The voltage

The voltage or potential difference between two points of an electric field is the energy that is dissipated when moving a unit of electric charge between those points. Its unit of measurement is the volt (V). 

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Together with the frequency and intensity, voltage is part of the three basic properties of electricity transmission, which must be in constant balance from the point of origin to the point of destination.

According to current regulations, high voltage is considered that which is greater than 1 kilovolt (kV) and low voltage is considered that which is equal to or less than 1 kilovolt. However, in electricity jargon and in the press, other classifications or terms are used. Thus, in Spain, the range of legal high voltage up to 36 kV is generally known as medium voltage. Also, very high voltage is used as a term when referring to the highest levels of voltage of the transmission grid (400 kV in Spain).

Domestic consumption usually uses low voltage, which, in Spain, includes the levels of 230 and 400 V. In order to reach homes, the voltage is reduced from the level used in the transmission grid – 220 or 400 kV on the Spanish Peninsula and 66, 132 or 220 kV in non-peninsular systems – through several voltage levels in the distribution network, until it reaches the suitable voltage necessary for final consumption. Transformers are used to reduce or increase the voltage.

Electricity generation is incorporated into the grid at different voltages; however, the use of high voltages brings with it technical and economic benefits as it reduces losses during transmission, therefore most of the generation is either connected directly to the transmission grid or to the higher voltage levels of the distribution network.

The main connection levels of the generators are often associated with the voltage of the transmission grid, which is different for non-peninsular systems, whose smaller distances justify lower voltage levels for this function. Thus, in the Spanish peninsular system, most of the generation is connected at 220 or 400 kV. However, for non-peninsular systems that are smaller, the transmission grid reaches the level of 220 kV in the larger islands (Majorca, Gran Canaria, Tenerife) and stays at 132kV or even 66 kV in the rest of the islands, depending on their size.

The constant balance that the electricity system requires also imposes narrow margins on the voltage levels of the lines and substations, which must be constantly monitored.

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