Root cause analysis of generator failures

The failure rate in one of Hydro-Quebec´s power plants is about three times higher than in the rest of the fleet. Over the years a total of 10 failures occurred for the six units of this plant. Spare bars were put in after every failure and the units returned to service. Apart from a few incidents, the failures were almost exclusively on bottom bars. Failed bars were retrieved and cross-section slices were obtained from them for inspection. The same was done with one un-failed bar removed from service and with a spare bar. Microscopic observations revealed differences between top and bottom bars with regard to curvature of the copper strand radii, the quality of the conductor stacking, the thickness of the groundwall insulation, the type of transposition filler material and the void content. The paper analyses the differences and their impact on the electrical field calculation in the groundwall insulation and the relative contribution of each factor to the high failure rate.