The choice of high power frequency depends on three factors; two that change over time and one that does not change:
A specific application.
Technology.
Basic laws of physics.
Let's start with # 3. The efficiency of telephone transmission decreases with increasing volume for two main reasons:
The skin effects force the AC currents to the top of the conductor.
Cables emit energy efficiently in high frequency waves. This is good for building antennas, not so good for building a transmission line.
So from a basic physics point of view, the frequency of the appropriate AC power line is zero Hz, that is, DC.
DC also has a peak-to-RMS volume ratio of 1: 1. Since the electrical insulation must withstand its high voltage, DC uses insulation insulation more effectively than AC. (Yes, the square-wave AC also has a voltage ratio of 1: 1 up to RMS, but this includes carrying an infinite number of harmonics - which re-introduces the recently reported disadvantages of high frequencies.)
Now why has DC not become a standard despite basic physics? As a result of considerations # 1 and # 2. The advantages of high power efficiency and (comparatively) low current transmission apply to both AC and DC, but during the Current War between Edison's DC and Westinghouse's AC there was no active DC transformer. So AC won automatically.
But what is the frequency of AC? It's too low, and the lights will flash. (Without DC, of course, but that was not an option without an active transformer.) High frequency transformers are also lighter and smaller than the low frequency AC transformer with the same power, which is why the unusually high frequency of 400 Hz became standard in aviation. Aircraft are also much smaller than the earth's power grid, so transmission losses are not a major problem.
Large electric motors work very well at low AC frequencies, especially the “AC / DC” brush type which has long been used in power grids (railways) due to the need for continuous speed variation. Many power lines live in low frequencies for this reason, e.g., 25 Hz of the Southern Northeast Corridor in the US and 16 2/3 Hz in most of central Europe. DC is even better, and many urban trains (e.g., subways and trams) use it, but also the benefits of high power AC wins when significant distances are involved.
But 50 and 60 Hz were both logical issues for many users for general purposes, which is why they became international standards. Why not one? Because one was as good as the other, and there was no real reason to throw away so many wonderful things that could last so long.
If we could do it again and again from the beginning with modern technology, the strongest case could be made that power systems could and should be completely DC. Thanks to the high power of semiconductor electronics, we now have an effective “DC transformer”. In fact, they “cut” the DC into AC at a very high frequency so that it can be lowered up or down by a transformer (very small and light), and then quickly converted back to DC at a new voltage.
This has already been done for decades on some long-distance transmission lines, especially those that carry very high distances for long distances, below sea level or below.
The same electronics make it possible to drive a simple and powerful AC import engine at any speed you want from a power source at any frequency, including DC. This technology is the basis of modern electric and hybrid vehicles, and it has taken over the railways.
And as the incandescent lamp is quickly replaced by CFL and now LED lights, both of which use electricity, DC is also natural - though it can also easily adapt to any AC supply.
