Stress grading for high voltage motor and generator coils

Stress grading on high voltage motor and generator stator windings is becoming more important as the electrical stress in the ground insulation is increased. While there are a variety of possible ways to relieve surface stress, non-linear resistive tapes have been almost universally adopted because of their simplicity and efficiency. However, selection and quality control on these materials has been problematic, as the means to characterize these materials has not been clearly set out. Most types of material can be characterized in terms of two parameters, a threshold stress and a gradient index, which can be determined from simple current and voltage measurements. It is hoped, therefore, that manufactures of grading products will adopt such measurements and publish these values as a matter of course. This will enable coil manufacturers to improve the specification and quality control of stress grading coatings.<>