Examples of stator winding insulation deterioration in new generators

Modern hydrogenerators rated a few hundred MVA and above are designed to operate at electrical, mechanical and thermal stresses that are higher than was customary for smaller, older machines in the past. The result is that some failure processes are becoming more common, and the expected life may no longer be the 50 year life experienced in the past. Certainly data indicates that the partial discharge activity is higher for machines made in the past 10 years, than for older (and more aged) machines. This paper reviews the experience with the premature deterioration and failure of large hydrogenerators from around the world. These failures have been due to poorly made electrical connections between stator bars, loose bars in the stator slot, vibration sparking, poorly designed corona suppression coatings and inadequate spacing of the stator bars in the end windings. These problems are reviewed, and ways to avoid premature failure with the better specification of new machines and modern diagnostic monitoring are discussed.