Transmission System

Nowadays, “electric energy” is the prime essential of life. Besides, electric energy development has pushed forward the changes of several dimensions, such as industry, medicine, public health, transport, education, tourism, and communication, etc.

The route of electric energy development has continued to advance further with mechanism of quality creation which prompts to give electric light from several fuel sources connecting through the lines of “transmission system.”

Transmission system plays an important role in supplying electric energy from producers to end users, enabling people to access basic utilities widely. Transmission system has brought prosperity to all regions of the country to enhance economic stability, which leads to reliability in investment in all sectors.

Electricity Generating Authority of Thailand (EGAT) is a state enterprise responsible for acquiring, generating, controlling the power system, and supplying electricity through EGAT transmission line network system across the country by connecting the transmission lines from the power sources to the supplying system of the Metropolitan Electricity Authority (MEA) and Provincial Electricity Authority (PEA) in which the electric voltage will be adjusted properly before reaching the end users of all sectors.

Transmission Line

High voltage transmission line of EGAT is like an artery of Thai electricity system which has to be taken care of for security and availability all the time. If the transmission line is cut off or defected even only one point, the outage or blackout may occur in a wide area.

Transmission line is therefore an important component of the power system being in charge of supplying electricity from the power plants to electricity center and connecting several transmission systems together in order to supply electricity from one system to another system or transferring electric energy to each other in case of emergency or the peak hours are different from each other. It is necessary that electric poles and transmission lines have to be in a good condition and available all the time.


As most power plants are constructed far away from community, electricity transmission from the remote area will face the problem of voltage drop, high power loss, and less amount of electricity supply. On the contrary, if supplying electricity with high voltage, the power loss will be lower and the amount of electricity supply will be more. Therefore, having a substation to change the voltage is necessary for reducing the power loss by increasing the voltage to be higher so that more amount of electricity can be sent in a long distance, while a substation will decrease the voltage of electricity when supplying to the city for convenience and safety for use.

A substation is the place equipped with circuit breaker, voltage transformer, electric flow control device, and the point where transmission lines at several points are connected. The protective equipment for the distribution system is installed to cut off the transmission line which has a short circuit from the power supply.


A switchyard will transform the voltage generated from a power generator to be higher in order to transfer it to the substation in the remote area to reduce power loss in the power system which comprises a power transformer to increase the voltage and electrical protection system.


As the transmission line has a certain dangerous space which electric power can jump across; therefore, it needs to have a cable gripper made from insulated material called “insulator” in an appropriate number of layers of insulators. The number of layers of insulators will indicate the level of voltage too.

Insulators are devices used to support high voltage transmission line wires. Insulators perform the function of insulating and protecting against leakage current to the structure of electric pole or short circuit to the ground. EGAT insulators are made of ceramic or glass with special characteristics of having very good electrical insulating properties. They are strong and durable. But to use them very long time outdoor, they are dusty, which can cause electric conductivity. Therefore, insulators have to be regularly cleaned.    

Types of high voltage transmission towers

Have you ever noticed that when we travel on the roads in the city or upcountry we will see different types and sizes of high voltage transmission towers? Each type and size of high voltage transmission towers has different function depending on the level of voltage, the number of transmission lines used in the system, and location of high voltage transmission towers installed so that the power distribution is secure, resulting in power system stability without power dropout or blackout.

The power system with high voltage is able to distribute electricity to the long distance efficiently. However, due to the high voltage, it has a very high voltage compared to the low voltage used by ordinary household appliances and buildings (220V); the high voltage is therefore able to jump through the air or electrically insulating objects or living things without touching the wire. (We can see birds have an electric shock without standing on electric wire.) With the higher voltage, the greater the distance that electricity can jump over. 

Nowadays, there are 4 types of high voltage transmission towers used by EGAT ranging from 69 kV (used very few), 115 kV, 230 kV, and 500 kV. In the future, if the demand for electricity is higher and electricity has to be distributed at longer distance, the power voltage may be more than 500 kV.

The transmission tower is designed for different types to be suitable for power voltage and the number of transmission lines in EGAT system. At present, 3 types of them are used, namely concrete, steel, and monopole. Their function depends on the condition and appropriateness of locations where the transmission tower is installed.

Announcement of Electricity Generating Authority of Thailand on Requirements for Safety in Electrical Wiring Areas

  1. Forbid anyone to do any act which may be dangerous to the electrical system, for example,
  • Bringing materials, equipment or machinery such as cranes, forklifts, loaders, and excavators near the high voltage transmission line in the distance of at least 4.00 meters
  • Burning sugarcane fields, paddy fields, forests, or any other materials in the electrical wiring boundary.

2. Construction of buildings, houses, or other structures of any kind is prohibited in the electrical wiring area.

3. Planting any trees or crops in the electrical wiring area as follows:

          3.1 In the area, the location of the transmission towers, and the area surrounding the base of the towers within a distance of 4 meters from the transmission lines; no trees or crops of any kind shall be planted.

          3.2  Planting trees or fruit trees which when fully grown will have a height of more than 3 meters in the electrical wiring area other than Clause 3.1 is prohibited.

          3.3 Sugarcane planting is prohibited in the electrical wiring area of the power transmission lines of voltage level 500,000 V.

4. Any action to change the ground conditions in the electrical wiring area, such as adjusting the ground to be higher, digging the soil or a well, or road construction must obtain written permission from EGAT. For the construction or something else created or made, trees or crops planted without complying with the conditions or without permission from EGAT, EGAT has the power to demolish, destroy, or cut them as appropriate without paying compensation.

Requirements of the Width of Electrical Wiring Field

Transmission line voltage
Distance from the center
of the tower to each side
Including electrical wiring
699 m18 m
11512-25 m24-50 m
23020-25 m40-50 m
50035-40 m70-80 m

If you wish to request for permission for construction or adjusting the area in the electrical wiring area, or you have any doubts or find any action which may be harmful to power distribution system, dangerous to life or to property of the nation, please notify the staff of EGAT at the nearest or EGAT Information Center 1416

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