Partial discharge measurements and life estimation in DC electrical insulation during voltage transients and steady state

•Analysis of partial discharge inception under DC steady state and voltage transients is presented.•An automatic approach to the separation of PD and noise is reported, under both steady-state DC and voltage transients.•A life model for insulation life with and without PD during DC steady-state operation and voltage transients is presented. The design and operation of DC insulation systems is more complex than for AC sinusoidal supply. The most subtle issues are how the electric field in DC can vary with operation and affect the partial discharge inception voltage and partial discharge phenomenology. During operation, a DC insulation system is indeed subjected to voltage and load transients, the former due to energization and voltage polarity inversion. This paper sheds some lights on such issues, going through another fundamental problem, that is, how to carry out effective partial discharge detection under DC supply, both in steady state and during voltage transients. An algorithm that can enable automatic and unsupervised partial discharge measurements in DC electrical apparatus is presented and validated based on partial discharge monitoring performed on test objects representing defective insulation systems. Identification of the source generating partial discharges, which is the basis for diagnostic and condition maintenance, is achieved without the support of experts. The impact of voltage transients on the life and reliability of DC insulation systems is modelled in the presence and absence of partial discharges. It is shown that, depending on the repetition rate, the presence of partial discharges, even if only during voltage transients, can dramatically affect the insulation life and cause premature insulation breakdown.