This article focuses on the effect of annealing process on the electrical properties of polypropylene (PP) insulation containing the

$\beta $

-nuclear agent of WBG-II. The crystal form transformation during annealing at 140 °C is investigated by differential scanning calorimetry (DSC), X-ray diffraction (XRD), and small-angle X-ray scattering (SAXS). The results indicate that as the annealing time increases, part of

$\beta $

-crystals transforms into

$\alpha $

-crystals, followed by the increase in total crystallinity. As the annealing time increases from 0 to 8 h, the dc conductivity is reduced by an order of magnitude at 90 °C, the accumulation of space charges is suppressed, and the dc breakdown strength increases by 13.2%. During annealing, the molecular chains in some

$\beta $

-crystals and amorphous rearrange and form

$\alpha $

-crystals, with higher molecular ranging regularity and better thermal stability. It is demonstrated that the crystal form transformation reduces the physical defects within the aggregate structure and increases the trap level, which contributes to the suppression of space charge and the improvement of the breakdown characteristics.

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