SF6 gas density sensor

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An SF6 gas density sensor is used in gas-insulated switchgear to monitor the density of the pressurized insulating gas, usually sulfur hexafluoride or a mixture of sulfur hexafluoride and nitrogen .

Sulfur hexafluoride (SF 6 ) is used in encapsulated and compact systems in medium and high voltage technology. Application examples are gas-insulated switchgear (GIS) and their high-voltage switches and, in the case of gas-insulated pipelines (GIL), in completely encapsulated systems with operating voltages from 6  kV to 800 kV. In addition to electrical insulation, SF 6 also serves as an extinguishing gas to extinguish the switching arc during switching operations in SF6-filled high-voltage switches, such as the self-blowing switch or piston-blowing switch .

If the density of the insulating gas in the high - voltage system falls below a certain level, for example due to a leak, the breakdown voltage of the insulating gas decreases as a result of Paschen's law , and the high-voltage system must be switched off and serviced. Gas density sensors are used for the timely detection of this operationally critical condition.

technology

There are different technologies of gas density sensors. A group of sensors works by measuring gas pressure and temperature . The adiabatic equation can be used to determine the density using these two measured variables and information on the gas . However, when operated in GIS systems, gas decomposition products arise which have an influence on the gas composition. This leads to an error in the adiabatic equation and thus to a measurement error.

In SF6 switchgear, especially when used in the area of ​​SF6 circuit breakers where SF6 decomposition products arise during operation, the density measurement is carried out using two quartz oscillators . A quartz oscillator is operated in a controlled reference environment and a quartz oscillator in the switchgear's insulating gas. The resonance frequency of a quartz crystal is dependent on the density of the surrounding gas, but largely independent of the specific gas composition. By comparing the frequency of the two crystal oscillators , the density can be determined in the form of a differential density measurement in relation to the control medium.

literature

  • Gerhard Wiegleb: Gas measurement technology in theory and practice: measuring devices, sensors, applications . Springer Vieweg, 2016, ISBN 978-3-658-10686-7 .

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