Although ac electric power is produced at relatively low voltages, it is sent through transmission lines at very high voltages (as high as 500 kV). The same power can be transmitted at different voltages because power is the product I_rmsV_rms. (For simplicity, we ignore the phase factor cos Ø.) A particular power requirement can, therefore, be met with a low voltage and a high current or with a high voltage and a low current. The advantage of the high-voltage/low-current choice is that it results in lower I²_rms R ohmic losses in the transmission lines, which can be significant in lines that are many kilometers long

The rms voltage from a power plant eventually needs to be stepped down from 12 kV to 240 V so that it can be safely introduced into a home. A high-voltage transmission line allows a low current to be transmitted via a substation over long distances. Typically, the alternating emfs produced at power plants are “stepped up” to very high voltages before being transmitted through power lines; then, they must be “stepped down” to relatively safe values (110 or 220 V_rms) before they are introduced into homes. The device that transforms voltages from one value to another using induction is the transformer