50 and 60 Hertz, 230 Volt AC
Why do we use 50 or 60 Hz?
The choice of the frequency is not as arbitrary as one might think. Between 1885 and 1900, a diversity of frequencies was used in the United States: 140, 1331/3, 125, 83 1/3, 66 2/3, 60, 50, 40, 33 1/3, 30, 25, and 16 2/3 Hz [5–8]. Each frequency had its own field of application. The power frequency finally came out at 60 Hz in North America, Brazil, and Japan and at 50 Hz in most of the other countries. Nowadays, 162/3 (Europe) and 25Hz (North America) are in use for railway applications, and 400 Hz is a popular frequency on board of ships, airplanes, and oil rigs.
A too low frequency, such as 10 or 20 Hz, is useless for domestic lighting as the human eye records this as flicker. On the other hand, the frequency cannot be too high as:
- The hysteresis losses in the transformer core increase in proportion to the frequency while the eddy current losses increase in quadratic proportion to the frequency.
- The capacitive reactance of cables and transmission lines increases (X = −1/ωC).
- The inductive reactance and the related voltage-drop, increases(X=ωL).
- The electromagnetic interference with the radio traffic will grow.
Yet there is also an advantage in using a higher power system frequency: the power-to-weight ratio of transformers, motors, and generators is higher. In other words, the components can be smaller, while the power output is the same. The formula of Esson gives a generalized expression for the power of an electrical machine:
K the “output coefficient” [J/m3], which depends on the type of machine, the type of cooling, and the magnetic material used. D the diameter of the armature [m]l the axial length of the armature [m] and n the rotational speed of the machine [1/s]
From Equation 1.13 we see that when we increase the rotational speed, by choosing a higher system frequency, the dimension of the machine can be smaller for the same output power.
AC versus DC systems
The choice for AC systems over DC systems can be brought back to the “bat- tle” between Nikola Tesla (1856–1943) and Thomas Alva Edison (1847–1931). Edison managed to let a light bulb burn for 20 hours in the year 1879. He used a 100 V DC voltage and this was one of the main drawbacks of the system. At that time a DC voltage could not be transformed to another voltage level, and the transportation of electricity at the low voltage level of 100 V over relatively short distances already requires very thick copper conductors to keep the voltage drop within limits; this makes the system rather expensive. Nevertheless, it took quite some time before AC became the standard. The reason for this was that Edison, besides being a brilliant inventor, was also a talented and cunning businessman as will become clear from the following anecdote. Edison tried to conquer the market and made many efforts to have the DC adopted as the universal standard. But behind the scenes he also tried hard to have AC adopted for a special application: the electric chair. After having accomplished this, Edison intimidated the general public into choosing DC by claiming that AC was highly dangerous, the electric chair being the proof of this! Eventually AC became the standard because transformers can quite easily transform the voltage from lower to higher voltage levels and vice versa.
Nowadays, power-electronic devices make it possible to convert AC to DC, DC to AC, and DC to DC with a high rate of efficiency, and the obstacle of alternating the voltage level in DC systems has disappeared. What determines, in that case, the choice between AC and DC systems? Of course, financial investments do play an important role here. The incremental costs of DC transmission over a certain distance are less than the incremental costs of AC, because in a DC system two conductors are needed whereas three-phase AC requires three conductors. On the other hand, the power-electronic converters for the conversion of AC to DC at one side, and from DC to AC at the other side, of the DC transmission line are more expensive than the AC transmission terminals. If the transmission distance is sufficiently long, the savings on the conductors ove come the cost of the converters, as shown in Figure 1, and DC transmission is, from a capital investment point of view, an alternative to AC.
The following are a few of the examples of high-voltage DC (HVDC) applications.
• Long submarine crossings. For example, the Baltic cable between the Scandinavian countries and Germany and the 600 km cable connection between Norway and the Netherlands (the NorNed Cable Project).
• Asynchronous interconnection to interconnect networks that operate at different frequencies. For example, the HVDC inter-tie connection between the 50 Hz, 500 kV Argentinean system and the 60 Hz, 525 kV Brazilian system.
• Asynchronous interconnection to interconnect networks that operate at the same frequency but cannot be connected by means of AC due to stability reasons or operational differences. For example, the Scandinavian system is asynchronously connected to the western continental European system; the same applies for the US Eastern Interconnection and the US Western Interconnection.
Also in our domestic environment DC systems are present as the majority of our electronic equipment works internally with a DC voltage: personal computers, hi-fi equipment, video, DVD players, the television, and so on.