Aging diagnosis of large generator stator winding insulation based on AC dielectric characteristic parameters

The long-term aging test for stator winding insulation (SWI) of large generator is carried out with multi-stresses, including thermal, electrical, mechanical and thermal cycling stresses. During aging periods, the characteristics of SWI are measured for the determination of deterioration degree. Some characteristic parameters, included PD intensity index PDI, PD phase index PDP, ac current rapid increase rate m₂, ac current rapid increase index Δ₃ and so on, are proposed to diagnose the degradation condition of large generator SWI subjected to combined stresses. The definitions of PDI, PDP, m₂ and Δ₃ are given in this paper. It is theoretically concluded that the PDI and PDP are the functions of dissipation factor tip-up and capacitance tip-up. The m₂ and Δ₃ indicate the growth rate of ac current between the first rapid increase point and the second rapid increase point. The experimental results are given and the relationship between the new characteristic parameters and residual breakdown voltage of stator winding insulation are discussed. It has been verified that these parameters, especially PDI and m₂, have closely relations with the residual breakdown voltage of the specimen insulation