Detecting aging by-products of SF6 using quadrupole mass spectrometry

Quadrupole mass spectrometry (QMS) has been used to validate the condition of the operating gas in a SF₆ GIS 150 kV indoor power substation, running under periodic maintenance for almost 30 years. The switches of the examined system were of the dead tank, single phase, double pressure type. The employed analyzer head enabled detection of positive ions having maximum mass to charge ratio 200 and a minimum detectable concentration 0.5 ppm. According to the experimental results, before the periodic maintenance process the ³⁴S concentration had been increased by a factor of 2 (natural abundance of isotope ³⁴S is 4.5%). Additionally significant amounts of CO ₂ could be detected in gas volumes enclosing the current transformers. One month after the periodic maintenance process, i.e. gas dehydration, chamber cleaning and filter substitution, the S isotopic ratios were back to normal and the CO₂ concentration was not detectable. In the event of permanent damage of solid insulants (i.e. presence of aluminum fluorides diffused in the epoxy resin) CO₂ concentration was still increased at higher concentrations than the CO₂ in atmosphere while the dew point measurements did not reveal the weakness. Other SF₆ by-products and their concentrations could also be detected with the above measuring instrument (i.e. SF₅⁺, SF₄⁺, SF₃⁺, SF₂⁺, SF⁺, S⁺, F⁺, H₂O⁺, HO⁺, SOF_x⁺, SO₂F_x ⁺) and therefore it may be used as a quality control tool of GIS power plants