Contents I. Introduction
HVdc and MVdc cable insulation design is a challenge for several reasons. The most important one is that in steady state, the electric field is ruled by conductivity, varying with temperature (load), while during voltage transients, as energization or voltage polarity reversals, the electric field is driven initially by permittivity [1]–[3]. Such dynamically changing electric field distribution may result in different contributions to both intrinsic and extrinsic aging rates [4]. In particular, referring to extrinsic accelerated aging, partial discharges (PDs), phenomenology can change considerably from transient to steady-state conditions. This is due to electric field distribution that can incept PD in different defects or at different field levels depending on defect location because the amplitude of PD pulses and repetition rate can differ from the initial part of a transient when the electric field is rising rapidly, to the steady-state dc, when the field is constant. On the other hand, dc insulation, and particularly dc cables, may experience a large number of energizations or voltage polarity inversions during life. Although the latter is going to be prevented somehow using the modern converter technologies, the former is increasing considerably in modern grids and assets tending toward enhanced interconnections, hybrid grid concepts, and renewable integration [5].
