Generally speaking, electricity is generated at power plants operated by power companies. Electricity from a power plant passes through the power lines of transmission towers, also called pylons, then the voltage is decreased, or stepped-down, at transformer substations before being used to power factories and our homes.
The transmission lines that transmit electricity are usually in the form of overhead power lines. The standard and most common structure is the double circuit transmission line which is characterized by having two independent circuits aligned vertically on either side of the structure with each circuit made up of three sets of lines for a total of six transmission lines.
As the figure illustrates, double circuit transmission lines have an equal number of lines on each side. Rather than sending a “different” kind of electricity, each side sends the “same” electricity. There is a simple reason for this: reliability.
This mechanism enables one side to continue sending power even if an accident occurs where transmission is impossible due to damage caused by a natural disaster, a bird or other wildlife, or by human error. In other words, using two circuits to transmit power increases reliability with aim of preventing prolonged, wide-scale power outages.
Also if you look carefully, you can see that there is an electric wire at in the highest part of the tower. This electric wire is called an optical ground wire, and it is not for conveying electricity, but it serves as an arrester to attract lightning. It minimizes the likelihood of direct lightning strikes to the transmission lines.
Electric utilities make various efforts to increase the reliability of power transmission, but still, power outages occur. In the next session, we’ll introduce some types of power outages—some of which may surprise you!