When two or more transformers are operated in parallel, it is necessary to know not only the connection method of the primary and secondary windings, but also the phase relationship between the line potentials (or line voltages) corresponding to the primary windings and the secondary windings in order to determine whether each transformer No parallel operation. Transformer connection group is used to characterize the phase difference of a sign. Practice and show that for three-phase windings, no matter what connection method is used, the phase difference of the potential of the primary and secondary windings is always a multiple of 30 °. How to quickly and accurately determine the three-phase transformer connection group? People have been using clock representation for years, that is, the potential vector of the primary winding as a long needle, the potential vector of the secondary winding as a short needle, the long Needle to 12 o'clock, to see what the number refers to a short pin, put this number as the connection group number. If the short hand refers to the 4 o'clock position, the short hand lags the long hand 120 ° from the counterclockwise direction of the vector, which indicates that the line voltage of the secondary winding of the three-phase transformer is delayed by 120 ° with respect to the original winding line voltage. If the original and secondary winding Y / △ or △ / Y wiring, then called the odd connection group, if the original and secondary windings are △ / △ or Y / Y connection, then called the even connection group, and thus the three-phase transformer The connection group appeared on the 12 clock number 24 connection method. The method does bring great convenience to people, which can not only reflect the phase relationship of the line voltage corresponding to the primary and secondary windings, but also reflect the relationship between the magnitude of the values.
Standard coupling group of transformers There are many coupling groups of single-phase and three-phase transformers. In order to avoid confusion during manufacture and use, the national standards stipulate:
Single-phase two-winding transformer has a standard connection group I, i0
Three-phase double-winding transformers have five standard connection groups: Y, yn0, Y, d11, YN, d11, Y, z11, D, z0.
z means zigzag link.
Y, yn0 used as distribution transformers, the secondary side of the center line can be drawn as a three-phase four-wire system that can be used for power and lighting; (U1 <35kV high voltage side, low voltage U2 <400V, 230V single phase)
YN, d11 for high-voltage transmission lines above 110kV, high-voltage side can be grounded.
A z-shaped connection is suitable for transformers with high lightning protection performance.
