‎Three Phase Transformer‬...

Transformers used with three-phase power require three interconnected coils in both the primary and the secondary. These transformers can be connected in either a wye or a delta configuration. The type of transformer and the actual voltage depend on the requirements of the power company and the needs of the customer.
The following illustration shows the secondary of a wye-connected transformer and the secondary of a delta-connected transformer. These are only examples of possible distribution configurations, the specific voltages and configurations vary widely depending upon the application requirements.
Power, generated at a power plant and stepped up to a high transmission voltage, is brought to a local substation. Here, it is stepped down to a lower distribution voltage. When it reaches its final destination at a residential customer, it is stepped down to 240 volts. Only single-phase power is used in a typical residential application.