When designing ac insulation systems much knowledge and experience is
available concerning- partial discharges. However, for dc insulation t
here is a lack of knowledge of partial discharges and their effect on
reliability and lifetime. For this reason we conducted a study on the
distribution functions of the discharge magnitude and the time between
two discharges. A model has been developed to describe the discharge
behavior for dc voltage. The measurement results are in agreement with
the proposed model. The extinction voltage of the discharges is close
to the ignition voltage. The discharge frequency is mainly determined
by the statistical time lag (period between reaching a voltage suffic
ient to cause breakdown and the breakdown itself). The statistical tim
e lag depends strongly on the test voltage.