Assessment of stator bars by four diagnostic methods

Condition-based assessment of high-voltage components like hydro generator insulation is crucial for optimizing the lifespan and avoid unexpected outages, but the effectiveness depends on reliable results and cost-efficient implementation. Early detection of insulation degradation in stator bars provides significant benefits, yet no sole diagnostic method will provide full coverage of all types of insulation defects. This study evaluates four commonly used techniques: partial discharge (PD) measurement, loss angle measurement (tan δ), polarization index (PI) measurement, and DC ramp test (DCR), with the main focus on the DCR. Thirteen stator bars with artificial defects—such as moisture, punctures, cracks, and thermal degradation—were analysed. A reference baseline was established using old, defect-free non-service spare bars. The performance of each method was assessed based on sensitivity to specific defects, ease of use, and interpretability of results. Results reveal that combining DCR and PD methods enables the detection of all tested defects. DCR stands out for its simplicity and comprehensive defect information, whereas PD excels in defect origin insights but requires more complex execution and analysis. The study is limited to single stator bars.