In this issue - Technically

The application and interpretation of partial discharge measurements on HV coil insulation are discussed. When internal partial discharges are developed in an insulation system containing voids, rapid changes in voltage distribution over the solid and gaseous dielectrics take place. The measured total changes in charge transfer and losses of the insulation are probably more dependent on the dielectric properties and configuration of the solid than on the actual corona discharge pulses occurring in the voids. Therefore, the analysis and interpretation of partial discharge test data must be based on a complete knowledge of the insulation systems and its intended function in a rotating machine. A better understanding of the effects of internal ionization in HV coils can only be gained by functional endurance testing at voltages above corona inception points. Quality control methods for HV coil insulation systems and control standards must be based on such experiences.