The first ever public electric power distribution system appeared in the late 19th century and mostly supplied direct currents, with multiple output voltages, or under the form of alternating current, which reverses the polarity of the current every 20 milliseconds. However, the energy demand and the advantages that derive from the use of standard voltage outputs quickly impose the alternating current as the preferred type for production, transport and distribution.
Most of the electric energy production process takes place in power plants burning fossil fuel, such as coal, natural gas or crude oil, which accounts for the massive amounts of greenhouse gases emitted daily in the Earth's atmosphere. Heat produced during fuel burning is used to evaporate large amounts of water and create steam that powers a steam turbine, turning the energy extracted during burning fuel into mechanical work. By connecting a mechanical generator to the turbine's drive shaft, the mechanical work can be turned efficiently into electric energy.
However, the electric energy cannot be directly supplied to the consumer in this form, thus it must undertake certain transformations in order to meet the required standard, which state that the electric current must have parameters ranging in certain imposed values. For example, in Europe, most of the countries use alternating electric current that has a nominal voltage of 220 volts and an oscillating frequency of 50 hertz, while the United States prefer an electric current having 110 volts, and a frequency of 60 hertz.
The frequency of the alternating current is usually determined by the construction of the generator, however voltage of the current represents a key aspect in the transport and distribution of the electric energy, thus it is uneconomical and unpractical to build generators that produce different voltages.
Instead, transformers are used to vary the voltage of the electric current in the required domain. Transformers consist basically of two electric circuits formed of two coils, separate electrically by an inductive coupling used to transfer energy between the two. There are two primary types of electric transformers: high-voltage transformers and low-voltage transformers. High-voltage transformers are used to increase the voltage output, while the low-voltage ones are used to decrease it.
The electric energy transport is usually greatly affected by the loss of power in long conductors, as a result of the presence of electrical resistance. Even the best electrical conductors having low electrical resistance can produce enormous power loss over relative small distances. To solve this problem, the electrical current's voltage is being increased with the help of high-voltage transformers, meaning that for a voltage increase of a 10 factor, the current intensity will drop by the same amount, thus reducing the power loss by over one hundred times.
Most of the outlets present in our houses have a mono-phase current configuration, consisting of two electrodes, one used as ground and the other to supply the alternating electric current. However, the transport systems mostly imply the use of three-phase currents that consist of four metal conductors, three for alternating electric current supply, while most of the time the fourth conductor is used as ground, though this is not necessarily required.
The electric currents through the three-phase metal system are out of phase by 120 solid degrees and voltages between the lines usually measure an average of 220 volts.
The use of fossil fuel for electric energy production has determined massive emissions of greenhouse gases; however, in the last decades, attention shifted towards alternative energy sources, such as wind power, solar power and, last but not the least, nuclear power that is quickly becoming the base of electric power production.