Due to the high power, transformers operate at relatively high voltages (110 kV or more). Electricity is usually transmitted over long distances by overhead lines. Underground lines are used only in densely populated urban areas, due to the high cost of commissioning and maintenance, as well as the production of additional reactive power on such lines.

Today, voltage transmission lines mostly include lines with a voltage of more than 110 kV. Lower voltages, such as 33 or 66 kV, are seldom used to power light loads over long distances. Voltages below 33 kV are commonly used to distribute electrical energy. Voltages greater than 230 kV are referred to as "extra high voltages" because most of the equipment required at these voltages is completely different from low voltage equipment.

Network input power:

In power plants, electrical power is generated at a relatively low voltage (eventually 30 kV) and then by substation transformers depending on the length of the path and other network considerations up to a voltage between 115 to 765 kV (in Iran this voltage is usually 400 kV) Increases to allow it to be transported over long distances.

Transmission network output:

As transmission lines approach cities and population centers for safety, the voltage drops in several stages. The voltage reduction stages in standard Iranian networks are 400/3030 kV, 230/132 kV, 132/63 kV and 63/20 kV, respectively. In the final stage or distribution stage, voltage distribution transformers are reduced from 20 kV to power consumption or 231/400 volts. In other countries, the voltage of consumers is between 100 and 600 volts.

Limitations:

The amount of transferable power in a transmission line is a limited value, and this limitation varies especially with respect to the length of the transmission line. For a short transmission line, the heat generated by the passage of current creates a constraint because the higher the temperature of the wires, the more they bend and get closer to the ground, which can eventually be dangerous. Conductors may also be affected by the passage. Melt the high flow.

For medium length transmission lines (about 100 km) there is more limitation in terms of voltage drop along the line and in long transmission lines the most important issue is maintaining stability in the network. The angle between the phases in a three-phase system is a constant value that too much change in a part of the network can lead to instability in the entire electrical network and during very long transmission lines the phase difference changes according to the power and output of the network. It will limit the amount of transferable current in a long transmission line. To improve the power factor along transmission lines, power factor correction equipment such as capacitors are used. In HVDC transmission lines, there is no limit on the power factor of the line and the only limit is the voltage drop and joule losses of the line.