Delta-wye transformer
A three-phase AC transformer, three-phase or three-phase transformer transformer sums necessary for the transformation to a three phase system of three individual transformers together into one. Typically, power transformers are designed as a three -phase transformer. The first three- phase transformer was founded in 1890 by Mikhail Dolivo - Dobrovolsky built by AEG in Berlin after he had there previously developed a three-phase system. The first practical use of three- phase transformers for electrical transmission of energy with hochtransformiertem three-phase alternating current was the following year at the AC transmission Lauffen -Frankfurt as part of the International Electrotechnical Exhibition in 1891.
Construction
Three phase transformers are constructed similarly single-phase transformers. In contrast to these primary and secondary side there are however three separate windings - usually with U, V, W for the high-voltage side and u, v, w for the low-voltage side called - which on a soft magnetic iron core, which is usually a " closed e " similar, are housed (see adjacent diagram). The three wound with the coil parts are called legs, the connections between the legs of the yokes. The upper and lower voltage winding of each phase is in this case normally is mounted on the same arm, separated by corresponding insulating material. There are also five leg core ( see below).
Assuming first of three single-phase transformers for each phase, so these have a closed annular core. The non- wound limbs, the return leg, serves to close the magnetic circuit. Now you can see the three transformers star-shaped put together at reflux leg, so that a joint return leg for all three transformers produced. In a symmetrically loaded three-phase system, ie with the same load on all three phases, the magnetic fluxes cancel each other in the common return leg of the iron core due to the phase shift on each other, which is why the return leg can be omitted. Thus, the core may be as shown in the diagram can be implemented as a three- leg core. The symmetric case is, however, not always in operation ensure one then speaks of an unbalanced load, which makes some of the switching groups to undesirable neutral point shift and a strong magnetic flux leakage. To avoid this condition, some three-phase transformers, depending on the switching group (eg dome transformers in the switch group " Yy0 " ) are either provided with a five-limb core with two additional external return shanks or carried out at three-limb core version with an additional tertiary winding to compensate.
Such as those on each side of a three-phase transformer connected in total six ports to one another is determined by the switch group. Usual interconnections are star and delta connection that can be combined on both sides in principle. This results from the phase voltages of the upper and lower voltage side different phase shifts, which can be not only 0 ° or 180 ° as in single-phase transformers. In these cases, the ratio is expressed by a complex factor, which additionally includes the phase shift. For this reason, the vector group is observed during parallel operation of several three-phase transformers.
Example
The group Dyn5 a three-phase alternating current transformer means:
- D = high-voltage windings in delta connection
- Y = voltage winding in star connection
- N = lead-out star point ( neutral)
- 5 = phase shift between upper and lower voltage is: 5 * 30 ° = 150 °