Have you ever wondered why power transmission lines sometimes have three cables or a set of three?
The law of electricity states that for electricity to flow through a conductor, the conducting material must provide a closed path for the current to flow. A closed path can be created with only two wires. Look at the plug of your computer or the plug of your TV. There are two metallic conductors.
Sometimes there is a third for ground.
Is it the Ground Wire?
No, it’s not the ground wire. There is a fourth separate wire for the ground. Usually it is thinner and distant from the set of three. The fourth ground wire is also used to prevent lightning from striking the main power cables.
The Polyphase system
The polyphase system is a system used for generating alternating current. The polyphase system is the most efficient type of generating process. It generally has multiple core windings in the generator which makes the system “have” multiple terminals.
The most commonly used polyphase system is the three-phase system. Thus, it explains why some electrical power transmission cables have three wires.
How does it work?
Inside the generator, there are three sets of coil windings. The difference in angle distribution of each coil is 120 degrees. So, the alternating current given off by each terminal has a difference of either +120 degrees or -120 degrees with respect to the ground.
This type of system allows the power grid to supply three different systems of circuits. One phase can supply a household, the other phase can supply a factory and the other phase can supply a mall. This can be done without compromising the delivered power to other phases.
The term phase used here denotes the combination of two wires. There are three phases in a three-phase polyphase system.
Another advantage of this system is that the power supplier will be able to use less wires to supply a given power grid. That means laying the wires will be cheaper. And there will also be less resistance in the system thus, reducing the voltage drop. This means that generating and transmitting power to households and industrial units is much cheaper.
So every time you see a set of three power cables, you can definitely say that they go directly from a power plant. That means they have not gone through any distribution units yet. The voltages in these wires are very high.