Phase resolved partial discharge measurement at dc voltage obtained by half wave rectification

Abstract:
Partial discharges and their evaluation at dc voltage are still an unsolved topic of diagnostics related to high voltage equipment. The main issue is that the well-known ac phase resolved partial discharge diagnostic method is often claimed to be inapplicable for direct voltage applications due to the assumed, missing phase angle. As dc voltages for high voltage applications are usually obtained due to a rectification of an ac voltage, a ripple will be present on the waveform, which bears a phase information. In this paper the ripple from a half wave rectification is used to record characteristic phase resolved partial discharge patterns for three typical model defects. These are corona in air, surface partial discharges and internal partial discharges. The outcome are clearly differentiable patterns, which can be used to identify the different defects. As the introduction of a ripple alters the field behaviour form a dc field to a combined ac and dc field, the influence of the ripple on the discharge frequency is analysed with respect to a varying ripple. The resulting ripple recommendations for dc PD test voltages that have been obtained by half wave rectification are: smaller than 3 % for negative corona, smaller than 2 % for positive corona, smaller than 0.4 % for internal PD and smaller than 0.8 % for surface PD.