Influence of Stress-Grading Systems on Insulation Resistance and Polarization Index

New, tougher, industry requirements for blackout and corona tests are driving improvement of the stress-grading systems of high-voltage, hydro-generator windings. The resulting changes in materials and bar-manufacturing processes have a strong influence on the Insulation Resistance (IR) of the stator bars and coils and of the entire generator winding. Therefore, the changes in the stress-grading system have also to consider the minimum, specified requirements for the IR and polarization index (PI) of the winding. This paper presents IR measurements on stater bars and coils that have been treated with different stress-grading systems. The stress-grading systems presented here are different in application processes, mainly in the length of the applied grading material. The results clearly show that these variations have a significant influence on the IR and the PI and that a balance between all insulation-system requirements must be considered. For windings with rated voltages greater than 16 kV, application of the stress-grading treatment to the entire winding overhang is recommended. For this case, the measured IR and PI are much lower than expected from historical data.