Evaluation of gas insulated substations and circuit breakers using partial discharge - IMS

Despite the high reliability of modern gas insulated switch gear and substations (GIS), failures still occur in their long term operation. Some of these failures are preceded to small variations of gas compositions within the shielded compartments of a GIS. Thus the prediction of the gas quality within the compartment should be attainable on the basis of a sensitive analytical monitoring system, by sensing the insulating gas continuously. Such a monitoring system for GIS and SF₆-filled circuit breakers was realised on the basis of ion mobility spectrometers (IMS). Especially, partial discharge IMS provides power utilities a diagnostic tool able to perform on-site monitoring and event controlled inspections of the SF₆ gas. The results of on-site monitoring as well as on-line analysis of SF₆ contained in a circuit breaker of a GIS in Germany, before and after operation are described and the relation between switching and the remaining number of decomposition products in SF₆ is shown and discussed on the basis of the signal delivered by the PD-IMS used. Conclusions arising from investigations on different circuit breakers in different substations in Germany are presented. The details of some case studies are considered, indicating that a full recombination of decomposition products was not achieved in all cases. Results of investigations of different disconnecting switches in a substation in Germany are presented also. In a special case at one switch the prevention of a failure is presented. Finally, the structure of an automatic system for continuous on site sensing of SF₆-decomposition in a hydroelectric power plant in Brasil is discussed.