HV and MV DC Insulation System: Unexpected Aging Mechanisms and Metrics for Life Prediction

Design and operation of DC insulation systems are faced with challenges related to the electric field distribution in DC, which can vary with operation and affect the partial discharge inception voltage and partial discharge phenomenology. Energization and voltage polarity inversions in DC insulation systems result in electric field transients that may last many hours, and load fluctuations can generate variable temperature profile transients. Inception of partial discharge during such repeated transients can significantly reduce insulation life. This paper addresses the behavior of electric field and partial discharge during voltage transients, the influence of such transients on life and how to measure partial discharges during transients and DC steady state. The effect of voltage transients on life of DC insulation systems is modelled with and without partial discharges, and it is shown that the presence of partial discharges can affect dramatically the life, even if it is restricted only during voltage transients. The issues related to measurement of partial discharges in DC insulation systems are also discussed, and an automatic and unsupervised partial discharge measurement approach is presented.