Classification of Overhead Transmission Lines

Types of Transmission lines:

we need to transport the electrical power generated at the power generating plant to the points of utilization or load points Electrical power is produced at the power generating plants which are located far away from the load centres. For this purpose, two major systems of conductors are developed called the transmission system and distribution system.

There are two types of Transmission Lines:

Overhead Transmission line
Underground Transmission Line

1. Overhead Transmission Line:

Classification of Overhead Transmission Lines

An AC transmission line consists of three constant parameters Resistance(R), Inductance(L) and Capacitance(C) which are distributed along the length of the transmission line uniform R and L act as the series impedance of the transmission line while C and G(=1/R) act as shunt impedance An overhead power line is an electric power transmission line suspended by towers or poles. Since most of the insulation is provided by air, overhead power lines are generally the lowest-cost method of transmission for large quantities of electric power. Towers for support of the lines are made of wood (as-grown or laminated), steel (either lattice structures or tubular poles), concrete, aluminium, and occasionally reinforced plastics. The bar wire conductors on the line are generally made of aluminium (either plain or reinforced with steel or sometimes composite materials), though some copper wires are used in medium-voltage distribution and low-voltage connections to customer premises. as shown in the figure below.

Note- capacitance value is taken in transmission line depending upon the manner.

2. Underground Transmission Line:

Underground transmission lines are electrical or telecommunications cables that are installed beneath the ground. These types of electrical power transmissions have their own benefits, pitfalls and also applications where it is frequently used. The selection of the method mainly depends on the factors like cost, voltage, safety, application, etc. The transmission of electrical power can be done overhead rather than underground using different types of transmission cables. Sometimes, due to factors like physical, cost and environmental, the two types of cables are combined together for electrical power transmission.

Difference between Types of Transmission lines:

Characteristic	Overhead lines	Underground Cables
Initial cost	Less	For the same power transfer, the cost is double as compared to overhead lines
Public safety	Less	More
Maintenance cost	More	Less
Occurrences of faults	More	Less
Frequency of accidents	More	Very less
Appearance	Not good	Good looking
Identification of faults	Easy	Very difficult
Interference to communication lines	Affected	No effect
Voltage drop	More	Low
Damage due to lightning and thunderstorms	Affected	Free from lightning and thunderstorms
Jointing of wire/cables	Easy	Very difficult
Operating voltage	Used for any range i.e., 756kV or even higher	Up to 132kV
Charging current	Less	More (since the spacing between conductors is less)
Surge effect	More	Less