Evaluation of diagnostic measurements used on stator insulation of hydrogenerators

Measurements of Dissipation Factor (tanδ/Δtanδ), Polarization Index (PI) and DC High Potential Insulation Currents are widely used diagnostic tools to assess the condition of stator insulation in hydro generators. This paper presents long-term diagnostic data gathered from three 60 MVA generators in a Swiss hydropower station. The measurements were performed in 5-year intervals as part of regular maintenance since the rewind of the stators with a mica/epoxy insulation system in the early seventies. Measurement results of Dissipation Factor, Polarization and DC High Potential tests are discussed with regard to their effectiveness in assessing the ageing condition of the stator insulation. After a brief review of the theoretical background, the strengths of each diagnostic method in detecting symptoms of insulation deterioration are identified. It is concluded that Polarization Index and DC High Potential Insulation measurements are best suited for the detection of humidity and contamination problems. Dissipation Factor measurements below partial discharge inception voltage are a robust indicator of long-term ageing caused by oxidative degradation.