Key Takeaways

• According to environmental conditions, geography, sensitivity, and cost, transmission lines are either made overhead or underground. 

• The allowable voltage range in medium transmission lines ranges from 20 to 100 kV.

• Low tension cables are used for voltages up to 1kV in underground transmission line systems.

Transmission lines are the connectors running between generating stations and distribution stations

Transmission lines are the connectors running between generating stations and distribution stations. Transmission lines carry high voltages from the generating stations to primary transmission stations, secondary transmission stations, primary distribution stations, and secondary distribution stations.

These lines are classified based on their location (overhead or underground), length, and voltage rating. Among these three characteristics, understanding how the classification of different types of transmission lines based on voltage works is particularly important for choosing the right cable for a given voltage level. Outside of characteristics like power of distribution lines and transmission cables, from a design perspective there are also features like characteristic impedance, propagation delay, induction, and reflected waves among other transmission line effects to track. 

Let’s explore two classifying characteristics, the location of the line and the voltage rating, and see how they relate to each other. 

Overhead and Underground Transmission Lines 

Transmission lines can either be located overhead or underground.

Overhead transmission lines are bare conductors above the ground level, supported by pylons and poles. The major parameter classifying overhead transmission lines is their length. For each length classification of overhead cables, there is a maximum voltage limit beyond which they are not permitted.

Underground transmission lines are insulated cables that are buried under the ground inside vaults and trenches. Voltage levels and insulation classify underground cables. There is a specific type of underground cable for each class of voltages.

When deciding on whether a transmission line should be overhead or underground, environmental conditions, geography, the line’s sensitivity, and costs should be taken into consideration. 

Types of Transmission Lines Based on Voltage

Both overhead and underground transmission lines have subclassifications based on voltages.

Overhead Transmission Lines

1. Short transmission lines - In short transmission lines, the length is within 50km and the voltage is limited to less than 20 kV. In short transmission lines, the effect of line resistance and inductance is more predominant than capacitance. 
2. Medium transmission lines - These lines have an overhead cable length of greater than 50km and less than 150km. The allowable voltage ranges from 20 to 100 kV. The analysis of medium transmission lines considers the three lumped line constants: resistance, inductance, and capacitance. 
3. Long transmission lines - Overhead transmission lines with lengths greater than 150km and voltages above 100kV form long transmission lines. Line constants are considered distributed elements in the analysis of long transmission lines. 

Underground Transmission Lines 

Unlike overhead cables, underground cables consist of one or more conductors with insulation and protective covering over it. The basic construction of underground transmission lines consists of parts such as the core or conductors, insulation, metallic sheath, bedding, armoring, serving, etc. 

There are several types of underground cables available on the market. Selecting the appropriate underground cable involves considering the working voltage and service requirements.

The classification of underground cables is done in two ways:

1. Classifying based on the voltage for which the underground cables are manufactured.
2. Classifying based on the insulation used in the cable’s construction. 

The table below gives the classification of underground cables based on voltages.