Moisture measurement in SF6-insulated switchgear is essential – and WIKA has improved the performance of the GDHT-20 sensor specifically for this industry. The transmitter, which also provides values for gas density, pressure and temperature, now measures the dew/frost point with an accuracy of ±2 K.
Andre Dahlheimer – Product Manager, WIKA Alexander Wiegand SE & Co, says moisture, together with density, are the crucial parameters for assessing the condition of the SF6 gas used in the switchgear. The GDHT-20 monitors moisture content in the gas continuously. The information from the online monitoring forms the input for a downstream trend analysis. The more precise the density and the moisture measurements are, the more precise this analysis will be.
Enabling predictive maintenance
The analysis shows the operator when the SF6 will reach a critical state and when remedial measures must be taken. As a result, maintenance work or gas treatment can be undertaken only when action is needed. Predictive maintenance thus replaces the sequence of regular maintenance previously necessary to avoid ecological and safety risks.
Too much moisture in the gas compartment is one of these risks. In older plants, moisture increases mainly due to permeation through sealing materials. In newer plants, it is usually a consequence of moisture remaining on metal surfaces or in insulation materials during tank assembly, due to incomplete evacuation before initial filling with SF6.
Excessive moisture content leads to corrosive reaction products that can damage internal components and shorten the service life. If such substances escape, there is an additional danger to people’s health.
SF6 moisture measurement parameters
Measurement of SF6 moisture is therefore indispensable. In most cases, the dew or frost point is used as a parameter. This is the temperature at which the humidity in the air begins to condense. In the case of SF6, the atmospheric dew point specified by IEC 60376 is -36°C.
The accuracy of the GDHT-20 of ±2 K for moisture measurement is validated at the frost point with an internationally valid reference standard. However, the measuring instrument can also be configured for other parameters (dew point, ppmv, ppmw and relative humidity).
The new version of the sensor also features greater application flexibility. The GDHT-20 is now suitable for monitoring density of the alternative insulating gases N2, CF4, O2, CO2 and 3M™ Novec™ 4710.
For more information visit: www.wika.co.za