Three-phase transformer

A three-phase transformer can be imagined as consisting of three individual single-phase transformers connected to each other. In a single-phase transformer, the primary and secondary windings are each arranged on one leg of the closed iron core. The function of the transformer is maintained if the windings are arranged one above the other on only one of the two core legs. These three single-phase transformers can be joined together with their free core legs, whereby the magnetic fluxes, each shifted by 120 degrees, are superimposed.

Magnetic flux and design shape

If the three phases of a star connection are loaded in the same way, the common neutral conductor remains current-free, as the current sum is zero on average over time due to the phase positions. The magnetic flux is proportional to the current flow, so the magnetic fluxes cancel each other out in the common core leg. The winding-free common core leg can therefore be omitted, so that the three-phase transformer only requires a three-legged EI core.

Voltage designations and winding arrangement

The voltages in a three-phase transformer are referred to as high voltage on the primary side and low voltage on the secondary side. The input and output voltage can be connected in a delta or star connection. The higher upper voltage is usually on the outer winding, which provides better electrical insulation from the core.

Switching groups and connection types

The circuit type, also known as the switching group, describes the connection of the primary and secondary sides. Three-phase star-connected generators have four connections: three phase conductors and one neutral conductor. The neutral conductor can be omitted for energy transport in the high-voltage grid if the secondary windings of the transformer are connected in a delta connection. In the low-voltage or household grid, the voltage is transformed down. If the secondary side is connected in a star connection, a four-wire three-phase system is created with the neutral conductor. This means that three independent single-phase networks can be operated simultaneously.

Designation of the switching groups

The switching types of the three-phase transformers are identified with capital letters for the primary side and lower-case letters for the secondary side, followed by a number.
Y / y: Star connection
D / d: Delta connection

The number, multiplied by 30 degrees, indicates the phase angle between the primary and secondary sides.

Star connection – properties

The star connection offers the advantage that the three conductor voltages on the linked windings lead to lower phase voltages.
Phase voltage = conductor voltage / √3
Phase current = conductor current

However, if a segment fails, this results in a very unfavorable power distribution, which often leads to the failure of the entire transformer.

Delta connection – properties

The following applies in the delta connection:
Conductor voltage = phase voltage
Conductor current = √3 × phase current

If a segment fails, the transformer remains largely operational with around 66% of its three-phase power.

For further information, see switching group