Detecting characteristics of SF6 decomposed gas sensor for insulation diagnosis on gas insulated switchgears

Gas insulated switchgears (GISs) with SF₆ gas with superior insulation and interruption performances used as an insulating medium have realized significant downsizing of substation facilities. GISs have been in use in substation facilities for more than 30 years and are expected to be used for an even longer period of time in the future. To realize effective operation and maintenance of highly aged apparatuses, it is necessary to establish insulation diagnosis technology that can be used to detect and accurately assess a discharge phenomenon occurring in a GIS. An effective diagnosis method for the prevention of a facility contingency is to detect a partial discharge (PD) that occurs as a precursory phenomenon of dielectric breakdown. Whereas many of the diagnosis technologies for a PD are based on electrical measurements, the focus in this study was placed on the insulation diagnosis for decomposed gases generated by the energy of a discharge. Chemical measurements for detecting decomposed gases are less influenced by noise and more reliable than electrical measurements. Furthermore, an abnormality can be detected even if a PD does not occur at the time of measurement because decomposed gases build up. This method can also be applied to a fault location. This paper describes creating the prototypes of decomposed gas sensors based on solid electrolytes to detect fluoride gases. The sensors with different output types, i.e., "voltage-output type" and "current-output type", were examined. The "voltage-output type" is suited for diagnosis in a wide range of concentrations whereas the "current-output type" has a good detection sensitivity in a low-concentration range. Furthermore, the "hybrid" sensor, which features a combination of the "voltage-output type" and "current-output type", was prototyped and evaluated, resulting in a finding of a good development possibility for a sensor that has both the advantages of these two types without impairment to either - of them.