Identification of partial discharge phenomena in HVDC apparatus

Detecting insulation flaws in HVDC apparatus, where partial discharge (PD) may occur can help preventing failures, even if the degradation rate under PD is much slower than in AC conditions. In order to improve reliability of HVDC systems and, in particular of HVDC transmission lines, it would be important to distinguish defects of different harmfulness. However, under DC voltage the concept of phase-resolved PD pattern loses meaning and conventional techniques for PD source identification are no longer applicable. Techniques based on the statistical correlation between subsequent PD events have been proposed to identify PD sources under DC voltage. In this paper, the search for novel features able to characterize different phenomena (that can be described in broad terms as internal, surface and corona PD) is carried out. In particular, besides additional markers that have not been presented yet in literature, it is proposed to exploit the polarity- and voltage-dependent characteristics of different PD phenomena.