Difference between Transmission Lines and Distribution Lines

Differences Digital Electronics Electronics & Electrical

Electrical power is produced at the generating stations which is then transported to the points of utilization with the help of electric power lines. The electric utility companies install two different types of electric power lines viz. transmission line and distribution line for conveying the electric power from power plant to consumer’s premises for utilization.

This article is meant for describing the key differences between transmission line and distribution line. We also briefly explained that what the transmission line and distribution line are.

What is a Transmission Line?

Transmission lines are the power lines that carry the electric power at high voltage from generating station to a number of substations. Generally, the transmission lines carry the electricity over long distances such as across the state. The typical transmission voltage is more than 69 kV.

The transmission lines are usually supported by tall steel towers and uses a three-phase, three-wire AC system for power transmission. The transmission lines are segmented in two parts that is primary transmission line (transport power from power plant to the receiving station) and secondary transmission line (which transports electric power from receiving station to the substations).

What is a Distribution Line?

The power lines that are used for carrying electricity from substations to the consumers are known as distribution lines. Generally, the distribution lines carry the electricity over short distances (within a city). The support to the distribution lines is usually provided by the wooden poles, RCC poles, steel poles, etc.

In practice, the distribution lines may be single-phase or three-phase. The AC system adopted at distribution stage is 3-phase 4-wire system, where three wires are provided for lines conductors and one wire is for neutral. The distribution lines carry the electric power at lower voltage (less than 69 kV).

The distribution lines are also of two types viz. primary distribution lines (carry power from substation to distribution station) and secondary distribution lines (carry power from distribution station to the end consumers).

Difference between Transmission Lines and Distribution Lines

Both transmission and distribution lines are used for carrying the electric power. However, there are many differences between distribution and transmission lines which are highlighted in the following table −

Basis of Difference	Transmission Lines	Distribution Lines
Definition	The high voltage power lines used for carrying electricity from generating station to the substations are known as transmission lines.	The low voltage power lines used for carrying the electricity from substations to the distribution stations and end users are known as distribution lines.
Length	The length of transmission lines is more, hence carry electricity over long distances.	The length of distribution lines is less, i.e. they carry electricity over short distances.
Voltage level	The voltage level of transmission lines is very high, usually more than 69 kV.	The distribution lines operate at lower voltages, usually less than 69 kV (primary distribution), 440 kV (3-phase distribution), 230 kV (1-phase distribution).
Phase system	The transmission lines always use three-phase system.	Distribution lines can be three-phase or single phase.
Number of wires	Transmission lines always use only 3 phase wires.	Distribution lines use 4-wires, where 3 wires for phases and 1 for neutral.
Current carrying capacity	Transmission lines have high current capacity	The current carrying capacity of distribution lines is low.
Line conductor size	The size of conductors used for transmission lines is more (thicker conductor used).	The distribution lines uses thinner conductor.
Line support	The transmission lines usually supported by constructing high length steel towers.	The support to the distribution lines is usually provided by wooden poles, RCC poles and rail poles, etc.
Power handling capacity	The transmission lines have comparatively high power handling capacity.	The power handling capacity of distribution lines is less than that of transmission lines.
Directionality	Transmission lines are usually bi-directional, i.e. the electricity can flow in both directions. It is a required feature for balancing of the power grid.	The distribution lines are traditionally made mono-directional, i.e. electricity can flow only in one direction.
Line maintenance	Since the transmission lines require operating more actively than distribution lines. Therefore, they require more maintenance which must be done through automatic and manual interventions.	The distribution lines require less maintenance as compared to transmission lines.
Applications	Transmission lines are used for carrying electric power from power stations to the substations.	Distribution lines are used for carrying the electricity from substations to load points such as home, businesses, industries, etc.

Conclusion

In the above table, we have enlisted all the major differences between transmission line and distribution line. The most significant difference is that transmission lines operate at high voltages and they are used to transmit the power from the generating station to the substations, whereas distribution lines operate at low voltages and transport electric power from substations to the consumer’s loads.