Calculation of the electrical field constraint and the potential breakdown risk in partially humid high voltage generator stator bar insulation

During recent years, one of the main causes of large generators failure reported worldwide is related to the stator insulation defaults. For the majority of studied cases, the insulation weakness is the result of cooling system problems, generally water micro-leakage located between the water joint box and the end of the stator bars. Experimentations have been conducted over more than one year on several stator bars to determine the water propagation law in the main insulation. The goal of this article is to estimate the potential breakdown risk of high voltage generator stator bars presenting different levels of humidity content. The risk estimation is based on electrical field constraint calculation using a finite element model.