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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.
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.
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.
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-effectively with a conventional three-phase current system.
The Cabora Bassa plant could handle 2,000 amperes per thyristor. The converters were remote-controlled via light signals over fiber-optic cables – another technical milestone.
In 2010, Siemens delivered the world's largest, most powerful 800 kilovolt (kV) converter transformer, intended for the high-voltage DC (HVDC) transmission segment then under construction between Xiangjiaba and Shanghai in China.
The single-phase 800 kV converter transformer had a rated capacity of 321 megavolt amperes (MVA), and weighed 380 metric tons to ship. Siemens delivered ten converter transformers in all for the Fulong converter station near the Xiangjiaba hydroelectric plant, five of which were in the 800 kV version. The HVDC transmission systems were the backbone of the power grid of the ultimate operator, the State Grid Corporation of China – the world's largest electric power utility, with about a billion customers.
At more than 2,000 kilometers and with a transmission capacity of 6,400 megawatts (MW), this HVDC connection was the longest and most powerful in the world.
Smart grids make the power supply more stable and allow distributed power generation sources to be connected in. Since 2011, Siemens has provided smart grids worldwide, to ensure the right balance between power generation and demand for electricity.
Wachtendonk is a small German municipality with a very large number of units feeding power into the system – mainly photovoltaic units. To keep the distribution grid stable in spite of the large share of renewable sources (around 80 percent), a smart grid was set up for a model experiment in 2014, with smart local grid stations and meters as well as measuring, monitoring and communications equipment.
Five new smart local grid stations played a special role in ensuring greater stability. They were equipped with adjustable local grid transformers, which took care of stabilizing the grid. If smart meter data showed a rise in voltage on the grid because of high photovoltaic feeds combined with low power consumption – for example on a cloudless day – the transformer automatically adjusted the grid voltage.
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