Effect of Thermal Aging on Space Charge Distribution in BOPP Films

Metallized polypropylene film capacitors (MPPFCs) are widely employed as critical components in advanced energy systems. Thermal aging can degrade the dielectric properties of polymer films, ultimately leading to capacitor failure, making it imperative to evaluate the impact of thermal aging on dielectric performance. Space charge accumulation and internal electric field distortion have become key indicators for assessing the electrical behavior of dielectric films. This study investigates the evolution of space charge and electric field distortion within <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$5.8 \mu ~\mathrm{m}$</tex>-thick BOPP films after isothermal aging at 90° C for varying durations, using the Thermal Pulse Method (TPM) for space charge measurements. After 72 hours of thermal aging, the injection and migration characteristics of space charge in BOPP films shifted from positive to negative polarity. With prolonged aging, the injection and migration of negative charges intensified, and the total accumulated charge within the films exhibited a U-shaped trend. The results of the residual space charge inside the film after removing the electric field show that the residual electric field distortion ratio of the unaged BOPP film is 2.85 %. For the samples aged for 24 hours and 72 hours, this ratio decreased to 1.385 % and 2.29 %, respectively. For samples aged beyond 72 hours, the distortion ratios increased to <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathbf{3. 8 7 5 \%}$</tex> (<tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathbf{1 2 0}$</tex> hours) and 8.8% (240 hours). Overall, the residual electric field distortion ratio also exhibits a U-shaped trend with the increase in thermal aging time. Short-term thermal aging helps to suppress the injection and accumulation of space charges in the BOPP film, thereby improving its insulation performance under high electric fields.