The power plants (e.g. hydro, thermal, nuclear or geothermal) are usually built far away from the large cities. Therefore the electrical energy needs to be transported over long distances, from the point it is produced to point of it will be used (and not consumed). For the transfer pillars and high voltage cables are used.
When the electric current moves through cables, some energy is converted into heat. The more energy is converted into heat during the transfer the less useful energy reaches to the users. To reduce the losses, electrical energy is transported in the cables in high voltage. Transformers convert AC power produced by  the generators in the factories in high voltage (several thousand Volts) alternating electric current. This very high voltage electric current is suitable for transport over long distances because it has fewer losses (in the form of heat).

When the electrical energy reaches the point of use, or homes (factories are another category of users) should be converted into low voltage 220-240 Volt (110 Volt for USA). If you notice the columns of PPC, you will see that they are not all the same. Some of them have in the top some strange "equipment" and make noise. These machines are transformers that convert high voltage electricical energy that comes from factory to electrical energy suitable for domestic use (e.g. suitable for our homes).

Column of PPC with transformer
Column of PPC
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more info
© 2003, Despina M. Garyfallidou Athens-Greece