Case Study on the Condition Assessment and Drying of a Hydrogenerator After Flooding

Inhalt:
During a flood in 2018, a global VPI-insulated 20 MVA generator of a storage power plant was completely flooded. This article aims to share and analyse the experience gained in dealing with this incident. Additionally, the corresponding physical background, which is particularly relevant for the interpretation of the diagnostic measurements as well as for the optimization of remedial actions, is also discussed. From a scientific point of view, this case is of particular interest due to two reasons. On the one hand, regular diagnostic measurements were carried out on the generator, which was commissioned in 1984, so that the condition of the insulation system (e.g., insulation resistance, polarization index, insulation time constant, dissipation factor) and the corresponding trends before the damage occurred are well known. On the other hand, regular diagnostic measurements were carried out after the incident. These measurements showed contradictory trends. Initially, there was a rapid recovery of the insulation resistance as a result of the immediate drying measures. Subsequently, however, the insulation resistance deteriorated increasingly so that extensive drying measures had to be taken again. In the end, the machine could be returned to regular operation. In the course of the subsequent analysis of the corrective measures, it was shown that the observed behaviour can be explained by the typical saturation mechanisms of epoxy-mica insulation. From this, it was possible to derive recommendations for drying contaminated winding insulation that take these physical relationships into account.