How does the voltage affect the current?

Why does current transfer take place at high voltages?

In most cases, electricity is not used where it is generated. It often has a long way to go before electricity flows out of our sockets. The technological challenge lies in keeping the energy loss during power transmission as low as possible.

Electric current consists of the movement of electrons in electrical conductors. The electrons can transfer their energy in two different ways. In the case of direct current, the electrons always flow in one direction: They move through a line to the user, give off part of their energy there and then flow back to the power generator via a second line. In the case of alternating current, on the other hand, the electrons change their direction - around fifty times per second in Europe. In this way, too, the electrons can give their energy to the user.

If a current flows, part of the energy of the electrons is converted into heat due to the so-called ohmic resistance of the lines. This principle is quite useful - for electric heating, for example. For electricity transport, however, it means that the electrical energy at the end of the line is less than at the beginning. In order to keep heat losses as low as possible, the flow of electricity is reduced - but this means that less electrical energy is transported. This effect can in turn be compensated for by increasing the voltage of the current [for details see box below the text].

In order to be able to achieve the required high voltages at all, scientists developed the transformer towards the end of the 19th century. With a transformer, on the one hand, alternating current with high voltage can be generated, which on the other hand can be transformed back down to a lower voltage at the destination. It was only with this technology that electricity grids could be built over ever greater distances, especially in America and Europe. Over time, transformer technology has been further developed and our power grid is still mainly based on alternating current.

However, the transport of energy with the help of alternating current has some disadvantages: In addition to heat losses, there are three other phenomena through which electrical energy is lost - caused by capacitive resistance, inductive resistance and the so-called skin effect. The first phenomenon is caused by the rapid change in the direction of the current, which has a similar effect to the charging and discharging of a capacitor. This effect becomes noticeable as an additional resistance in the circuit - as a capacitive resistance.

In addition, electrical currents always generate a magnetic field around them. This is constantly built up and reduced depending on the frequency of the alternating current, which in turn becomes noticeable as an inductive resistance. These two effects increase with the length of the electrical lines, until they finally make the transmission of alternating current uneconomical over longer distances. On the other hand, the skin effect - the third phenomenon - is caused by the fact that the electrons move almost exclusively on the surface of the power line due to the rapid change of direction. This behavior requires ever thicker cables or several parallel lines, which is also uneconomical for long transport lengths.

Such sources of loss do not occur when the current is transmitted with direct voltage. That is why many scientists are currently researching what is known as high-voltage direct current transmission. To do this, the alternating current must first be converted into direct current at so-called converter systems in order to be converted back into alternating current at the destination. At the moment, however, it is only worthwhile to transport electricity with DC voltage from great distances - for example, to transmit electricity from offshore wind farms to the mainland.