Transmission Line

Transmission lines transmit bulk electrical power from sending end stations to receiving end stations without supplying any consumers route. It transmits the wave of voltages and current from generating station to various distribution substations. The transmission system of an area is known as grid. The different grids are interconnected through tie lines to form a regional grid and different regional grids are further  interconnected to form a national grid.

Transmission Lines

HISTORY

The first transmission of electrical impulses over an extended distance was demonstrated on July 14, 1729 by the Physicist Stephen Gray. However the first practical use of overhead line was in context of telegraphy. By 1837 experimental commercial telegraph systems ran as far 13miles. Electric power transmission was accomplished in 1882 with the first high voltage transmission between Munich and Miesbach. In 1912 the first 110kV overhead power line entered service followed by the first 220kV overhead power line in 1923. In 1920s RWE AG built the first overhead line for this voltage.

TRANSMISSION VOLTAGES

The generation and transmission of electrical energy has been increasing at a tremendous rate, throughout the world during the past many decades. The increasing need of transmitting and greater amounts of power has led to a continuous increase in transmission voltages. A high voltage transmission line is used to transport large amounts of electricity over long distances. In context of Nepal usually, lines over 110kV (such as 132kV 220kV, 400kV) are used for long distance and are called high voltage transmission lines. In contrast, lower voltage lines of 11kV or 33kV are used for shorter distances and are called distributions line. Voltages between 300kV and 765kV are termed as Extra High Voltages (EHV) and those greater than 765kV are classified as Ultra High Voltages (UHV). 

TRANSMISSION LINE CLASSIFICATION

The classification of the transmission lines depends on its voltage and the length of the conductor. 

1. AC Transmission Line

The transmission line has resistance R, inductance L, capacitance C and the shunt or leakage conductance G. These parameters along with the load and the transmission line determine the performance of the line.

- Short Transmission Line

If the line not more than 80 KV or if the voltage is not over than 66 KV then the line is known as the short transmission line. The capacitance of the line is governed by their length. The effect of capacitance on the short transmission line is negligible.

- Medium Transmission Line

The line which is ranging from 80 to 240 km is termed as a medium transmission line. The capacitance of the medium transmission line cannot be ignored. The capacitance of the medium transmission line is considered to be lumped at one or more point of the lines.

• Pi – model
• T – model

~ Pi Model of a Medium Transmission Line

In nominal Pi model, it is assumed that the half of the capacitance concentrate at the each end of the line.

Pi model of Medium Transmission Line

~ T Model of a Medium Transmission Line

In T model, it is assumed that the capacitance is concentrated at the center of the line.

T model of Transmission Line

- Long Transmission Line

The line having a length more than 240 km is considered a long transmission line. All the four parameters (resistance, inductance, capacitance, and leakage conductance) are found to be equally distributed along the entire length of the line. 

Long Transmission Line

2. DC Transmission Line

The DC transmission is mainly used for the bulk power transmission. For long distance transmission, the DC is less expensive and have low electrical losses. The cost of the DC transmission systems is higher for short distance transmission line because it requires more convertible equipment as compared to an AC system.

- Overhead Transmission Line

 It is a structure used in electric power transmission and distribution to transmit electrical energy across large distances. It consists of one or more non insulated electric conductors suspended by towers or poles. 

Overhead Transmission Line

Advantages    

• High power can be transmitted over long distances.
• Fault locating, maintenance  is easier.
• Extension or joining on overhead lines can be performed easily and also facilitates easy replacing.

Disadvantages

• As it is exposed to the surroundings the safety risk is high.
• A continuous pathway for the line creates obstructions. 
• Vulnerable to lightning strikes.    
• Right of way problems are frequently occuring. 

- Underground Transmission Line

It is the replacement of overhead cables providing electrical power or telecommunications, with underground cables. This is typically performed for aesthetic purposes, but also serves the additional significant purpose of making the power lines less susceptible to outages during high wind thunderstorms or heavy storms. 

Underground Transmission Line

Advantages:

• Safer than overhead lines.

• They are not subjected to lightning problems and creates no obstructions.

Disadvantages:

• Difficult installation process through various geographic areas.

• High installation cost. 

•  Heating problems arises.

• Natural cooling as in overhead lines is not possible and special cooling provisions should be made.

• High complications in faults detections and maintenance.

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