Ageing Behaviors of Polypropylene Film under the Operating Condition of Oil-Impregnated Capacitors

With the increasing demand of energy storage density ($U_{\mathrm{e}}$) of capacitors in power system, the performance of biaxially oriented polypropylene (BOPP) film faces gradually increasing gap to meet the demand of capacitors. Researches on the dielectric materials with high$\boldsymbol{U}_{\mathbf{e}}$have achieved remarkable results, but mainly focusing on the dry-type capacitor. In the present study, the aging characteristics of BOPP film under the practical operating condition of oil-impregnated capacitors were studied. BOPP film samples from two different manufacturers were immersed in benzyl toluene (M/DBT), under 80° C and 100 kV/mm for 1000 h. Examinations of the samples showed that the impregnant molecules could permeate into the voids between PP molecules, and thus reduce the activation energy of molecular chain segments. Besides, the coupling effect of electrical, thermal and impregnant could decline the regularity of molecular chains alignment, causing the breakdown strength decreased by 14.07 % and 14.85 % respectively. The study provided experimental and theoretical supports for the understanding of ageing behavior of BOPP films, as well as the further development of dielectric materials applied for oil-impregnated capacitors.