Impulse Breakdown of Prestressed Polyethylene Films in the Ns Range

This paper describes the effect of prestressing on the breakdown strength and time lag of lowdensity polyethylene films subjected to square pulses of 100 ns width. When prestress and impulse fields are opposite in polarity, the 50% breakdown strength Eb decreases from 4.9 to 3.4 MV/cm as the prestressing field is increased from 0 to 4.6 MV/cm. When prestress and impulse fields have the same polarity, Eb increases to 5.4 MV/cm as the prestress is increased to 4.1 MV/cm. The dc break-down strength is 5.8 MV/cm. The formative time lag tf decreases with increasing applied field. The value of tf is insensitive to the level of prestress within a given mode. The mode of opposite prestress yields considerably larger values of tf than the other two modesof no and same prestress, and at relatively low applied fields. The apparent electron mobility calculated from tf increases from 2.2×10-2 to 2.4×10-1 cm2/Vs as the total (prestress +pulse) applied field is changed from 3.0 to 6.0 MV/cm. The results are attributed to a modification of the field intensity near the electrodes due to space charge accumulation during the period of prestressing. The breakdown mechanism is discussed on the basis of an avalanche-streamertransition in the gaseous breakdown theories.