  It is state-of-the-art to describe the dielectric behavior of an insulation
material by its permittivity and its specific electric conductivity in order to
estimate the dielectric stress of an insulation system. Thus, the electric
field at DC voltage stress is determined according to the stationary electrical
conduction field with the electric conductivity of the insulation materials.
However, at oil-insulated arrangements a higher field strength in front of bare
metal electrodes at high DC voltage stress occurs, which is not explainable
with this model.
  Therefore, a charge carrier-based approach is presented to describe the
dielectric behavior of the oil-paper insulation. It describes the movement of
charge carriers and their effect on the electric field strength. Their drift
leads to an accumulation of charge carriers in front of electrodes which
results in a higher field strength in these areas, which can be calculated
numerically using the Poisson-Nernst-Planck equation system.
  Compared to the conductivity-based model fundamental differences can be
shown. Breakdown experiments qualitatively confirm the expectations according
to the charge carrier-based approach.
  The results show that the charge carrier-based field distribution has to be
considered for modelling the electrical field strength distribution at high DC
voltage stress. They also show, that the dielectric behavior of these
arrangements cannot be explained according to the state-of-the-art model.
