Opinions Resisting the Mainstream

The Variety of AC Systems

by Fred Steinhauser, OMICRON electronics GmbH, Austria

The early electrical systems before 1900 were developed independently and each of these islands was using a different nominal frequency. It took until the 1920s for the systems to converge to the two main nominal frequencies 50 Hz in Europe and 60 Hz in the US. Besides that, Edison’s DC system was still there, although it did not become widely spread due to the known reasons. Nevertheless, the last DC system in New York City was shut down in 2007.

Besides the common mains frequencies, a multitude of alternate frequencies are used in different fields. And the applications of DC will expand as well.

Japan is special because both nominal frequencies are in use there. The “east side” uses 50 Hz and the “west side” 60 Hz, the boundary is west of Tokyo. The two systems are connected by DC short links. After the Fukushima disaster, the east side suffered from undersupply because several nuclear power plants were shut down. There was generating capacity on the west side to compensate for this, but the capacity of the DC links was insufficient to transfer the required power.

Within AC systems, not only the actual frequency is controlled, but also the sum of cycles over time. There are still many synchronous clocks in use, which in fact count the cycles of the electrical power network. In 2018, there was a dispute between Kosovo and Serbia about the supply of power for frequency control, which led to a sustained underfrequency situation in the ENTSO-E network, causing the synchronous clocks to become noticeably retarded. The error peaked to -359 s (6 minutes late). In the US, a corresponding procedure is in place, known as Time Error Correction.

When looking at electric trains, the frequency patchwork gets even more diverse. The Arlberg Line in western Austria, opened 1884, needed lots of expensive coal and the smoke in the 10km long Arlberg Tunnel caused severe ventilation problems, so electric traction became desirable.

The considerations started around 1900, the electrification was completed in 1925. The limiting factor for the frequency selection was the commutation of the currents in the motors which becomes more difficult when the frequency increases. It was not feasible at that time with 50 Hz. After some evaluation, 162/3 Hz was chosen, which is the standard for the main train lines in Germany, Austria, Switzerland, Sweden, and Norway. The railways in Germany, Austria and Switzerland also operate their own transmission networks, which are two-phase networks. The 110 kV network of the Deutsche Bahn is by the way the largest compensated network.

Other countries went for different solutions, such as Italy choosing DC. All kinds of different frequencies and voltages can be found at railways and trams around the world. Latecomers in electrification with AC could make use of the advances in motor controls and then run their trains with 50 Hz.

DC systems exist in dedicated niches, most prominently as HVDC connections. Besides minimizing losses, DC links can improve the stability of the power system. Stability is always more difficult to maintain when the networks become larger.

For comparison: the east-west extension of the ENTSO-E network (Portugal to Turkey) is more than half of the wavelength at 50 Hz, and the north-south extension of the WECC equals roughly the wavelength at 60 Hz. Such conditions present a real challenge.

But also, the idea of DC for homes has been gaining ground since the 2000s. A single high efficiency DC power supply instead many small power adapters could save energy. Today, a central DC supply for driving all the LED lights is an option in private homes.

What didn’t work for Edison 100 years ago could experience a renaissance for a reason.


Fred Steinhauser studied Electrical Engineering at the Vienna University of Technology, where he obtained his diploma in 1986 and received a Dr. of Technical Sciences in 1991. He joined OMICRON, and worked on several aspects of testing power system protection. Since 2000 he worked as a product manager with a focus on power utility communication. Since 2014 he is active within the Power Utility Communication business of OMICRON, focusing on Digital Substations and serving as an IEC 61850 expert. Fred is a member of WG10 in the TC57 of the IEC and contributes to IEC 61850. He is one of the main authors of the UCA Implementation Guideline for Sampled Values (9-2LE). Within TC95, he contributes to IEC 61850 related topics. As a member of CIGRÉ he is active within the scope of SC D2 and SC B5. He also contributed to the synchrophasor standards IEEE C37.118.1 and IEEE C37.118.2.