Power transformers:
Any transformer are used in the 60-Hz utility line, intended to give a certain rms alternating current voltage for operation of electrical circuits, is a power transformer. The size of Power transformers ranges from smaller than a tennis ball to as big as a room.
At the generating plant
The largest transformers can be used right at the place where electricity is getting generated. Not astonishingly, high energy power plants have bigger transformers which develop higher voltages than the low energy, local power plants. These transformers should be able to handle not huge voltages only, but also large currents. Their primaries and secondaries should withstand the product EI of the volt-amperes which is equal to power P ultimately delivered by transmission line.
When electrical energy should be sent over the long distances, very high voltages are used. This is because, for the given amount of power ultimately dissipated by loads, the current is lower when voltage is higher. Lower current translates into the reduced loss in transmission line. The formula P = EI, w=here P is power in watts, E is voltage in which is given in volts, and I is current in amperes. If you make the voltage 10 times larger, for the given power level, then the current can be reduced to 1/10 as much. The ohmic losses in wires are proportional to the square of current; also remember that P =I2R, here P is power in watts, I is current in amperes, and R is resistance in ohms. Engineers cannot do much about the wire resistance or power used by the loads, but they can adjust the voltage, and so the current. Increasing voltage 10 times will cut current to 0.1 its previous value. This will render I2R loss (0.1)20.01 (1 percent!) as before.
For this very reason, regional power plants have massive transformers which are capable of generating hundreds of thousands of volts. A few can generate 1,000,000 V rms. A transmission line which carries this much voltage requires gigantic insulators, at times several meters long, and tall towers.