Large hydro-generator endwinding clearance limits for the inhibition of external PDs

The present study was undertaken to determine the minimum distance that allows discharge-free operation in the endwinding area with or without various types and sizes of laboratory and numerically simulated defects on a specific size of bar with a known insulation thickness. The defects were chosen to simulate the presence of metallic foreign objects, epoxy drops and conductive contamination, respectively. The test objects were subjected to high voltage to determine the discharge inception voltage (DIV) in each configuration. In parallel, the electric field enhancement was calculated by finite-element modeling (FEM) and the results were validated by experimental data. Once the model was proven to correspond to reality, a variety of defect sizes could be tested by FEM alone. The combined experimental data, FEM and correction for high temperature curve show that, for our test bars, PDs can be avoided in the endwindings when the clearance between adjacent endarms is 9 mm or more. However, if conductive defects like metallic objects are present, discharges can appear for clearances of less than 9 mm.