Condition Assessment of Hydro Generator Insulation Using Partial Discharge Measurements

More than half of all hydro generator failures are caused by insulation breakdown. Insulation failures inflict serious damage on the generator and represent significant operating costs for utility companies. One of the main reasons for insulation breakdowns is the irreversible degradation caused by internal partial discharges (PD) resulting from voids in the insulation system. Therefore, it is crucial that methods for condition assessment of generator insulation can identify and analyze the partial discharge activity in these voids. The main purpose of this thesis is to facilitate condition assessment of generator bar insulation by measuring partial discharges resulting from voids in insulating materials. The aim is to clarify how important PD parameters relate to the void size and the discharge mechanisms in the voids. A part of the work also involves comparing the experimental results with the theoretical model of internal partial discharges: the Abc Model. In addition, possibilities and limitations of partial discharge measurements as a diagnostic tool are examined. The test samples consist of both real generator bar insulation and specially designed laboratory samples containing disk-shaped voids of different diameters. A thin layer of aluminum was applied by vacuum evaporation to the cavity surfaces of one sample group to investigate the effects of increased surface conductivity. Electrical detection of partial discharges was performed using a conventional measuring circuit, and the PD activity was analyzed in the form of inception voltage, as well as the discharge magnitude and discharge frequency at a voltage frequency of 50 Hz. The inception voltage is found to decrease with increasing void diameter due to lower field enhancement in larger voids. In the case of the laboratory samples, the discharge magnitude is generally increasing with increasing diameter. The discharge magnitude is larger when the void surfaces are conducting since the discharge area then is equal to the void surface area. However, for samples made of aged generator insulation, the discharge magnitude is constant regardless of void diameter. More importantly, it was not possible to conclusively distinguish the PD activity in the voids from the PD activity inherent in the insulation. The discharge frequency tends to increase with increasing void size. This can be explained by the differences in the electric field strength in the different cavities at the specific voltage level, and the occurrence of parallel discharges. Higher void surface conductivity generally leads to lower discharge frequency. In conclusion, the PD activity is seen to depend on the void size. The theoretical model is successful in describing the PD activity in voids with conducting surfaces, but fails to describe the PD activity in aged generator insulation. PD measurements performed on the laboratory samples can detect the voids and assess the relative void size. However, the voids in aged generator insulation cannot conclusively be detected using the chosen PD approach. This represents an important limitation of the PD method