Air and moisture versus paper insulation

THIS is the third of a series of studies into the still open question of the influence of residual air and moisture upon the properties of impregnated-paper insulation. The influence of residual moisture was found to be universally bad and increasing amounts of such moisture are immediately reflected in higher values of power factor and loss and in other evidence of instability. One of the most striking results of the earlier studies was the relatively small effect of residual air upon the shape of the voltage-power factor curves. The shape of these curves was not appreciably affected in the range of evacuation pressure from 1 to 10 cm. Hg. abs. and furthermore the curves were flat up to 300 volts per mil and 50 deg. cent. Only at impregnation pressures above 25 cm. Hg. do the typical rising power factor-voltage curves become evident. Upon dissection, dryness and gas voids always were associated with rising power factor-voltage curves, and apparently-excellent impregnation with the flat curves.