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How to Determine the Voltage Level of Overhead Lines?

When looking at overhead lines, it is very important to distinguish between voltage levels. The voltage level approximately determines the amount of energy and the distance to be transmitted. 

How do you know the voltage of a transmission line? It is mainly determined by the number of conductor splits, the number of insulators and the height of the towers.


Number of conductor divisions


For each of the three phases of the ABC conductor, are divided into several strands. The reason why a phase of wire is split into several strands is to expand the "equivalent diameter" of the wire. 

The use of several strands of thinner wire to form an approximate circle can reduce the loss of power during transmission.


Ultra-high-voltage 750 kV transmission lines (líneas de transmission de 750kV) typically consist of six wires. 500 kV lines typically have four wires. But more lines are now split into six wires, especially around large cities, probably because of the high transmission power. 

The 220 kV cables are usually split into two wires. As for 110 kV cables and below, there is no need to split the wires. It is sufficient to use only one wire. Because corona is no longer serious at this voltage level. However, in some places you can also see transmission lines with four splits for 220 kV and two splits for 110 kV.


Number of insulators


What is an insulator? An insulator is a special type of insulating fittings, usually made of glass or ceramic. Its shape is saucer-shaped and one saucer counts as one insulator. Insulator strings are strung on several pans and serve to insulate the tower conductor. The approximate thickness of one insulator is 15 cm and 7 pieces are almost one meter.


Number of insulators corresponding to voltage levels (in general):

750 kV, 32

500 kV, 23-25

330 kV, 17

220 kV, 13

110 kV, 7

66 kV, 5

35 kV, 3


In case of high altitude or heavily polluted areas, the number of insulators has to be increased. Basically 1 insulator can withstand 1-15,000 volts. 

However, as the voltage level increases, the stronger the non-linearity of voltage bearing. In fact, the number of insulator pieces may vary greatly from region to region and from environment to environment. Of course, different types of insulators require different numbers of pieces.


Height of the pole tower


In addition to the number of conductor splits and insulator pieces, the voltage level can also be judged by the height of the pole tower. 

Although the design regulations do not specify how high the towers should be, they specify in detail the minimum distance of the transmission conductors (conductors de transmisión) from other objects. This is because the cables of transmission lines are subject to sagging. 

Especially when the weather is hot and the electricity load is large, the wire thermal expansion and contraction, sagging more powerful.


In residential areas the minimum distance of conductors to the ground are:

35 to 110 kV, 7 meters

220 kV, 7.5 m

330 kV, 8.5 m

500 kV, 14 m

750 kV, 19.5 m


When considering the conductor arc sag and insulator length:


110 kV, about a dozen meters

220 kV and 330 kV, about two dozen meters

500 kV, thirty to forty meters

750 kV, more than fifty meters

1000 kV, seventy to eighty meters.


Overall, the easiest way to distinguish voltage levels is to rely on the number of conductor splits, the most accurate way to determine the number of insulator pieces, and the most intuitive way to determine the height of a pole tower.