Premature failure of modern turbine generator rotor windings

Abstract

Rotor windings of modern turbine generators (TG) suffer early damage due to deficiencies in design, manufacture, abnormal operating conditions etc. During the last decade, trivial to serious dielectric problems had been noticed untimely resulting in unexpected down time of power plant, revenue loss and huge rectification cost. This paper deals with various failure modes of latest rotor windings, degradation of dielectric elements, diagnostic methods and severity mitigation process. Dielectric model of field winding is developed to have an understanding of turn-turn short circuits, field grounds and polluted overhangs. Problems experienced by present day rotor windings are detailed. Diagnosticians approach to assess the condition of windings and its dielectrics with conventional and modern diagnosis technique of recurrent surge oscillograph (RSO) along with operational electro mechanical characteristics are presented. Case studies on 32MVA, 11kV TG rotors associated with load related high vibrations at infant stage are reported. Results of diagnosis tests, interpretation of resultants for root cause identification and modalities of fast refurbishment are narrated. Recommendations to improvise the reliability of rotor windings are offered to producers and users.