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. 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 be omitted, so the three-phase transformer only needs a three-legged EI core. The voltages on the primary side of the three-phase transformer are referred to as the high voltage and those on the secondary side as the low voltage. The input and output voltage can be connected in a delta or star connection. The higher upper voltage is usually the outer winding, thus achieving better electrical insulation from the core. The switching type, also known as the switching group, can be changed between the primary and secondary side. Three-phase generators in star connection have four external connections, three for the phases and a common neutral conductor. The neutral conductor can be dispensed with for energy transport in the high-voltage grid if the secondary windings of the transformer are connected in a delta connection. For the low-voltage or household grid, the transformation is downward. If the secondary side is connected in a star connection, a four-wire three-phase system is created with the neutral conductor. This allows three independent single-phase networks with all three conductors to operate a three-phase network together. The switching types of the three-phase transformers are designated with capital letters for the primary side and lower-case letters for the secondary side followed by an integer. The letters Y and y stand for the star connection, D and d for the delta connection. The number indicates the phase angle between the primary and secondary side multiplied by 30 degrees. The star connection has the advantage that the three conductor voltages in the transformer lead to lower phase voltages at the linked windings. If a segment fails, this results in a very unfavorable power distribution and often the entire transformer fails. The string voltage is 1/√3 of the conductor voltage. The string current is equal to the conductor current. In the delta connection, the conductor voltage is equal to the phase voltage. The conductor current is higher than the linked string current by a factor of √3. If a segment fails, the transformer remains largely operational with around 66% three-phase power.

For further information, see switching group