After leaving the generator, the electricity travels to a substation equipped with transformers. A step-up transformer increases the voltage of the electricity, giving it the force it needs to travel efficiently through high-voltage distribution lines. Finally, at its destination substation, a step-down transformer reduces the voltage to a level needed to operate customers' lights, equipment, and appliances.

Figure 4 Large power system An electrical power plant and transmission system is an example of a very large power system.