The rectifiers are used to convert alternating current (AC) to direct current (DC). The advantages of increased number of phases of the AC supply are given as follows −
A smoother waveform is obtained on the DC side as the number of phases of the AC supply is increased.
The objectionable harmonics in the AC being reduced with the increase in the number of phases.
With the increase in the number of phases, the efficiency of the converter unit (rectifier) is also increased.
Therefore, because of these advantages 6-phase is preferred instead of 3-phase for rectification. In order to obtain three to six phase supply, the double delta connection is used.
The connection arrangement of the double-delta connection for three-to-six phase transformation is shown in the figure.
In the double delta connection of 3-to-6 phase transformation, three identical single phase transformers T1, T2 and T3 are used. The primary windings of the three transformers can be connected in star because the two secondary windings are connected in delta and provide required path for third harmonic currents. Each transformer has its secondary winding split into two equal sections. One set of three secondary windings r1r2, y1y2, b1b2 being connected in delta by connecting starting end r1 of the first winding to the finishing end y2 of the second winding, starting end y1 of the second winding to the finishing end b2 of the third winding and the starting end b1 of the third winding to the finishing end r2 of the first winding. The other set of three secondary windings r3r4, y3y4, b3b4 being connected in reverse delta by connecting finishing end r4 of the first winding to the starting end y3 of the second winding, finishing end y4 of the second winding to the starting end b3 of the third winding and the finishing end b4 of the third winding to the starting end r3 of the first winding. The phasor diagrams of the primary and secondary windings are shown in the figure below.
The main advantage of the double-delta connection is that, it has good harmonic elimination. But, the main problem associated with the double-delta connection is, it is not suitable for rectifier circuits, due to the absence of the secondary neutral.
Therefore, a true 6-phase supply is obtained when the six terminals are connected to a 6-phase load.