key takeaways
Depending on environmental conditions, geography, sensitivity and cost, transmission lines are built either overhead or underground.
The permissible voltage range for medium-sized transmission lines is 20 to 100 kV.
Low voltage cables are used for voltages up to 1kV in underground transmission line systems.
transmission line
Transmission lines are the connectors that run between power stations and substations
Transmission lines are the connectors that run between power stations and substations. Transmission lines carry high voltage from power stations to primary transmission stations, secondary transmission stations, primary distribution stations, and secondary distribution stations.
These lines are classified according to their location (overhead or underground), length and voltage rating. Of these three characteristics, understanding how the classification of different types of transmission lines based on voltage works is especially important for choosing the right cable for a given voltage level. In addition to characteristics such as power distribution lines and transmission cables, from a design perspective, characteristics such as characteristic impedance, propagation delay, induced and reflected waves, and other transmission line effects need to be tracked.
Let's explore two categorical features, the location of the line and the rated voltage, to see how they relate.
Overhead and Underground Transmission Lines
Transmission lines can be located overhead or underground.
Overhead transmission lines are bare conductors above ground, supported by towers and poles. The main parameter for classifying overhead transmission lines is their length. For each length classification of overhead cables, there is a maximum voltage limit that must not be exceeded.
Underground transmission lines are insulated cables buried in basements and trenches. Underground cables are classified by voltage class and insulation. There is a specific type of underground cable for each voltage class.
Environmental conditions, geography, line sensitivity and cost should be considered when deciding whether a transmission line should be overhead or underground.
Voltage-Based Transmission Line Types
Both overhead and underground transmission lines have voltage-based sub-categories.
Overhead Transmission Line
Short transmission lines - In short transmission lines, the length is less than 50km and the voltage is limited to less than 20kV. In shorter transmission lines, the effect of line resistance and inductance is more important than capacitance.
Medium Transmission Lines - These lines have overhead cable lengths greater than 50 kilometers and less than 150 kilometers. The allowable voltage range is 20 to 100 kV. The analysis of dielectric transmission lines considers three concentrated line constants: resistance, inductance, and capacitance.
Long Transmission Lines - Overhead transmission lines with lengths greater than 150km and voltages over 100kV constitute long transmission lines. In the analysis of long transmission lines, the line constants are considered as distributed elements.
