Enhancing Hydrophobic Stability of Silicone Rubber Insulators Under High‐Voltage Direct Current Stress Through Optimised Nano‐Fumed Silica Surface Area: A Dynamic Drop Test Study

The long‐term hydrophobic stability of silicone rubber (SiR) insulators is essential for their reliability in high‐voltage direct current (HVDC) applications. This study investigates the effect of nano‐silica fillers with varying specific surface areas (SSA = 120, 200 and 300 m 2 /g) and different concentrations of alumina trihydrate (ATH) fillers (5–50 wt.%) on the dynamic hydrophobicity of SiR using the dynamic drop test (DDT) under HVDC stress. Key performance indicators—hydrophobicity retention, recovery and transfer—were evaluated. Experimental results indicate that nano‐silica, particularly with an SSA of 200 m 2 /g, (SFS200) significantly improves hydrophobic performance, offering the best balance of retention time, recovery, transfer and reproducibility. In contrast, high ATH loadings hinder the migration of low molecular weight and reduce recovery. Moderate ATH content (10 wt.%) showed optimal performance among ATH‐filled samples but was still inferior to silica‐filled composites. Although higher SSA enhanced retention time, an excessively high SSA (300 m 2 /g) resulted in agglomeration and consequently reduced recovery. These results confirm the superiority of fumed silica in improving the hydrophobic stability of SiR insulators and highlight the critical role of filler characteristics—particularly SSA, porosity and dispersion—in optimising SiR performance under HVDC stress. The study also demonstrates that additional experimental precautions improved DDT reproducibility, supporting its use in future standardisation.