Switchgear: Stopping Flashovers with Clean Air

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The space-saving, gas-insulated Siemens switchgears form the fuse box for any plant. These are capable of switching extremely high currents in fractions of a second. | Download

Siemens has built the world’s first metal-encapsulated, gas-insulated switchgear unit for up to 145 kilovolts (kV) that uses chemically pure air as an environmentally compatible insulating gas. Until now, such units have relied on sulfur hexafluoride (SF6), a greenhouse gas. The gas insulates current-carrying conductors in switchgear units and is also used to extinguish the electric arc that forms when circuit contacts are opened. The new clean-air technology unit uses a mixture of nitrogen and oxygen for electrical insulation. However, because this gas mixture is not suitable for extinguishing an arc, Siemens has also developed a vacuum interrupter for high voltages.

Gas-insulated high-voltage switchgear is used in transformer substations and at large industrial facilities that are directly connected to the high voltage grid. In the event of a short circuit, the switchgear must be able to interrupt its electrical connection extremely quickly at currents of up to 40,000 amperes in order to ensure the short circuit doesn’t damage other equipment and network areas. When the circuit is broken, an arc forms between the contacts. Current switchgear extinguishes this arc by blowing SF6 into the system.

SF6 is the most powerful greenhouse gas known and is used mainly in high-voltage technology. There is extremely little SF6 in the atmosphere today, but levels are rising. Siemens has therefore established a closed loop for SF6 that covers everything from production to its use in switchgear and disposal. This closed loop ensures that virtually no SF6 escapes into the atmosphere. Nevertheless, because many countries and industries around the globe are now looking to minimize the use of SF6, Siemens decided to develop an alternative.

New drive-system concept required
With the new clean-air technology for its high-voltage switchgear systems, Siemens is using an insulating gas that prevents flashovers between current-carrying conductors. The gas is a mixture of 80 percent nitrogen and 20 percent oxygen, which closely corresponds to the natural composition of air. Siemens engineers also developed a high-voltage vacuum interrupter to extinguish the arc.

This technology is already well established for medium-voltage applications (up to 40 kV), but it can not be transferred directly to higher voltage applications. Siemens therefore developed a completely new drive system for moving the electrical contacts, which, due to the higher voltages involved, have to be pulled much further apart and therefore also have to move much faster. The contacts used for higher voltages are also larger and have a much higher mass than those used with lower voltages. This means that a very large amount of kinetic energy has to be transferred to the contacts very quickly. The switching technique previously used in SF6 units differs fundamentally from the technology in the new vacuum interrupters, and so could not serve as a basis for the new system.

However, the nitrogen-oxygen mixture has a lower dielectric strength than SF6, which means arcs form even at low field strengths. For this reason, the new switchgear unit is somewhat larger – although its carbon footprint is more than 30 percent lower than that of the previous unit. Siemens has also already developed a vacuum interrupter for 72.5 kV that can be used in converter stations at offshore wind farms. With the new 145-kV unit, Siemens is expanding the use of this environmentally compatible technology known as Blue GIS to include standard applications as well.

Contact:
Mr. Dr Norbert Aschenbrenner
Editorial Office
Siemens AG
norbert.aschenbrenner@siemens.com

Original Internet Article:
https://www.siemens.com/innovation/en/home/pictures-of-the-future/energy-and-efficiency/power-transmission-environment-friendly-high-voltage-switchgear-unit.html

Pictures of the Future
https://www.siemens.com/innovation/de/home/pictures-of-the-future.html