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In practical power apparatus, the insulation under high electrical stress is exposed to recurrent partial discharge pulses, and the utility of the insulation is largely determined by its resistance to aging by partial discharges. The occurrence of partial discharges is largely governed by a combination of the applied field and the space charges that are generated due to these discharges. Since the space charge profile generated by a time-varying voltage and, thus, the resultant field would be much different from that of a constant ac voltage, this work investigates how the partial discharge characteristics, the resulting erosion, and remnant breakdown strength change when samples are aged with a step voltage profile. Since incorporation of nanofillers in controlled quantities is known to alter the space charge formation and transportation, this work also focuses on understanding the effect of incorporation of different quantities of natural (cloisite 20 A° natural montmorillonite clay) and synthetic (tetrasilisic fluro mica) nanofillers into a pure polypropylene matrix on its partial discharge characteristics and the resulting effect when aged under the application of step voltages. Isotactic polypropylene films with different filler concentrations of natural and synthetic nanofillers were aged. The cumulative effect of the partial discharge behavior was investigated through analysis of the erosion depth produced by these pulses on sample surface and remnant breakdown strengths of aged samples. Results indicate that maximum partial discharge magnitudes become prominently lower in nanofilled samples as compared to that of base polypropylene only when the applied voltage is beyond a certain level. Up to that voltage, the average discharge magnitude is significantly lower in samples whose filler content is below a percolation threshold, but above that voltage, the average magnitude depends on the aspect ratio of the nanofillers. Partial discharge resistance and characteristic breakdown strength increases with increase in filler content in both types of nanofilled samples, but excessive filler content is not effective in enhancing these parameters.
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