Modelling the electric field transients in DC insulation systems upon energization and voltage polarity inversion

Designing insulation systems for DC application is not straightforward as under AC, both because electric field can distribute inside the insulation in significantly different ways, and due to the effect on field distribution of voltage transients, such as energizations and the polarity inversions. During and after each voltage variation, the electric field in the insulation is mainly driven by permittivity, as in AC, while at steady-state the electric field profile depends on conductivity and, hence, on dielectric material and load. This can impact on aging phenomena and rate, thus on the electro-thermal life of an insulation system. It is, therefore, important to estimate how long it takes for the electric field to reach its steady state condition (i.e. the transient time) upon voltage-time variations. Different methods for estimating the electric field transient time are discussed in this paper, from conductivity and permittivity measurements at high or low fields, as a function of temperature, to partial discharge time evolution. Specimens made by polymeric materials having different conductivities, and containing artificial defects, are used for the experimental validation of those methods.