Detection of decomposition products in SF6: A comparison of colorimetric detector tubes and ion mobility spectrometry

Despite the high reliability of modem gas insulated switches and substations (GIS), failures still occur during extended operations. Arcs and partial discharges arise in normal operations of these high voltage switches and the insulating gas (SF₆) is decomposed eventually to corrosive and partly toxic impurities. The impurities worsen switch performance and a method to monitor gas composition in these switches, particularly the accumulation of impurities, should allow preventative maintenance in a controlled and timely manner. Decomposition products can be detected using fast and inexpensive methods such as colorimetric detector tubes and ion mobility spectrometers. In this work, parallel experiments were made using colorimetric detector tubes and an ion mobility spectrometer in order to compare directly analytical performance of both. Samples were created under controlled laboratory conditions where SF₆ was aged in a high voltage chamber using differing quantities of electrical discharges. An ion mobility spectrometer provided automated, high speed response with detection limits sufficient to detect the impurities formed in a GIS at values of 20 ppm_v. In contrast, the colorimetric detector tubes showed response to impurities at 65 ppm_v for SO₂ and 75 ppm_v for HF and involved consumable reagents or materials. All measurements were validated using Infrared Spectrometry (FTIR) as an independent method to determine and decomposition products.