Reliability of HVDC and MVDC Electrical Asset: The Challenge of Insulation Design

Reliability of DC electrical asset components has not been investigated as broadly as for sinusoidal AC supply. On the other hand, DC assets are more and more common, and they are forecasted to grow in number, power, and voltage in the near future. This is not only in transmission grids, but also in distribution/generation (renewables), industrial application and electrification transportation. To complicate the framework, it has to be recognized that DC asset components are not only subjected to DC steady-state voltage, but also to voltage and temperature transients, as those coming from energization, voltage polarity inversion, ripple, repetitive voltage impulses and load variations. The major issue to asset component reliability can come, in these conditions, from electrical insulation. While designing insulation under AC sinusoidal voltage is a century-long practice, with many feedbacks from field installations, the same does not hold for modulated sinusoidal (i.e., power electronics) and DC supply. Electrical stresses can be different in magnitude and distribution from AC to DC, and load variations in DC can contribute to electric stress variations much more than in AC. All of this may impact significantly on aging rate and reliability of electrical insulation.This paper investigates the difference between electric field distribution, and consequent aging mechanisms and rate, from AC sinusoidal to DC supply, considering, in particular, the real DC operating conditions during which voltage and load transients can occur frequently. The contribution of partial discharges to aging rate will be also taken into account, bringing to the derivation of a probabilistic life model that can allow reliability estimations in the design of DC insulation systems to be achieved.