Enhancing Dielectric Properties of Polypropylene Insulation for HVDC Cables by Annealing at an Appropriate Temperature

Thermal annealing at specific temperatures stabilizes the microstructure of polypropylene (<tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathbf{P P}$</tex>) products into a thermodynamically favorable state, enhancing their stability. To meet application requirements for green non-crosslinked PP cables in HVDC transmission, this study investigates annealing temperature effects <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\left(100^{\circ} \mathrm{C}, 120^{\circ} \mathrm{C}\right.$</tex>, and 140°C) on DC characteristics of two blend systems: elastomer-toughened PP/polyolefin elastomer (POE) insulation and crystal-structure-modified PP/POE/WBG (<tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\beta$</tex>-nucleating agents) insulation. Results reveal that during PP crystallization, <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\beta$</tex> crystals form first at higher temperatures followed by <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\alpha$</tex> crystals at lower temperatures. Based on distinct crystallization behaviors of these crystal forms, annealing near 120°C facilitates <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\alpha$</tex>-crystal recrystallization, improving the crystallization behavior of the PP/POE modified system. At 140°C, the nucleating agent simultaneously enhances crystallization capabilities of both <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\boldsymbol{\alpha}$</tex> - and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\boldsymbol{\beta}$</tex>-crystals, refining the crystalline and phase structures of the PP/POE/WBG system. This reduces dielectric constant and loss under high-temperature/low-frequency conditions while improving dielectric constant temperature stability.