Experience with advanced in-service condition monitoring techniques for GIS and transformers

Gas insulated substations (GIS) and EHV transformers are examples of equipment on which monitoring is being introduced. Many failures in EHV plant are preceded by partial discharge (PD) activity. This may for example, be due to free metallic particles in the case of GIS and partial dielectric breakdown in the case of EIN transformers. In both cases warning of the presence of a fault will allow the operators to plan the removal of the plant from service before complete failure occurs. A number of techniques have been implemented to detect the presence of PD signals. It is accepted internationally that the ultra high frequency (UHF) technique is the most sensitive and practical method for assessing the dielectric condition of a GIS. The application of this technique for on-line monitoring of GIS is discussed and tests on its use for the detection of PD in EHV transformers is also reported.