DC Voltage Supply in TD: What about Electrical Insulation Reliability and Maintenance?

The growth of DC supply in distribution and electrical transport raises the issue of assessing the long-term behavior of electrical insulation. Type and prequalification tests are specified for cables, but ensuring a life at low failure probability may be not as obvious as under AC. Besides caring of reliable operation under DC steady-state, design must take into account that DC insulation systems are subjected, during operation, to numerous energizations and, sometimes, voltage polarity inversions, during which the electric field distribution is as under AC. This may favor partial discharge occurrence and, thus, increase degradation rate, because, in general, the partial discharge inception voltage in AC is lower than that in DC. The potential sources of accelerated degradation of DC insulation, and how they can affect reliability and life, are discussed in this paper, focusing on partial discharges. Issues related to partial discharge measurements under DC power supply and diagnostic tools are also examined, highlighting a worrisome lack of focus on DC electrical insulation.