The continuous increase in demand for electrical energy has led to the further development of three-phase transmission up to 220 kV since the memorable first high-voltage transmission of 20 kV from Laufen to Frankfurt a. M. (1891). The Big Creek facility in Los Angeles was operated at this voltage in 1920, and between 1923 and 1929, the Rheinland line was established, connecting the Alpine hydroelectric power with the steam power plants of the Ruhr area. The transmission from Boulder Dam to Los Angeles required the use of even higher voltages, and in 1936, this transmission was put into operation at 287 kV, initially designed for a transmission capacity of 240 MW over a distance of 430 km. The operational reliability of these three-phase transmissions reached a high level, supported by careful further development of the electrotechnical fundamentals.