SMART-DT Dew Point Meter in Transmission and Distribution System (High Voltage Switchgear and Sulfur Hexafluoride)

High voltage switchgear widely used in transmission and distribution systems uses sulfur hexafluoride (SF6) as the insulating arc extinguishing gas. When the water content of this gas exceeds a certain limit, rapid and severe damage to switchgear can occur.

During transmission and distribution (ranging from 100 to 400 kV), power switching or switching with load or tripping leads to arcing discharges between contacts. the insulating properties of SF6 are used to seal the switchgear enclosure to inhibit arc formation. However, moisture entering the interior causes the arc to grow larger and larger, causing SF6 to decompose into its by-products, such as hydrolyzable fluorides.

These fluorides are good dielectrics and their presence does not impair the operating efficiency of the equipment. However, this only applies when the moisture content of the gas inside the switchgear is very low. In the presence of water vapor, arcing produces by-products including highly corrosive acid-hydrogen fluoride that can accelerate switch contact corrosion leading to physical breakdown. The corrosive nature of hydrogen fluoride means that it spreads to the area around the switch enclosure causing further damage. Therefore, it is important to keep the water content of the gas to a minimum to avoid the formation of this acid. 

Additionally, it is also important to keep the moisture content low to avoid moisture condensation in the housing during cold weather, which can discharge the surface of the insulated parts. 

Moisture measurement is important at the following stages.:

◆ Check the water content of SF6 gas in cylinders supplied by industrial gas manufacturers before commissioning by switchgear manufacturers or transmission power companies. 

◆ SF6 cylinders seem to be particularly susceptible to moisture in storage. SF6 should have less than 10 ppm water content (V) under good conditions, which is equivalent to -60°C at the dew point. 

◆ In the final stage of manufacture or subsequent maintenance, purge new switchgear with dry gas before pressurizing and sealing with SF6. Usually use high purity nitrogen or another inert gas, repeatedly pressurize and vent until the internal water content has been less than 10ppm (V) equivalent to the dew point of 60℃.

◆ During maintenance inspection and continuous monitoring, ensure that the moisture content in SF6 gas is less than the critical level of 100 ppm (V) equivalent to the dew point of 40°C.

All these applications can also use the MZD SMART-DT portable dew point meter. This instrument is easy to use, quick to respond and can display the moisture content in a variety of engineering units. If you need more information, you can contact us (info@mzdd.de).