The Impacts of Chemical Modification on the Initial Surface Creepage Discharge Behaviors of Polyimide Insulating Film in Power Electronics

In this study, a series of polyimide (PI) films containing both phenyl sulfide and phenyl ether functional groups in different ratios were prepared via a two-step thermal imidization method. The creepage discharge behaviors, including creepage discharge inception voltages (CDIVs), as well as phases, amplitudes, and numbers of discharges in the initial stage, were investigated under 15 kHz high-frequency sine high voltage on specimens. Combined with the dielectric analysis, the inception voltage tests indicate that the high-frequency dielectric losses of the prepared PI films cause the variation of CDIVs, although not obviously in the short-term effect. The phase-resolved creepage discharge results show that most creepage discharges occurred in the positive half cycle of the high-frequency sine stress. The postdischarge negative potential, captured from the surface of all the films, reveals the reason for the polarity effect. Besides, the maximum discharge amplitudes decrease first and then increase with the percentage of sulfide, which means that the partial sulfur modification induces an inflection phenomenon. The synergistic action of sulfide and ether groups on the microcrystal properties affects the trap characteristics in the polyimide films. Thus, the dissimilarity of trapped charges, which would reform the resultant negative potential on the films, is the reason for the inflection. Noticed from the experiments, the trend of the discharge number per cycle was opposite to that of the maximum discharge amplitude. This work may provide a guideline in developing modified polyimide insulating materials oriented for high-frequency and high-voltage applications in power electronics.