltage. If the neutral point is connected to separate terminals, the code should be YN or ZN for the high voltage side and yn or zn for the low voltage side.

Common to all the connection types is that the transformer’s phase terminals are marked 1U, 1V and 1W on the high voltage side and 2U, 2V and 2W on the low voltage side. Any connection point which represents the windings’ neutral point is marked 1 N or 2N.

Let’s discuss about three most common transformer connections:

  1. Star connection
  2. Delta connection
  3. Zigzag connection

Star connection

In the case of star connection the three windings are connected together at their end points.The connection point represents the windings’ neutral point. Star connection is indicated by Y on the high voltage side and y on the low voltage side.

USAGE: This connection type is used for both low and high voltages and for low currents.

Transformer star connection
Transformer star connection

Delta connection

In the case of the ends of the windings are connected together as shown in the diagram. We can see that both the ends are connected together. Delta connection is indicated by D or d.

USAGE: This connection type is used mainly for high rated currents and low voltages.

Transformer delta connection
Transformer delta connection

Zigzag connection

Each phase consists of two equal windings on unequal limbs. There will therefore be parts of two phases on each limb, with one winding on each limb being connected together at the end points. Zigzag connection is indicated by Z or z. This connection type requires 15.5% more windings than star or delta connection, resulting in a larger and more expensive transformer.

USAGE: It is mainly used where load unbalance can occur between the phases and neutral.

Transformer zigzag connection
Transformer zigzag connection

Vector groups

The connection of all the windings in a three-phase transformer is indicated by a vector group symbol. This symbol indicates the winding connections and their relative phase displacement by means of a numerical index (e.g. Dny11).

The “clock method” is used to produce the numerical index of the vector group, with each hour representing 30 electrical degrees.

The numerical index of the vector group comes from the clock hour gure the phase voltage hand (2U) is at when the phase voltage hand of the high voltage winding (1U) is at 12 o’clock. The system’s phase sequence should be 1U, IV, 1W, or R, S and T.

The following are the most common three-phase connections: Dd0, Dyn5, Dyn11, Yyn0, Yd1, Yd11, Dz0, Yz1 and Yz11.


Dd0

Dd0 vector group
Dd0 vector group

Dyn5

Dyn5 vector group
Dyn5 vector group

Dyn11

Dyn11 vector group
Dyn11 vector group

Yyn0

Yyn0 vector group
Yyn0 vector group

Yd1

Yd1 vector group
Yd1 vector group

Yd11

Yd11 vector group
Yd11 vector group

Dz0

Dz0 vector group
Dz0 vector group

Yz1

Yz1 vector group
Yz1 vector group

Yz11

Yz11 vector group
Yz11 vector group

Reference // The three-phase transformer by Noratel

About Author //

author-pic

Edvard Csanyi

Edvard - Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV high power busbar trunking (<6300A) in power substations, buildings and industry fascilities. Designing of LV/MV switchgears.Professional in AutoCAD programming and web-design.Present on

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