3 PHASE Transformers – different connections

What are transformers?

The transformer is a static piece of apparatus which transfers electrical energy from one circuit to another, which are electrically disconnected by means of mutual induction.

A Transformer consists of two or more stationary electric circuits where the source supplies the power to the primary winding and the coil delivers power to the Load through the Secondary winding.

3 Phase transformer:

A three-phase transformer is constructed by winding three single-phase transformers in a single core. These transformers are placed in an enclosure that is then filled with dielectric oil.

The dielectric oil performs several functions. Since it is a dielectric, a non-conductor of electricity, it provides electrical insulation between the windings and the housing. It is also used to help provide cooling and to prevent the formation of moisture, which can deteriorate the insulation of the winding.


3 phase transformers are constructed in two ways; 3 single phase transformers connected in a 3–phase bank or 3 windings are wound on a common core.

Three phase transformers connections: 

  • Star Connection ( Y ):.

Line current  is same as phase current , Line-Line  voltage is Ö3 phase-neutral voltage,


Delta connection:

Line-Line voltage is same as phase voltage., Line current is Ö3 phase current.

Zigzag (Z) connection:

A zigzag transformer is more effective for grounding purposes because it has less internal winding impedance going to the ground than when using a wye-type transformer.

The zigzag winding provides an easy path for in-phase currents but does not allow the flow of currents that are 120° out of phase with each other.

Three phase transformers connection combinations:



Rarely used for large amount of power transmission, used for the special type of application



Used at substations where the voltage is stepped down


Used at the generator station, where the voltage is stepped up


If one transformer is removed, the remaining two are able to supply three-phase power to the load terminals.

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