Condition assessment study on stator bars, after 40 years of operation

The knowledge of the condition of a stator bar insulation after a long period of operation is a key point for enabling a reliable operation and for the decision on an investment for a rewind. The Insulation Competence Center (ICC) of Alstom had the opportunity to investigate the condition of operation-aged Micadurreg bars, which originate from the beginning of the modern vacuum pressure impregnated manufacturing using synthetic resin. Accompanying the life expectancy studies, as they are consulted for the qualification of new insulation systems, periodically diagnostic investigations were carried out according to the Alstom winding diagnosis program. These investigations were done in order to register the ageing pattern by measurement, among others for assessing the meaningfulness of these studies during the periodically carried out investigations. The studies of the residual lifetime behaviour were carried out in the same manner and under same load conditions as how the new bars of this insulation system also must pass this life test. In addition these bars were monitored permanently through a partial discharge monitoring system. All operation-aged bars with nearly 40 years in service passed the criteria-required for new bars-brilliantly, which underlines the excellent quality of the MICADUR-insulation system. The diagnostic investigations carried out simultaneously point out the benefits and meaningfulness of supplementary diagnostic investigations-but likewise their limits also. The partial discharge monitoring is the only suitable tool to detect reliably the quality of the stator insulation during operation and to detect forthcoming breakdown early