What is an electric transmission line?
A transmission line is the channel or medium through which the transmission and distribution of electric power occurs. It consists of a metal structure that acts as a support for electrical conductors through which energy is transmitted over long distances.
Electrical transmission lines are high voltage and, along with substations, they make up the electric transport network. Energy supply is still possible through them even if there is some incident in the transmission process; this is because the energy can get there from another line that the system is interconnected with.
What elements make up a transmission line?
- Conductors: materials that serve as a means of transporting energy, from the beginning to the end of the line. They offer little resistance to the flow of energy so that it can circulate without difficulty. In a transmission line, the number of conductors will depend on the number of circuits per phase. Copper is typically used, though over time it has been replaced, given its lightness and the same resistance capacity.
- Support structures: these are usually reticular structures that support the overhead lines and cables of an electrical transmission line.
- Insulators: these elements mechanically fasten the conductors, keeping them isolated from each other and the ground.
- Ground wires: these are used in high voltage lines to protect the rest of the conductors against possible atmospheric incidents, such as lightning strikes.
What characteristics does a transmission line have?
Every transmission line has elements that affect the transmission capacity of the electrical system, such as:
- Resistance (R): opposition to the passage of an electrical current. In a transmission line, the material offers resistance to the passage of electromagnetic signals.
- Inductance in series (L): the opposition to the current change of a coil, which stores the electrical energy inside the electromagnetic field, affecting the surface part of the transmission line.
- Shunt capacitance (C): the transmission line’s ability to receive and store energy as it flows through it.
- Shunt conductance (G): the ability of the material used in the transmission line to withstand the passage of the signal. This property is the opposite of resistance.
Resistance and inductance in series occur along the transmission line, while the shunt capacitance and conductance occur between the conductors.
What types of transmission lines are there?
Transmission lines can be classified into two types, according to their electrical balance:
- Balanced: those where each conductor carries a current, one for signal transmission and one for return. The signal moving along the cable is measured as the difference in potential between the conductors. Any cable can operate in a balanced way as long as none of them is grounded.
- Unbalanced: one of the conductors is grounded, making this the opposite of the previous model. While one is at the ground potential, the other is at the signal potential.
- According to the geometry of their cables, transmission lines can be classified as:
- Single wire: a single line where all the conductors of an electrical section are included.
- Double transmission line: a transmission line where the distance between parallel conductors is made possible by an insulating material of low electrical conductivity.
- Coaxial: these allow high-frequency electrical signals to be transported between concentric conductors: a central one, in charge of carrying the signal, and an external one in the form of a tube, which serves as a ground reference and current return.
- Radiant cables: coaxial cables with small transverse slots in the external conductor that allow energy to radiate outside the cable, functioning as an antenna in the path of the electric current.