The power system mostly operates with alternating current (AC); the other possible mode, that is, by direct current (DC), is used for some specific applications. The amount of electric power, which is transmitted and used, results by the voltage multiplied by the current and is proportional to the amount of energy (through the time). The values of voltage and current in a point of the system are related by Ohm's and Kirchhoff's laws. In the alternating way, the current and the voltage are not constant over time like in the DC supply, but vary sinusoidally with time. These time functions oscillate with a frequency which is typically 50 Hz (e.g. in Europe) or 60 Hz (e.g. in the United States). These values result from a compromise between different techno-economic choices concerning the various system components.
Historically, at the beginning (end of 19th century) the electric power system was operated in DC. However, after the introduction of AC electrical machines (like power transformers, induction motors and synchronous generators) the operation of the system in AC became preferable. In fact, the utilisation of power transformers, which are able to change the voltage level in a smooth way, allows a power transmission and distribution in AC at higher voltages. Thus, at constant power the currents and then the electrical line losses (which are proportional to the square of the currents) can be reduced. To minimise the losses in the system, the aim is consequently to transmit power at the highest possible voltage compatible with techno-economic limits. This feature of power transformers, together with the lower maintenance required by these machines and their longer life, is very important for their utilisation. In fact, on one side, power generators are constrained (due to conductor insulation) to operate at a voltage level not higher than a certain value (generally 30 kV maximum). On the other side, the loads, depending on their constructive characteristics and also for safety reasons, have to operate at lower voltage. Then, power transformers step the voltage up from generation to transmission level and then step it down to distribution and utilisation levels.
Furthermore, AC rotating components such as induction motors and synchronous generators are undoubtedly more reliable and suitable for industrial applications and power generation, respectively, than the corresponding DC ones. Another reason which led to prefer the AC supply is that an alternating current in its sinusoidal variation crosses the zero value and can be more easily interrupted than direct current.