Polarization of vacuum-pressure-impregnated high voltage epoxy-mica insulations [electric machines]

In this paper, the electrical behaviour of modern epoxy-mica high voltage electric machine insulation is investigated with the help of a two-layer insulation model. The simplified model helps to understand the charging phenomenon in actual high voltage insulation. The behaviour of vacuum pressure impregnated (VPIed) epoxy-mica insulation is compared with Shellac-Micafolium insulation. It is illustrated that because of higher volume resistivity, the rate of change of the field distribution due to charging is considerably slower in epoxy-mica insulation. This tends to result in lower polarization index (PI) value for the epoxy-mica insulation than typically obtained for Shellack-Micafolium insulations. It is also illustrated that the faster PI measurement method (ratio of the 60 seconds value to the 15 seconds value) gives clearly lower PI value than the method defined in IEEE 43 (1974) (ratio of 600 s value to 60 s value). Finally, the effect of moisture on insulation resistance and PI of VPIed epoxy-mica winding insulation is discussed