Partial discharge testing of defects in dielectric insulation under DC and voltage ripple conditions

This paper details testing conducted on selected void defects in dielectric insulation samples under various HVDC voltage conditions. The aim of this work is to illustrate the type of PD activity that could be observed in HVDC insulation systems. Initially the samples were subject to HVAC excitation to confirm the defect type and establish the PD inception voltage. HVDC testing was then conducted using a ‘ramp and hold’ test technique with different hold voltages in the ramp determined from the measured HVAC inception voltage. The effect of harmonic ripple superimposed on the HVDC voltage waveform, which is typically a result of the converter switching operation, was also investigated. The findings from this study should provide network operators and insulation manufacturers a greater insight into the behavior of PD phenomena in solid insulation under HVDC conditions, enabling greater confidence in the diagnosis of defect type and severity in such systems.