Analysis of voltage transformer failures at Jirau GIS with the EMTP

The presentation will discuss the analysis of successive dielectric failures of inductive voltage transformers (VT) at a 500 kV GIS of Jirau hydroelectric plant in Brazil. All the failures involved the same region of the high voltage winding and the cause was undetermined. These incidents motivated a series of actions to identify the cause and improve the reliability of these equipment in the field. One possibility the effect of very fast transients (VFTO) expected to occur during frequent switchings of disconnector and/or circuit-breaker at GIS. It well known that VFTO may stress inductive equipment insulation leading to a dielectric failure. In this context, different switching conditions were simulated with the EMTP to determine the transient overvoltage at the potential transformer terminals. A special high frequency model of the VT was developed based on its frequency admittance matrix measured in the field. The result of the simulations was discussed together with the VT manufacturer and a final diagnosis of the failures was achieved.