Why do we use delta-star in the transformer? The low voltage side of the main transformer is triangulated to eliminate the third harmonics. Prevent a large number of harmonics to the system, resulting in power grid voltage waveform distortion.

Delta-star connected transformers were widely used in low power supply distribution with the primary windings providing a 3-wire balanced load to the utility company while the secondary windings provide the required 4th-wire neutral or the earth connection.

the delta connection, the line voltage was equal to the phase voltage. The line current is equal to √3 times of phase current supply. The delta-connected 3ϕ transformer could be used for both balanced and unbalanced loading. In the delta connection, the 3rd harmonic component flows rounds the closed loop of the delta.

The delta/delta connection, as shown in Figure 1, was used for applications where only one 2nd voltage was required or where the majority of the load consists of 3-phase equipment, such as factories that have large 3-phase 480 V or 240 V motor loads or relatively small 120 V lighting and receptacle the loads.

3 conditions must be met before you can connect transformers in parallel. 1. The transformers must have the same primary and 2nd voltage ratings. If the voltage ratings of the transformers were not the same, large circulating currents would flow in both the primary and secondary windings.