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Laying the 110 kilovolt oil-filled cable in Nuremberg, 1931

1928: The 110 kilovolt oil-filled cable

In 1923, Siemens had already built the first 60 kilovolt (kV) triplex cable with conventional technology, and in 1925 it delivered a 100 kV cable with mass filler to Braunschweig Technical University in Germany. That same year the company began developing a 110 kV oil-filled cable, and within only about two years, the company had built a reliable version that was far superior to the former mass-filled design.


The heart of the cable was the hollow-core conductor. It was surrounded with a layer of paper insulation, which was impregnated with transformer oil conveyed via the hollow-core conductor. At the same time, the hollow-core conductor collected the surplus oil that developed as the cable heated up during operation, and channeled it to compensation reservoirs. As the next layer, the cable was wrapped in a thick lead sheath, which in turn was covered with a sheath of asphalt and fiber material to protect it from chemical reactions and stray currents.


The cable was a milestone in the history of German cable technology, and made it possible to connect long-distance transmission lines safely with urban centers.

Expansion circuit breaker, 1930

1930: Expansion circuit breaker

The continuous rise in power grid voltage in the 1920s also increased demands on switching equipment. The first solution to become established was the oil circuit breaker, which has its contacts inside an oil-filled chamber. But this kind of circuit breaker was vulnerable to occasional horrific explosions, because a gas mixture could form between the surface of the oil and the cover, and could ignite when incandescent gas rose during switching. So electric utility companies were looking for oil-free switches.


In 1930, Siemens introduced a fluid circuit breaker that used water as the arc quenching medium. In this "expansion circuit breaker," the arc that occurred during switching evaporated some of the surrounding water. This caused high pressure to develop, which would cool or extinguish the arc if the flow was designed right.


Siemens was the first company to develop an oil-free circuit breaker that used water as an arc quenching medium. The expansion circuit breaker opened a new chapter in high-voltage circuit breaker construction. It became a foundation for the interconnection of various grids to create an interregional power supply. 

SF6 high-voltage circuit breaker in a test field, 1980

1964: SF6 High voltage circuit breaker

By World War II, the "expansion" circuit breaker, using steam (or later, oil) as an arc quenching medium, had been developed for switching high voltages. After that point, “minimum oil” circuit breakers started giving way to other forms. Vacuum circuit breakers became established for the medium-voltage range (1 to 50 kilovolts, kV), while SF6 high-voltage circuit breakers came in for applications above 72 kV.


In 1964, Siemens was the first company in Europe to introduce a 220 kV SF6 high-voltage circuit breaker. Ten years later, the Siemens puffer circuit breaker brought the second generation of SF6 circuit breakers onto the market. The modular system used in all Siemens SF6 circuit breakers ensured very reliable operation and easy maintenance.


These circuit breakers used sulfur hexafluoride (chemical formula: SF6) as the arc quenching medium. It was blown onto the contacts and interrupted the arcs caused by switching. The breaker units were only half the size of units using compressed air, saving considerable space.

Apollo station of the HVDC connection from Cabora Bassa to South Africa, 1975

1975: High-voltage DC transmission from Cabora Bassa

High voltage DC transmission (HVDC transmission) is a way of transmitting high-voltage direct current electricity. The heart of any HVDC system is the converter. As modern power electronics developed, thyristors came to be used for this purpose. In 1975, Siemens initiated the world's first thyristor-operated long-distance HVDC transmission system, between the Cabora Bassa power plant, in what is now Mozambique and the Republic of South Africa.


The Cabora Bassa hydroelectric plant project was ideal for this kind of HVDC transmission, because transmitting electricity from the Cabora Bassa dam in northern Mozambique to the Johannesburg metropolitan area in South Africa called for a line 1,420 kilometers long – a distance that could never have been covered cost-